Oncol Rev (2007) 1:81–90 DOI 10.1007/s12156-007-0010-8
Honey V. Reddi Alicia Algeciras-Schimnich Bryan McIver Norman L. Eberhardt Stefan K.G. Grebe
Received: 1 June 2007 Accepted in revised form: 20 June 2007
S.K.G. Grebe (쾷), Department of Laboratory Medicine/Pathology, 200 First Street SW, Mayo Clinic, Rochester, MN 55905, USA. Tel.: +1-507-284-3345 Fax: +1-507-284-9758 e-mail:
[email protected]
H.V. Reddi • B. McIver N.L. Eberhardt Department of Medicine, Division of Endocrinology Mayo Clinic, Rochester, MN 55905, USA A. Algeciras-Schimnich S.K.G. Grebe Department of Laboratory Medicine and Pathology, Mayo Clinic, Rochester, MN 55905, USA N.L. Eberhardt Department of Biochemistry and Molecular Biology, Mayo Clinic, Rochester, MN 55905, USA
REVIEW
Chromosomal rearrangements and the pathogenesis of differentiated thyroid cancer
Abstract The majority of thyroid cancers arise from the follicular cells of the thyroid gland, which yield a wide variety of distinct morphotypes, ranging from relatively indolent lesions to the most malignant forms of cancer known. The remaining primary thyroid cancers arise from C cells within the gland and result primarily from mutations of the RET protooncogene, germ line mutations of which give rise to the various forms of multiple endocrine neoplasia. The most common of the follicular cell-derived cancers are papillary carcinomas, (PTC), followed by follicular carcinomas (FTC) and its Hurthle cell variant (HCC) and finally anaplastic carcinomas (ATC). The pathogenesis of many thyroid cancers, of both PTC and FTC morphotype, involves chromosomal translocations. Rearrangements of the RET protoconcogene are known to be involved in the pathogenesis of ca. 50% of PTC. A similar proportion of FTC have been associated with a t(2;3)(q13;p25) translocation, fus-
Differentiated thyroid cancer Endocrine glands are uniquely susceptible to neoplastic transformation. Based on autopsy and radiographic surveys, incidental adrenal, pituitary, and thyroid adenomas and car-
ing the thyroid-specific transcription factor PAX8 with the peroxisome proliferator-activated receptor gamma (PPARγ) nuclear receptor, a ubiquitously expressed transcription factor. These rearrangements have analogy with translocations in erythropoetic cells, which form the only other known group of human malignancies that are largely the result of chromosomal translocation events. In this review we compare and contrast the oncogenic properties of thyroid and erythroid chromosomal transformations and speculate on mechanisms leading to their formation.
Key words Follicular thyroid carcinoma • Papillary thyroid carcinoma • RET protooncogene • Peroxisome proliferators activated receptorgamma • PAX8 • Chromosomal translocations
cinomas affect 5%–20% of the population [1–6]. While most tumours in endocrine glands remain undetected and are benign, there are significant numbers of clinically evident malignant tumours of the thyroid. Indeed, thyroid carcinoma represents the most common endocrine malignancy, with an estimated 33,500 new cases, predicted to cause 1,500 deaths
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in the USA during 2007 [7]. During this time period, all other endocrine malignancies will account for only 2,000 new cases and around 800 deaths [7]. Over the last several years the rates of thyroid cancer have increased, whereas most other cancer incidence rates have either remained unchanged or have decreased. The reasons for this increased incidence are unknown, though improved detection has certainly played a role. Nevertheless, there is a clear need for improved understanding of the mechanisms involved in thyroid tumour pathogenesis, since knowledge of these may help to identify factors that influence incidence rates, provide clinically useful molecular prognostic factors, and assist in tailoring an approach to treatment that prevents unnecessarily aggressive treatment for the low-risk majority, while avoiding under-treatment for the minority of patients with more aggressive disease variants. Follicular cells of the thyroid give rise to several distinct cancer types, including papillary (PTC), follicular (FTC), Hurthle cell (HCC) and anaplastic (ATC). These subtypes are phenotypically distinct and exhibit extremes of malignant potential, from relatively indolent (PTC) to highly aggressive (ATC), with distinctive patterns of growth and spread. Follicular carcinoma (FTC) may arise in an adenoma-carcinoma sequence from follicular adenoma (FA), most of which are cytologically difficult to distinguish from FTC [8], The pathogenesis of HCC remains unclear, and there is controversy whether it represents a distinct morphotype or evolves on the background of PTC or FTC. ATC is the least common of the follicular cell derived thyroid cancers, but certainly the most aggressive, with >90% 1 year mortality. ATC probably arises in most cases from pre existing PTC, FTC, or HCC, through a mechanism that involves loss of p53 function by mutation or loss of expression [9, 10], impaired Rb pathway function [11], or abnormalities in beta-catenin signaling [12]. ATC are among the most aggressive of all human tumours, but fortunately only account for ca. 1%–5% of all thyroid cancers. The well-differentiated thyroid cancers include two distinct morphotypes, PTC and FTC that are readily distinguished by their distinct microscopic appearance. In contrast, Hurthle cell carcinoma is classified as a variant of FTC, which exhibits at least 80% of its cells with Hurthle cell features. The distinct biology of HCC may warrant a unique classification, however, and improved understanding of the molecular genetic basis of thyroid carcinoma may provide a means to improve the current classification system for these tumours. PTC and FTC (including HCC) account for 80%–90% of all thyroid cancers. The majority of these are PTC, while FTC accounts for approximately 10%–20% of all well-differentiated thyroid cancers, but up to 40% of cause-specific deaths in this group. Follicular cell-derived thyroid cancers (PTC and FTC) differ from most other solid malignancies in their high
propensity to undergo chromosomal rearrangements, which result in the creation of fusion oncogenes. While numerical and structural chromosomal abnormalities are common in all malignancies, the formation of fusion oncogenes is an almost unique feature of thyroid cancer amongst epithelial tumours. In carcinomas other than thyroid cancer, chromosomal abnormalities tend to represent a manifestation of chromosomal instability; while such changes may contribute to the progression of the malignant phenotype, it does not appear to be a major mechanism for tumour initiation in other epithelial malignancies. In particular, only a very small proportion of the various translocations that have been described in non-thyroid epithelial tumours are frequent, occurring in only a small proportion of tumours. Moreover, virtually none of the few known recurrent rearrangements have been associated with specific and defined pathogenic tumour-genetic abnormalities, such as tumour suppressor gene deletion, oncogene over-expression, or creation of a fusion oncogene. By contrast, soft tissue tumours, and, to an even greater extent, hematopoetic-lymphatic malignancies, exhibit frequently chromosomal rearrangements that are associated with well-defined tumour genetic events. Indeed, there is almost no hematologic or lymphatic tumour that does not show some chromosomal rearrangement [13]. Certain genes with oncogenic potential are either transferred out of their normal promoter context into a different location, where they are expressed outside their physiologic transcriptional control, or fusion genes are created. In functional terms, the resultant oncogenic rearrangements fall into three broad categories: (i) those that lead to aberrant activation of kinase signaling cascades, (ii) those that cause over-expression of genes with anti-apoptotic properties, and (iii) those that bring about the abnormal expression of transcription factors, leading to either dysregulated over-expression of a potentially oncogenic transcription factor, or formation of an abnormal transcription factor, usually a fusion oncogene, which interferes with the function of its wild-type counterpart. The specific type of rearrangement is predictive of tumour morphotype and the functional category predicts tumour behavior. Tumours with somatic genetic changes that affect transcription factors are usually the most aggressive, while those with kinase-activating changes tend to be indolent. Thyroid tumours exhibit disease and morphotype-specific chromosomal rearrangements in ~40% of both PTC and FTC, not quite as frequent as in hematologic malignancies, but far more common than in other epithelial tumours. All of the known changes create fusion oncogenes, which result in either aberrant kinase signaling (in PTC), or novel transcription factors (in FTC). Thyroid cancer therefore behaves in a manner analogous to hematopoetic tumours in terms of the functional categories of rearrangements that occur. This may have important implications, especially for therapy.
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Understanding the mechanisms and pathways by which these fusion proteins operate might improve thyroid cancer diagnosis and allow the development of novel, targeted therapeutic approaches. In this review we will briefly cover the known PTC-specific oncogenic chromosomal rearrangements, about which a good deal is known, and then cover in greater detail the more recently described FTC specific rearrangements, which remain much less well studied. Finally, we will speculate what might predispose thyroid tumours to develop recurrent and highly specific oncogenic chromosomal rearrangements. Analogies to hematological malignancies will be drawn whenever the actual data on thyroid neoplasms are incomplete or inconclusive.
PTC and oncogenic chromosomal rearrangements Chromosomal rearrangements of the RET (10q11.2) and NTRK1 (1q21-22) proto-oncogenes are frequently involved in both sporadic and radiation-induced PTC (reviewed in [14–16]). Both RET and NTRK1 proto-oncogenes encode tyrosine kinase receptors that are located in the plasmalemma [17]. The RET extracellular domain contains a binding site for several members of the glial cell line-derived neurotrophic factor (GDNF) family, which mediate its normal functions in the development of neural crest-derived cells [18]. The NTRK1’s extracellular domain binds nerve growth factor (NGF), which mediates its actions in both central and peripheral nervous system development [19]. Like many tyrosine kinase receptors, ligand binding induces receptor dimerization and autophosphorylation of the receptors’ cytoplasmatic domains [20]. The phosphorylated sites then serve as recognition sites for a number of intracellular adaptor proteins, including Shc, GRB2, FRS2, PI3K, and PLCγ which activate downstream signaling pathways, including the Ras/MAPK and PI3K signal cascades [21]. RET is physiologically expressed in parafollicular C cells, but not in thyroid follicular cells. Sporadic and familial mutations of RET within the parafollicular C-cells, or in the germ line, give rise to sporadic and familial MTC respectively, and to the multiple endocrine neoplasia syndromes type 2A and 2B (MEN2A and MEN2B), while loss of RET function is involved in Hirschsprung disease [22]. In addition to neurotrophic functions, NTRK1 signaling is active physiologically in a number of other cell types, including lymphocytes, keratinocytes and prostate cells [23–25]; however, it is not normally expressed at appreciable levels in mature follicular thyrocytes. Aberrant expression of RET or NTRK1 in differentiated thyroid follicular cells occurs through one of a large number of different chromosomal rearrangements. All of these rearrangements juxtapose genes that are normally expressed
in follicular cells just upstream of the coding sequence to the tyrosine kinase domains of RET or NTRK1 (reviewed in [14, 26, 27]). The various RET or NTRK1 fusion partners all retain their respective promoters, leading to constitutive expression of the fusion oncogenes. The breakpoints within RET or NTRK lie 3’ to the coding sequence of the extracellular and transmembrane domains but 5’ to those of the tyrosine kinase domains. The resultant fusion oncogenes therefore consist of a variable length of the fusion partners’ 5’ portion coupled to a full-length RET or NTRK1 tyrosine kinase coding region. Since all the known fusion partners in these rearrangements contain sequences that code for strong protein-protein interaction domains – either coiled-coil domains [26, 28–30], or reactive cysteine residues [27] – the resultant fusion proteins dimerize spontaneously, resulting in ligand-independent autophosphorylation, and activation of downstream signaling cascades, typically through the ERK/MAPK pathway [31–36]. Support for the importance of this signaling cascade as the pivotal PTC oncogenic pathway comes from observations that activating mutations in BRAF, and to a lesser degree, RAS are also observed frequently in PTC and may be causative in some of these tumours [34, 36]. In addition, a novel rearrangement of BRAF has been observed in some radiation-induced papillary carcinomas [37]. This rearrangement involves a paracentric inversion of chromosome 7q, resulting in an inframe fusion between exons [1–8] of the AKAP9 gene and exons [9–18] of BRAF. This fusion protein contains the BRAF protein kinase domain, but lacks its N-terminal autoinhibitory domain. RET-rearrangements, RAS mutations and BRAF mutations collectively account for ~70% of PTC and appear to be mutually exclusive, indicating that a single oncogenic hit in this kinase-cascade is sufficient for PTC development [36]. It remains uncertain, whether the remaining ~30% of PTC are caused by inappropriate activation of other kinases in this pathway, or whether an alternative RAS-downstream signaling route, involving AKT/STAT [38, 39] might be involved in these cases. There are remarkable similarities between the RET and NTRK1 rearrangements in PTC and rearrangements that are observed in myeloid lineage cells involving activation of several tyrosine kinases. This is most commonly observed in chronic myeloid leukemia (CML). The t(9;22)(q34;q11.2) translocation is the most frequent recurrent rearrangement observed in CML, which results in the formation of the BCR-ABL1 fusion gene. In addition to BCR-ABL, BCRJAK2, TEL-ABL, TEL-JAK2 TEL-PDGFβR, HIP1PDGFβR, H4-PDGFβR, and RAB5E-PDGFβR fusions have been identified (reviewed in [40]). All of these fusion events, like the RET or NTRK1 fusions, create proteins with a N terminal dimerization motif – primarily coiled-coil domains – and a C terminal tyrosine kinase domain. Dimerization results in constitutive activation of the respec-
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tive tyrosine kinase. Paralleling the process seen in PTC, activation of tyrosine kinase signaling pathways appears sufficient for transformation, as each of the lymphoblastoid fusion tyrosine kinases is capable of transforming cultured hematopoietic cell lines like Ba/F3 and 32D to cytokineindependent growth. As in PTC, CML is also a relatively indolent and well differentiated malignancy, at least until additional oncogenic hits are accumulated, again analagous to the behavior of PTC, with a small subset of tumours that appear to become dedifferentiated, presumably through the accumulation of additional cumulative genetic changes. CML is characterized by leukocytosis with normal cellular maturation and differentiation. While PTC assumes a unique morphotype (papilliform structures), the individual cells retain most of their differentiated phenotype, including the capacity to concentrate iodine, and express and secrete proteins including thyroglobulin.
Oncogenic chromosomal rearrangements in FTC – abnormal transcription factors Until recently, understanding of the pathways involved in FTC pathogenesis have lagged behind that of PTC. This situation has started to change in the last 7 years, with the recent evidence of a unique recurrent chromosomal rearrangement. Numerical and structural chromosomal abnormalities are fairly common in FTC. In particular, anomalies involving the short arm of chromosome 3 have been noted in multiple studies [41–43]. Seven years ago a common translocation involving chromosome 3 was identified in a proportion of FTC [44]. Between 25 and 70% of FTC, and a lesser proportion of follicular adenomas (FA), carry a balanced translocation of most of the q-arm of chromosome 2 to the p-arm of chromosome 3, while in exchange the 3p25-pter region is translocated to 2q13 (t(2;3;)(q13;p25) [15, 45–48]. This translocation creates a fusion transcript in which the 5’region of the thyroid-specific transcription factor PAX8 gene (2q13) is fused in-frame with exon1 of the peroxisome proliferator-activated receptor gamma (PPARγ) gene (3p25), a member of the thyroid hormone-steroid hormone nuclear receptor super family [44]. The PAX8 promoter, which is active in thyroid follicular cells, appears to drive expression of the fusion gene [49]. PAX8 is a member of the paired-box family of transcription factors, which is necessary for normal thyroid development [50], while PPARγ is involved in adipocyte differentiation, lipid and carbohydrate metabolism as well as cell proliferation and differentiation (reviewed in [51, 52]). Studies in our laboratory have demonstrated that transient or constitutive expression of full-length PAX8/PPARγ fusion protein (PPFP) in immortalized human thyrocytes
generate a growth advantage compared to cells expressing control vector [53], supporting an oncogenic role for PPFP, an observation that has been confirmed by others [54]. The impact of PPFP on the observed increase in cell numbers is mediated through a substantially reduced rate of apoptosis and to a lesser effect on accelerated cell cycle kinetics [53]. In addition to increased cell growth, PPFP expression is also associated with a significant increase in the rate of soft agar colony formation [53], strongly suggesting that PPFP is a true transforming agent in these cells. What remains unclear is the mechanism through which PPFP acts. Comparisons with sarcomas and hemato-lymphatic neoplasms, might shed light on this question. Fusion oncogenes involving paired box genes have been implicated in several subgroups of these malignancies. In alvelolar rhabdomyosarcoma PAX3 (2q35) or PAX7 (1p36) are translocated to 13q14.1, joining them to the forkhead family transcription factor FKHR (forkhead homolog rhabdomyosarcoma) (reviewed in [55–57]. The fusion gene consists of a truncated portion of FKHR, missing its transactivation domain, and full-length PAX3 or PAX7 sequences. As a consequence, PAX3 or PAX7, which are normally only expressed during embryogenesis, are expressed extopically and in altered, fusion gene form, from the FKHR promoter. The resultant fusion proteins show increased activity on PAX3 and PAX7 response elements and induce proliferation, de-differentiation and eventually transformation of mature myocytes. An analagous recurrent translocation involving PAX genes is observed in B-cell Nnon-Hodgkin’s lymphomas, in which a t(9;14) (p13;q32) translocation transfers the PAX5 gene into the IgH locus (reviewed in [58, 59]). In adult humans, PAX5 acts as a B-cell specific transcription factor that induces development and expansion of B-cell precursor clones. It is switched off in mature B-cells before they transition into plasma cells. Following the translocation, PAX5 continues to be constitutively expressed from promoters in the IgH locus, leading to clonal expansion of affected cells and malignant transformation. All the translocations involving PAX genes in these neoplasms therefore represent gain of function somatic genetic changes. Another interesting parallel between FTC and erythropoietic neoplasms is the finding that clustered homeobox-containing genes (HOX), which are critical for erythropoetic development, have been strongly implicated in leukemic hematopoiesis [60]. Like the PAX proteins, the HOX proteins are DNA binding transcription factors with a critical developmental function. The involvement of HOX genes in human leukemia is supported by their observed aberrant expression, translocations involving their cofactor PBX1 and chromosomal rearrangements involving their upstream regulator MLL and CDX2 (reviewed in [61]). HOX genes are deregulated in several leukemias and HOXA9 has been identified in AML as the single gene whose expression most cor-
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related with treatment failure [62]. ABD-B HOX genes are the most common fusion partners of the NUP98 gene, located on chromosome 11p15.5, detected in AML and CML [63]. A subset of human acute leukemias, possess a chromosomal translocation involving the mixed-lineage leukemia gene (MLL, HRX, ALL1), a regulator of HOX gene expression, on chromosome segment 11q23 [64]. While fusion genes or oncogenic translocations involving PPARγ have not been observed in soft tissue tumours or hematologic malignancies, translocations of other liganded transcription factors from the thyroid hormone-steroid hormone nuclear receptor super family are also well recognized in these non-epithelial malignancies. The most prominent examples are the oncogenic chromosomal rearrangements observed in pro-myelocytic leukemias, which all involve the formation of fusion oncogenes of the retinoic acid receptor A gene (RARA) and other genes (reviewed in [65–67]. The fusion genes involving RARA are particularly interesting, as RARA utilizes the retinoic X receptor as a heterodimerization partner, a feature shared with PPARγ. The functional consequences of fusion gene expression are well described for the most common RARA fusion gene, PML/RARA, which is created by a t(15;17)(q22;q21). Its fusion protein has a dominant negative effect on the biological activity of the remaining wild-type RARA protein [68, 69]. RARA expression is required for maturation of promyelocytes to myelocytes, and the disruption of this process launches these cells down an oncogenic path. These neoplastic fusion genes therefore transform cells through a loss of wild-type allele function. Extrapolating these observations to FTC and PPFP, it is likely that either increased PAX8 function or diminished PPARγ function underlie the oncogenic role of the fusion protein. Early studies by Kroll et al. [44] suggested that PPFP exerted dominant negative effects on wild-type PPARγ. Further in vitro studies have confirmed these findings, indicating that PPFP function is mediated at least in part by inhibition of wild-type PPARγ function [44, 53, 54] or down-regulation of wild-type PPARγ expression [48]. in wild-type PPARγ, ligand binding and activation of the ligand-gated activation function 2 (AF2) domain increase the transcriptional activity of PPARγ, but these same processes also induce ubiquitination and subsequent proteosomal degradation of the transcription factor [70], providing a negative feedback system to balance PPARγ activity. We hypothesize that PPFP interferes in this balance, through direct inhibition of wild-type PPARγ transcription factor activity, competition for its heterodimeric partner RXR or for its ligand, or altering turnover of PPARγ. Ultimately, this process inhibits PPARγ action and pushes the cell towards proliferation. Gene transcription activation by PPARγ requires the formation of a heterodimer with retinoic X receptor (RXR),
followed by binding to a PPARγ response element (PPRE). Transactivation of the heterodimer is mediated by ligands of PPARγ, RXR, or both. Since PPFP has the ability to independently bind RXR [54] and probably to the PPRE, it is possible that PPFP competes with wild-type PPARγ for RXR, DNA binding sites, or both, thus preventing wild-type PPARγ-initiated transcriptional regulation. In addition, the PPFP-RXR complex might still recruit cofactors, which are bound, but do not initiate transactivation, further diminishing the necessary resources for wild-type PPARγ action. There is one report of a t(3;7)(p25;q31) rearrangement in FTC [71] that also involves the PPARγ gene, but a different fusion partner. This translocation has been reported to fuse the promoter and coding sequences of the CREB3L2 gene to most of the coding sequences of PPARγ [45]. However further studies of this putative fusion oncogene have not been reported to date. It is therefore difficult to make any conclusions on its pathogenic mechanism, but it appears to confirm that abnormal PPARγ function might play the dominant role in PPFP tumourigenesis. In contrast, we have not been able to demonstrate any definitive increase in PAX8 function, suggesting that PAX8/PPARγ is functionally more akin to PML/RARA than to the various PAX gene rearrangements observed in other malignancies. There remains, however, the possibility that PPFP could express some aberrant PAX8 function, or completely novel functions, under certain conditions and on certain response elements. Data reported by Au et al. [54] hint at this possibility, since PPFP stimulated transcription from PAX8-responsive thyroperoxidase and sodium-iodide symporter promoters, but failed to stimulate transcription from the thyroglobulin promoter and blocked the synergistic stimulation of this promoter by wild-type PAX8 and thyroid transcription factor-1. These data argue for a complex influence of PPFP on transcription from both PPARγ and PAX8 dependent promoters. Furthermore, analysis of gene expression array data of FTC that express PPFP, demonstrate that these cancers have a distinct transcriptional signature [72–74]. PPFP expression up-regulates genes associated with signal transduction, cell growth and translational control, while a large number of ribosomal protein and translational associated genes are concurrently under-expressed. Giordano et al. demonstrated that PPFP has unique transcriptional activities [72], with the potential to function in ways qualitatively similar to PAX8 or PPARγ, depending on the promoter and cellular environment [72]. PPFP has been shown to disrupt normal transcriptional pathways of PAX8 in a cell-type specific manner [54], presumably through some form of negative feedback. Based on these observations, the hypothesis that PPFP functions primarily through the control of PPARγ might require re-evaluation or refinement. Further analysis of the down-stream regulatory pathways of PPFP will be essential as we begin to design strategies to
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intervene in the fundamental mechanisms of FTC. In particular, it will be important to determine whether PPFPexpressing FTC will respond to high dose ligand treatment, analogous to the situation in promyelocytic leukemia, and whether PPARγ agonists or RXR ligands or both could be used. If additional pathways turn out to be involved, then it will be interesting to see if ligand treatment can reverse PPFP’s effect on these also, or whether additional interventions might be required to reverse the impact of this fusion oncogene on cellular proliferation and the malignant phenotype of FTC.
The basis of recurrent chromosomal rearrangements in thyroid cancer Chromosomal rearrangements are the result of non-homologous or homologous recombination (reviewed in [75–79]). The immediate upstream event is either a double stranded DNA break (DSB) or a Holliday junction structure. Outside of Meiosis, DSBs can be created directly by enzymes such as topoisomerase II, or by a variety of noxious agents, in particular high-energy ionizing radiation. They can also arise from replication-dependent triggers. For example, single stranded DNA breaks (SSBs), created either by direct SSB-inducing substances such as superoxide, or as a consequence of incomplete nucleotide excision or base excision repair, might be converted to DSBs during replication, while altered nucleotides or DNA adjuncts that are not cleared at all by nucleotide excision base excision repair, can result in replication fork stalling. A stalled replication fork can either be processed into a DSB, or reconfigured into a Holliday junction structure, which can be processed directly as a substrate for homologous recombination. By contrast, DSBs, by whatever mechanism they are created, form potent substrates for both homologous and non-homologous recombination. DNA-damage is therefore always the key initial event that results in non-meiotic recombination. Whether the final outcome of the DNA damage is a homologous or non-homologous recombination event, depends on a number of additional factors. These include: (i) the nature of the DNA damage – does the noxious stimulus directly induce DSBs? (ii) the replication status of the cell – is it a dividing cell that might end up converting a number of different DNA damaging stimuli into a DSB or a Holliday junction structure? and (iii) the capability of the cell to survive extensive DNA damage – is this a cell that has an active repair system and a downregulated apoptosis program? Based on these considerations the best candidate cells for induction of recurrent chromosomal rearrangements are organ stem cells [80, 81]. These cells are actively dividing,
have active DNA repair systems, and down-regulated or completely inactivated apoptotic pathways. For hematopoetic malignancies there is strong evidence that lineage specific stem cells are indeed the targets of tumour induction [82]. Many chromosomal translocations in these tumours have been shown to be acquired prenatally and affect genes that promote the continuation of the pluripotent stage in affected cells, eventually resulting in malignant transformation [82]. Some evidence has emerged that a similar process may be involved at least in radiation-induced PTC. In the Chernobyl population of cancers, RET/PTC rearrangements are very prevalent, and have affected younger individuals disproportionately, indicating a likely pre- and early post-natal tumourigenic event [83–85]. This age-dependent susceptibility argues for a developmentally dependent process, influencing either the process of DSBs or their repair, and would be consistent with the involvement of a thyroid stem cell. Subpopulations of cells capable of initiating AML in immunodeficient mice have been identified and found to have phenotypic characteristics of hematopoetic stem cells [86]. In solid tumours the presence of cancer stem cells have been described in breast, prostate, colon, and brain tumours [87–92]. Within the thyroid, candidate stem cells have been observed in solid cell nests, which are embryonic remnants of endodermal origin composed of main cells and C-cells [93, 94]. Main cells express P63, a homolog of P53, and a stem cell marker [95, 96]. Burstein et al. have proposed that P63-positive embryonic remnants, rather than mature follicular cells, are the cells that give rise to papillary carcinomas [93]. The actual trigger for PTC development appears to be ionizing radiation in many cases. Ionizing radiation, particularly through high-energy, short range beta particles from iodine isotopes, has been recognized for several decades as a strong risk factor for PTC. The radiation induces DSBs directly, but may also create indirect oxidative damage. Open chromatin regions, representing genes actively transcribed in affected cells, seem to be more susceptible, again implicating thyroid stem cells in PTC, as RET transcription is switched off in mature thyrocytes. Finally, the chromatin structure and the cell-specific physical proximity of various different DNA regions in interphase nuclei determines the possible recombination partners [97]. If the resultant rearrangement leads to loss of growth control or loss of control of organ stem cell maturation, as in RET/PTC, malignancy ensues. In keeping with this model, RET/PTC ooncogenes introduced into mouse thyroid stem cells result in PTC, while those that are introduced experimentally into mature thyrocytes fail to induce transformation or carcinogenesis [98, 99]. For FTC and the PAX8/PPARγ rearrangements, the triggers and predisposing events are not yet understood, so the possible involvement of stem cells in the process remains to be explored.
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To fully define the characteristics of the cells of origin of thyroid tumours and the basis of the recurrent chromosomal rearrangement, more work is needed to identify and characterize thyroid stem cells, and to induce various forms of DNA damage experimentally at different stages of stem cell maturation. This would allow unequivocal identification of
the cells and events that lead to various signature chromosomal rearrangements. Transplantation of these cell populations into immunodeficient mice to test their tumourigenic potential will then be the final step towards closing the loop to clinical observation.
References 1. Bertherat J, Mosnier-Pudar H, Bertagna X (2002) Adrenal incidentalomas. Curr Opin Oncol 14:58–63 2. Bondeson L, Ljungberg O (1981) Occult thyroid carcinoma at autopsy in Malmo, Sweden. Cancer 47:319–323 3. Delides GS, Elemenoglou J, Lekkas J et al (1987) Occult thyroid carcinoma in a Greek population. Neoplasma 34:119–125 4. Komorowski RA, Hanson GA (1988) Occult thyroid pathology in the young adult: an autopsy study of 138 patients without clinical thyroid disease. Hum Pathol 19:689–696 5. Molitch ME, Russell EJ (1990) The pituitary “incidentaloma”. Ann Int Med 112:925–931 6. Mortensen JD, Wollner LB, Bennett WA (1955) Gross and microscopic findings in clinically normal thyroid glands. J Clin Endocrinol Metab 15:1270–1280 7. Jemal A, Siegel R, Ward E et al (2007) Cancer statistics, 2007. CA Cancer J Clin 57:43–66 8. Goellner JR, Gharib H, Grant CS, Johnson DA (1987) Fine needle aspiration cytology of the thyroid, 1980 to 1986. Acta Cytol 31:587–590 9. Wynford-Thomas D (1997) Origin and progression of thyroid epithelial tumours: cellular and molecular mechanisms. Hormone Res 47:145–157 10. La Perle KM, Jhiang SM, Capen CC (2000) Loss of p53 promotes anaplasia and local invasion in ret/PTC1induced thyroid carcinomas. Am J Pathol 157:671–677 11. Onda M, Nagai H, Yoshida A et al (2004) Up-regulation of transcriptional factor E2F1 in papillary and anaplastic thyroid cancers. J Hum Genet 49:312–318
12. Kurihara T, Ikeda S, Ishizaki Y et al (2004) Immunohistochemical and sequencing analyses of the Wnt signaling components in Japanese anaplastic thyroid cancers. Thyroid 14:1020–1029 13. Mitelman F (2000) Recurrent chromosome aberrations in cancer. Mutation Res 462:247–253 14. Nikiforov YE (2002) RET/PTC rearrangement in thyroid tumours. Endocr Pathol 13:3–16 15. Tallini G (2002) Molecular pathobiology of thyroid neoplasms. Endocr Pathol 13:271–288 16. Arighi E, Borrello MG, Sariola H (2005) RET tyrosine kinase signaling in development and cancer. Cytokine Growth Factor Rev 16:441–467 17. Takahashi M, Buma Y, Iwamoto T et al(1988) Cloning and expression of the ret proto-oncogene encoding a tyrosine kinase with two potential transmembrane domains. Oncogene 3:571–578 18. Airaksinen MS, Titievsky A, Saarma M (1999) GDNF family neurotrophic factor signaling: four masters, one servant? Mol Cell Neurosci 13:313–325 19. Tessarollo L (1998) Pleiotropic functions of neurotrophins in development. Cytokine Growth Factor Rev 9:125–137 20. Longati P, Comoglio PM, Bardelli A (2001) Receptor tyrosine kinases as therapeutic targets: the model of the MET oncogene. Curr Drug Targets 2:41–55 21. Kaplan DR, Miller FD (2000) Neurotrophin signal transduction in the nervous system. Curr Opin Neurobiol 10:381–391 22. Smith DP, Eng C, Ponder BA (1994) Mutations of the RET proto-oncogene in the multiple endocrine neoplasia type 2 syndromes and Hirschsprung disease. J Cell Sci Supp 18:43–49
23. Otten U, Ehrhard P, Peck R (1989) Nerve growth factor induces growth and differentiation of human B lymphocytes. Proc Natl Acad Sci U S A 86:10059–10063 24. Djakiew D, Delsite R, Pflug B et al (1991) Regulation of growth by a nerve growth factor-like protein which modulates paracrine interactions between a neoplastic epithelial cell line and stromal cells of the human prostate. Cancer Res 51:3304–3310 25. Di Marco E, Mathor M, Bondanza S et al (1993) Nerve growth factor binds to normal human keratinocytes through high and low affinity receptors and stimulates their growth by a novel autocrine loop. J Biol Chem 268:22838–22846 26. Pierotti MA, Greco A (2006) Oncogenic rearrangements of the NTRK1/NGF receptor. Cancer Lett 232:90–98 27. Jhiang SM (2000) The RET protooncogene in human cancers. Oncogene 19:5590–5597 28. Greco A, Fusetti L, Miranda C et al (1998) Role of the TFG N-terminus and coiled-coil domain in the transforming activity of the thyroid TRKT3 oncogene. Oncogene 16:809–816 29. Ohan N, Sabourin D, Booth RA, Liu XJ (2000) Xenopus laevis TRK-fused gene (TFG) is an SH3 domain binding protein highly expressed in the cement gland. Mol Reprod Develop 56:336–344 30. Monaco C, Visconti R, Barone MV et al (2001) The RFG oligomerization domain mediates kinase activation and re-localization of the RET/PTC3 oncoprotein to the plasma membrane. Oncogene 20:599–608 31. Jhiang SM, Cho JY, Furminger TL, et al (1998) Thyroid carcinomas in RET/PTC transgenic mice. Recent Result Cancer Res 154:265–270
88
32. Powell DJ, Russell J, Nibu K et al (1998) The RET/PTC3 oncogene: metastatic solid-type papillary carcinomas in murine thyroids. Cancer Res 58:5523–5528 33. Tallini G, Asa SL (2001) RET oncogene activation in papillary thyroid carcinoma. Adv Anat Pathol 8:345–354 34. Nikiforova MN, Kimura ET, Gandhi M et al (2003) BRAF mutations in thyroid tumours are restricted to papillary carcinomas and anaplastic or poorly differentiated carcinomas arising from papillary carcinomas. J Clin Endocrinol Metab 88:5399–5404 35. Knauf JA, Kuroda H, Basu S, Fagin JA (2003) RET/PTC-induced dedifferentiation of thyroid cells is mediated through Y1062 signaling through SHC-RAS-MAP kinase. Oncogene 22:4406–4412 36. Kimura ET, Nikiforova MN, Zhu Z et al (2003) High prevalence of BRAF mutations in thyroid cancer: genetic evidence for constitutive activation of the RET/PTC-RAS-BRAF signaling pathway in papillary thyroid carcinoma. Cancer Res 63:1454–1457 37. Ciampi R, Knauf JA, Kerler R et al (2005) Oncogenic AKAP9-BRAF fusion is a novel mechanism of MAPK pathway activation in thyroid cancer. J Clin Invest 115:94–101 38. Saito J, Kohn AD, Roth RA et al (2001) Regulation of FRTL-5 thyroid cell growth by phosphatidylinositol (OH) 3 kinase-dependent Akt-mediated signaling. Thyroid 11:339–351 39. Ringel MD, Hayre N, Saito J et al (2001) Overexpression and overactivation of Akt in thyroid carcinoma. Cancer Res 61:6105–6111 40. Gilliland DG (2002) Molecular genetics of human leukemias: new insights into therapy. Semin Hematol 39:6–11 41. Herrmann MA, Hay ID, Bartelt DH et al (1991) Cytogenetic and molecular genetic studies of follicular and papillary thyroid cancers. J Clin Invest 88:1596–1604 42. Jenkins RB, Hay ID, Herath JF et al (1990) Frequent occurrence of cytogenetic abnormalities in sporadic nonmedullary thyroid carcinoma. Cancer 66:1213–1220
43. Grebe SK, McIver B, Hay ID et al (1997) Frequent loss of heterozygosity on chromosomes 3p and 17p without VHL or p53 mutations suggests involvement of unidentified tumour suppressor genes in follicular thyroid carcinoma. J Clin Endocrinol Metab 82:3684–3691 44. Kroll TG, Sarraf P, Pecciarini L et al (2000) PAX8-PPARgamma1 fusion oncogene in human thyroid carcinoma. Science 289:1357–1360 45. Kroll TG (2004) Molecular events in follicular thyroid tumours. Cancer Treat Res 122:85–105 46. McIver B, Grebe SK, Eberhardt NL (2004) The PAX8/PPAR gamma fusion oncogene as a potential therapeutic target in follicular thyroid carcinoma. Curr Drug Targets Immun Endocrin Metab Disorder 4:221–234 47. Reddi HV, McIver B, Grebe SKG, Eberhardt NL (2007) The paired box8/peroxisome proliferator-activated receptor-gamma oncogene in thyroid tumourigenesis. Endocrinology 148:932–935 48. Fuhrer D (2001) A nuclear receptor in thyroid malignancy: is PAX8/PPARgamma the Holy Grail of follicular thyroid cancer? Eur J Endocrinol 144:453–456 49. Mascia A, Nitsch L, Di Lauro R, Zannini M (2002) Hormonal control of the transcription factor Pax8 and its role in the regulation of thyroglobulin gene expression in thyroid cells. J Endocrinol 172:163–176 50. Pasca di Magliano M, Di Lauro R, Zannini M (2000) Pax8 has a key role in thyroid cell differentiation. Proc Natl Acad Sci USA 97:13144–13149 51. Fajas L, Debril MB, Auwerx J (2001) Peroxisome proliferator-activated receptor-gamma: from adipogenesis to carcinogenesis. J Mol Endocrinol 27:1–9 52. Fajas L, Debril MB, Auwerx J (2001) PPAR gamma: an essential role in metabolic control. Nutr Metab Cardiovas Dis 11:64–69
53. Gregory Powell J, Wang X, Allard BL et al (2004) The PAX8/PPARgamma fusion oncoprotein transforms immortalized human thyrocytes through a mechanism probably involving wildtype PPARgamma inhibition. Oncogene 23:3634–3641 54. Au AY, McBride C, Wilhelm KG et al (2006) PAX8-peroxisome proliferatoractivated receptor gamma (PPARgamma) disrupts normal PAX8 or PPARgamma transcriptional function and stimulates follicular thyroid cell growth. Endocrinology 147:367–376 55. Mercado GE, Barr FG (2007) Fusions involving PAX and FOX genes in the molecular pathogenesis of alveolar rhabdomyosarcoma: recent advances. Curr Mol Med 7:47–61 56. Xia SJ, Pressey JG, Barr FG (2002) Molecular pathogenesis of rhabdomyosarcoma. Cancer Biol Ther 1:97–104 57. Barr FG (2001) Gene fusions involving PAX and FOX family members in alveolar rhabdomyosarcoma. Oncogene 20:5736–5746 58. Ohno H, Ueda C, Akasaka T (2000) The t(9;14)(p13;q32) translocation in B-cell non-Hodgkin's lymphoma. Leuk Lymphoma 36:435–445 59. Al-Saleem T, Al-Mondhiry H (2005) Immunoproliferative small intestinal disease (IPSID): a model for mature B-cell neoplasms. Blood 105:2274–2280 60. Grier DG, Thompson A, Kwasniewska A et al (2005) The pathophysiology of HOX genes and their role in cancer. J Pathol 205:154–171 61. Abramovich C, Pineault N, Ohta H, Humphries RK (2005) Hox genes: from leukemia to hematopoietic stem cell expansion. Ann N Y Acad Sci 1044:109–116 62. Golub TR, Slonim DK, Tamayo P, et al (1999) Molecular classification of cancer: class discovery and class prediction by gene expression monitoring. Science 286:531–537 63. Lam DH, Aplan PD (2001) NUP98 gene fusions in hematologic malignancies. Leukemia 15:1689–1695
89
64. Dimartino JF, Cleary ML (1999) Mll rearrangements in haematological malignancies: lessons from clinical and biological studies. Br J Haematol 106:614–626 65. Zhang JW, Wang JY, Chen SJ, Chen Z (2000) Mechanisms of all-trans retinoic acid-induced differentiation of acute promyelocytic leukemia cells. J Biosci 25:275–284 66. Yamamoto K, Hamaguchi H, Kobayashi M et al (1999) Terminal deletion of the long arm of chromosome 9 in acute promyelocytic leukemia with a cryptic PML/RAR alpha rearrangement. Cancer Genet Cytogenet 113:120–125 67. Kalantry S, Delva L, Gaboli M et al (1997) Gene rearrangements in the molecular pathogenesis of acute promyelocytic leukemia. J Cell Physiol 173:288–296 68. Duprez E, Benoit G, Flexor M et al (2000) A mutated PML/RARA found in the retinoid maturation resistant NB4 subclone, NB4-R2, blocks RARA and wild-type PML/RARA transcriptional activities. Leukemia 14:255–261 69. Zhong S, Delva L, Rachez C et al (1999) A RA-dependent, tumourgrowth suppressive transcription complex is the target of the PMLRARalpha and T18 oncoproteins. Nature Genet 23:287–295 70. Hauser S, Adelmant G, Sarraf P et al (2000) Degradation of the peroxisome proliferator-activated receptor gamma is linked to ligand-dependent activation. J Biol Chem 275:18527–18533 71. Lui WO, Kytola S, Anfalk L et al (2000) Balanced translocation (3;7)(p25;q34): another mechanism of tumourigenesis in follicular thyroid carcinoma? Cancer Genet Cytogenet 119:109–112 72. Giordano TJ, Au AY, Kuick R et al (2006) Delineation, functional validation, and bioinformatic evaluation of gene expression in thyroid follicular carcinomas with the PAX8-PPARG translocation. Clin Cancer Res 12:1983–1993 73. Lui WO, Foukakis T, Liden J et al (2005) Expression profiling reveals a distinct transcription signature in follicular thyroid carcinomas with a PAX8-PPAR(gamma) fusion oncogene. Oncogene 24:1467–1476
74. Lacroix L, Soria JC, Bidart JM, Schlumberger M (2005) Oncogenes et tumeurs de la thyroide. Bull Cancer 92:37–43 75. Pastink A, Eeken JC, Lohman PH (2001) Genomic integrity and the repair of double-strand DNA breaks. Mutation Res 480–481:37–50 76. Cahill D, Connor B, Carney JP (2006) Mechanisms of eukaryotic DNA double strand break repair. Front Biosci 11:1958–1976 77. Bryant PE (2004) Repair and chromosomal damage. Radiother Oncol 72:251–256 78. Bryant PE, Gray LJ, Peresse N (2004) Progress towards understanding the nature of chromatid breakage. Cytogen Genom Res 104:65–71 79. Shaw CJ, Lupski JR (2004) Implications of human genome architecture for rearrangement-based disorders: the genomic basis of disease. Hum Mol Genet;13 Spec No 1:R57–R64 80. Takano T, Amino N (2005) Fetal cell carcinogenesis: a new hypothesis for better understanding of thyroid carcinoma. Thyroid 15:432–438 81. Zhang P, Zuo H, Ozaki T et al (2006) Cancer stem cell hypothesis in thyroid cancer. Pathol Int 56:485–489 82. McHale CM, Smith MT (2004) Prenatal origin of chromosomal translocations in acute childhood leukemia: implications and future directions. Am J Hematol 75:254–257 83. Nikiforov Y, Gnepp DR (1994) Pediatric thyroid cancer after the Chernobyl disaster. Pathomorphologic study of 84 cases (1991-1992) from the Republic of Belarus. Cancer 74:748–766 84. Nikiforov Y, Gnepp DR, Fagin JA (1996) Thyroid lesions in children and adolescents after the Chernobyl disaster: implications for the study of radiation tumourigenesis. J Clin Endocrinol Metab 81:9–14 85. Nikiforov YE, Rowland JM, Bove KE (1997) Distinct pattern of ret oncogene rearrangements in morphological variants of radiation-induced and sporadic thyroid papillary carcinomas in children. Cancer Res 57:1690–1694
86. Bonnet D, Dick JE (1997) Human acute myeloid leukemia is organized as a hierarchy that originates from a primitive hematopoietic cell. Nature Med 3:730–737 87. Al-Hajj M, Wicha MS, BenitoHernandez A et al (2003) Prospective identification of tumourigenic breast cancer cells. Proc Natl Acad Sci USA 100:3983–3988 88. Singh SK, Hawkins C, Clarke ID, et al (2004) Identification of human brain tumour initiating cells. Nature 432:396–401 89. Piccirillo SG, Reynolds BA, Zanetti N et al (2006) Bone morphogenetic proteins inhibit the tumourigenic potential of human brain tumour-initiating cells. Nature 444:761–765 90. Ricci-Vitiani L, Lombardi DG, Pilozzi E et al (2007) Identification and expansion of human colon-cancer-initiating cells. Nature 445:111–115 91. Bao S, Wu Q, McLendon RE et al (2006) Glioma stem cells promote radioresistance by preferential activation of the DNA damage response. Nature 444:756–760 92. O'Brien CA, Pollett A, Gallinger S, Dick JE (2007) A human colon cancer cell capable of initiating tumour growth in immunodeficient mice. Nature 445:106–110 93. Burstein DE, Nagi C, Wang BY, Unger P (2004) Immunohistochemical detection of p53 homolog p63 in solid cell nests, papillary thyroid carcinoma, and hashimoto's thyroiditis: A stem cell hypothesis of papillary carcinoma oncogenesis. Hum Pathol 35:465–473 94. Reis-Filho JS, Preto A, Soares P et al (2003) p63 expression in solid cell nests of the thyroid: further evidence for a stem cell origin. Mod Pathol 16:43–48 95. Parsa R, Yang A, McKeon F, Green H (1999) Association of p63 with proliferative potential in normal and neoplastic human keratinocytes. J Invest Dermatol 113:1099–1105 96. Pellegrini G, Dellambra E, Golisano O et al (2001) p63 identifies keratinocyte stem cells. Proc Natl Acad Sci U S A 98:3156–3161
90
97. Nikiforova MN, Stringer JR, Blough R et al (2000) Proximity of chromosomal loci that participate in radiationinduced rearrangements in human cells. Science 290:138–141
98. Portella G, Vitagliano D, Borselli C et al (1999) Human N-ras, TRK-T1, and RET/PTC3 oncogenes, driven by a thyroglobulin promoter, differently affect the expression of differentiation markers and the proliferation of thyroid epithelial cells. Oncol Res 11:421–427
99. Jhiang SM, Sagartz JE, Tong Q et al (1996) Targeted expression of the ret/PTC1 oncogene induces papillary thyroid carcinomas. Endocrinology 137:375–378
