Proximity of chromosomal loci that participate in radiation-induced rearrangements in human cells.

Rearrangements involving the RET gene are common in radiation-associated papillary thyroid cancer (PTC). The RET/PTC1 type of rearrangement is an inversion of chromosome 10 mediated by illegitimate recombination between the RET and the H4 genes, which are 30 megabases apart. Here we ask whether despite the great linear distance between them, RET and H4 recombination might be promoted by their proximity in the nucleus. We used two-color fluorescence in situ hybridization and three-dimensional microscopy to map the positions of the RET and H4 loci within interphase nuclei. At least one pair of RET and H4 was juxtaposed in 35% of normal human thyroid cells and in 21% of peripheral blood lymphocytes, but only in 6% of normal mammary epithelial cells. Spatial contiguity of RET and H4 may provide a structural basis for generation of RET/PTC1 rearrangement by allowing a single radiation track to produce a double-strand break in each gene at the same site in the nucleus.

Interphase chromosome folding determines spatial proximity of genes participating in carcinogenic RET/PTC rearrangements.

Recurrent chromosomal rearrangements are common in cancer cells and may be influenced by nonrandom positioning of recombination-prone genetic loci in the nucleus. However, the mechanism responsible for spatial proximity of specific loci is unknown. In this study, we use an 18 Mb region on 10q11.2-21 containing the RET gene and its recombination partners, the H4 and NCOA4 (ELE1) genes, as a model chromosomal region frequently involved in RET/PTC rearrangements in thyroid cancer. RET/PTC is particularly common in tumors from children exposed to ionizing radiation. Using fluorescence in situ hybridization and three-dimensional microscopy, the locations of five different loci in this region were mapped in interphase nuclei of normal human thyroid cells. We show that RET and NCOA4 are much closer to each other than expected based on their genomic separation. Modeling of chromosome folding in this region suggests the presence of chromosome coiling with coils of approximately 8 Mb in length, which positions the RET gene close to both, the NCOA4 and H4, loci. There was no significant variation in gene proximity between adult and pediatric thyroid cells. This study provides evidence for large-scale chromosome folding of the 10q11.2-21 region that offers a structural basis for nonrandom positioning and spatial proximity of potentially recombinogenic intrachromosomal loci.

Targeted overexpression of IGF-I evokes distinct patterns of organ remodeling in smooth muscle cell tissue beds of transgenic mice.

Smooth muscle cells (SMC) of the vascular wall, bladder, myometrium, and gastrointestinal and respiratory tracts retain the ability to proliferate postnatally, which enables adaptive responses to injury, hormonal, or mechanical stimulation. SMC growth is regulated by a number of mesenchymal growth factors, including insulin-like growth factor I (IGF-I). To explore the function of IGF-I on SMC in vivo, the mouse SMC alpha-actin promoter fragment SMP8 (-1074 bp, 63 bp of 5'UT and 2.5 kb of intron 1) was cloned upstream of rat IGF-I cDNA, and the fusion gene microinjected to fertilized eggs of the FVB-N mouse strain. Mating of hemizygous mice with controls produced about 50% transgenic offspring, with equal sex distribution. Transgenic IGF-I mRNA expression was confined to SMC-containing tissues, with the following hierarchy: bladder > stomach > aorta = uterus > intestine. There was no transgene expression in skeletal muscle, heart, or liver. Radioimmunoassayable IGF-I content was increased by 3.5- to 4-fold in aorta, and by almost 10-fold in bladder of transgenic mice at 5 and 10 wk, with no change in plasma IGF-I levels. Wet weight of bladder, stomach, intestine, uterus, and aorta was selectively increased, with no change in total body or carcass weight of transgenic animals. In situ hybridization showed that transgene expression was exquisitely targeted to the smooth muscle layers of the arteries, veins, bladder, ureter, stomach, intestine, and uterus. Paracrine overproduction of IGF-I resulted in hyperplasia of the muscular layers of these tissues, manifesting in remarkably different phenotypes in the various SMC beds. Whereas the muscular layer of the bladder and stomach exhibited a concentric thickening, the SMC of the intestine and uterus grew in a longitudinal fashion, resulting in a marked lengthening of the small bowel and of the uterine horns. This report describes the first successful targeting of expression of any functional protein capable of modifying the phenotype of SMC in transgenic mice. IGF-I stimulates SMC hyperplasia, leading to distinct patterns of organ remodeling in the different tissue environments.

Distinct pattern of ret oncogene rearrangements in morphological variants of radiation-induced and sporadic thyroid papillary carcinomas in children.

In this study, we compare the morphological and genetic characteristics of 38 post-Chernobyl thyroid papillary carcinomas from Belarussian children 5-18 years old with those of 23 sporadic papillary carcinomas from the same age children without history of radiation exposure from Los Angeles and Cincinnati. Among radiation-induced tumors, solid variant of papillary carcinoma was found in 37%, follicular in 29%, typical papillary in 18%, and mixed and diffuse sclerosing variants in 8% each. In the sporadic group, a typical papillary pattern was prevalent in 70%, follicular in 17%, diffuse sclerosing variant in 9%, and solid in 4%. In both groups, the prevalence of ret rearrangements was high, but the frequency of specific types of rearrangement was significantly different. Among radiation-induced tumors, ret/PTC3 was found in 58%, ret/PTC1 in 16%, and ret/PTC2 in 3%, whereas among sporadic tumors, ret/PTC1 was found in 47% (P < 0.05), and ret/PTC3 was found in 18% (P = 0.01). The morphological variants of papillary carcinoma showed different prevalence of the specific types of ret rearrangement. Seventy-nine % of solid variant tumors had ret/PTC3, whereas only 7% had ret/PTC1 (P = 0.0007). Among typical papillary tumors, ret/PTC1 was found in 38%, ret/PTC3 in 19%, and ret/PTC2 in 5%. Thus, ret rearrangements are highly prevalent in pediatric papillary carcinomas from children exposed to radiation and in those occurring sporadically. However, the types of ret/PTC vary between these two populations, with ret/PTC3 present more commonly in post-Chernobyl tumors. Furthermore, solid variants have a high prevalence of ret/PTC3, whereas typical papillary carcinomas do not, suggesting that the different types of ret rearrangement confer neoplastic thyroid cells with distinct phenotypic properties.

Prevalence of minisatellite and microsatellite instability in radiation-induced post-Chernobyl pediatric thyroid carcinomas.

Exposure to ionizing radiation induces different forms of genomic instability in cultured cells and experimental animals. A higher rate of germline mutations at human hypervariable minisatellite loci was reported in children born from parents exposed to radiation after Chernobyl, implicating genome destabilization as a possible mechanism responsible for late radiation effects in humans. To test if radiation-induced carcinogenesis in the thyroid gland may be associated with somatic minisatellite instability or microsatellite instability, we utilized a PCR-based approach to study normal and tumor DNA from 17 pediatric post-Chernobyl papillary thyroid carcinomas for mutations at three different minisatellite loci (D1S80, D17S30, ApoB), and 27 microsatellite loci of di-, tri-, or tetranucleotide repeats. Minisatellite instability was found in three (18%) tumors, with one of them exhibiting mutations in all three minisatellite loci, whereas two others showed mutations in one of two informative markers. By contrast, none of 20 sporadic thyroid cancers from patients with no history of radiation exposure was positive for minisatellite instability. Microsatellite analysis of post-Chernobyl tumors revealed a mutation in one (6%) tumor only at the locus of D10S1412, whereas all other 26 microsatellite markers showed identical patterns in each normal/tumor pair. Our results suggest that somatic cell microsatellite instability does not contribute to radiation-induced thyroid carcinogenesis. However, somatic minisatellite mutation events are present in a subset of radiation-induced, but not sporadic, thyroid cancers, suggesting that this type of genomic instability may play a role in radiation-induced tumorigenesis in the thyroid gland.

Prevalence of mutations of ras and p53 in benign and malignant thyroid tumors from children exposed to radiation after the Chernobyl nuclear accident.

Starting 4 years after the Chernobyl accident, a dramatic increase in incidence of thyroid carcinoma was noticed in children from contaminated areas. The incidence of benign thyroid lesions in the exposed population was also increased. To study the possible role of ras and p53 genes in radiation-induced thyroid tumorigenesis, 33 papillary carcinomas, one follicular carcinoma and 22 benign lesions removed from children aged 5-19 were screened for point mutations of H-, K-, and N-ras, as well as of p53 (exons 5-8) using single strand conformation polymorphism (SSCP) analysis. Ras point mutations were detected in 1/1 case of follicular carcinoma (N-ras codon 61 CAAgln-->AAAlys), and in 3/7 follicular adenomas (N-ras codon 61 CAAgln-->CGAarg x 2, CAAgln-->AAAlys). None of the cases of papillary thyroid carcinoma was positive for ras oncogene abnormalities. The lack of K-ras mutations was confirmed by allele-specific oligonucleotide hybridization (ASOH), and by sequencing in five cases. Somatic point mutations in p53 were found by SSCP in 2/33 papillary thyroid carcinomas, with one missense mutation (exon 5, codon 160 ATGmet-->GTGval) and another silent mutation (codon 182, TGCcys-->TGTcys). Immunohisto-chemically, focally positive p53 staining was found in four papillary carcinomas being primarily confined to solid and poorly-differentiated areas in tumors. These data demonstrate that as opposed to the few reports on tumors arising after therapeutic external irradiation, ras mutations are not primary events in the development of post-Chernobyl thyroid papillary carcinomas. p53 mutations do not appear to be important in the development of these tumors, but may in some cases have a role in progression to a more aggressive phenotype that has not yet fully manifested in these pediatric neoplasms.

Chromosomal breakpoint positions suggest a direct role for radiation in inducing illegitimate recombination between the ELE1 and RET genes in radiation-induced thyroid carcinomas.

The RET/PTC3 rearrangement is formed by fusion of the ELE1 and RET genes, and is highly prevalent in radiation-induced post-Chernobyl papillary thyroid carcinomas. We characterized the breakpoints in the ELE1 and RET genes in 12 post-Chernobyl pediatric papillary carcinomas with known RET/PTC3 rearrangement. We found that the breakpoints within each intron were distributed in a relatively random fashion, except for clustering in the Alu regions of ELE1. None of the breakpoints occurred at the same base or within a similar sequence. There was also no evidence of preferential cleavage in AT-rich regions or other target DNA sites implicated in illegitimate recombination in mammalian cells. Modification of sequences at the cleavage sites was minimal, typically involving a 1-3 nucleotide deletion and/or duplication. Surprisingly, the alignment of ELE1 and RET introns in opposite orientation revealed that in each tumor the position of the break in one gene corresponded to the position of the break in the other gene. This tendency suggests that the two genes may lie next to each other but point in opposite directions in the nucleus. Such a structure would facilitate formation of RET/PTC3 rearrangements because a single radiation track could produce concerted breaks in both genes, leading to inversion due to reciprocal exchange via end-joining.

Risk factors for thyroid cancer.

The potential risk factors for thyroid carcinoma development include genetic predisposition, exposure to therapeutic or environmental ionizing radiation, residence in areas of iodine deficiency or excess, history of preexisting benign thyroid disease, as well as hormonal and reproductive factors. In this review, we analyze some of the epidemiological data, as well as the possible molecular mechanisms by which certain environmental and genetic factors might predispose to thyroid tumorigenesis. (c) 1997, Elsevier Science Inc. (Trends Endocrinol Metab 1997; 8:20-25).

Overexpression of insulin-like growth factor-binding protein-4 (IGFBP-4) in smooth muscle cells of transgenic mice through a smooth muscle alpha-actin-IGFBP-4 fusion gene induces smooth muscle hypoplasia.

Insulin-like growth factor I (IGF-I) has been postulated to function as a smooth muscle cell (SMC) mitogen and to play a role in the pathogenesis of bladder hypertrophy, estrogen-induced uterine growth, and restenosis after arterial angioplasty. IGF-binding protein-4 (IGFBP-4) inhibits IGF-I action in vitro and is the most abundant IGFBP in the rodent arterial wall. To explore the function of this binding protein in vivo, transgenic mouse lines were developed harboring fusion genes consisting of a rat IGFBP-4 complementary DNA cloned downstream of either a -724 bp fragment of the mouse smooth muscle alpha-actin 5'-flanking region (SMP2-BP-4) or -1074 bp, 63 bp of 5'-untranslated region, and 2.5 kb of intron 1 of smooth muscle alpha-actin (SMP8-BP-4). SMP2-BP-4 mice expressed low levels of the exogenous IGFBP-4 messenger RNA (mRNA), which was not specifically targeted to SMC-rich tissue environments, and were therefore not analyzed further. Six SMP8-BP-4 transgenic lines derived from separate founders were characterized. Mating of hemizygous SMP8-BP-4 mice with controls produced about 50% transgenic offspring, with equal sex distribution. Expression of IGFBP-4 mRNA in nontransgenic littermates was maximal in liver and kidney. By contrast, transgenic IGFBP-4 mRNA expression, distinguished because of a smaller transcript size, was confined to SMC-containing tissues, with the following hierarchy: bladder > aorta > stomach = uterus. There was no transgene expression in skeletal muscle, brain, or cardiac myocytes. The abundance of IGFBP-4 measured by Western ligand blotting or by immunoblotting, was 8- to 10-fold higher in aorta and bladder of SMP8-BP-4 mice than in their nontransgenic littermates, with no change in plasma IGFBP-4 levels. Transgenic mice exhibited a significant reduction in wet weight of SMC-rich tissues, including bladder, intestine, aorta, uterus, and stomach, with no change in total body or carcass weight. In situ hybridization showed that transgene expression was targeted exclusively to the muscular layers of the arteries, veins, bladder, ureter, stomach, intestine, and uterus. Overexpression of IGFBP-4 was associated with SMC hypoplasia, a reciprocal phenotype to that of transgenic mice overexpressing IGF-I under control of the same promoter (SMP8-IGF-I). Double transgenic mice derived from mating SMP8-BP-4 with SMP8-IGF-I animals showed a modest decrease in wet weight at selected SMC tissues. Although we cannot exclude that the effects of IGFBP-4 may be IGF independent, these data suggest that IGFBP-4 is a functional antagonist of IGF-I action on SMC in vivo.

Thyroid lesions in children and adolescents after the Chernobyl disaster: implications for the study of radiation tumorigenesis.

Eight years after the Chernobyl nuclear accident, the most obvious effect is manifested by an increase in the prevalence of thyroid gland diseases in the exposed children and adolescents. In this study, we describe a comparative analysis of epidemiological, clinical, and morphological features of 92 malignant and 59 benign thyroid lesions from patients 5-18 yr of age exposed to radiation in Belarus as a result of the Chernobyl disaster. All of them were operated at the same institution during the period from September 1991 through December 1992. The highest number of patients that subsequently developed thyroid carcinomas was in the group that was less that 1 yr of age at the time of Chernobyl, and this number decreased progressively through age 12 yr. Conversely, none of the patients with benign lesions only was less than 2 yr old at the time of the accident, and an exposure age of 5-6 yr was a threshold separating significant prevalence of malignant tumors in younger children from the more frequent benign lesions in older patients (P < 0.001). Fifty-two percent of children with carcinomas and only 24% with benign lesions (P < 0.005) were residents of the Gomel region, which is the most contaminated in Belarus. The morphology of thyroid tissue adjacent to carcinomas showed a high prevalence of multinodular and diffuse changes, but not of adenomas or solitary adenomatoid nodules. There was a high prevalence of focal micropapillary hyperplasia with graded degrees of severity, which we hypothesize may correspond to precursors for papillary thyroid carcinoma in post-Chernobyl radiation-associated tumors.

Tyrosine kinase expression is increased in papillary thyroid carcinoma of children and young adults.

Tyrosine kinases (TKs) are important candidate genes for malignant transformation and at least 21 different TKs have been identified in the thyroid gland. We hypothesized that the collective activity of these TKs might be increased in thyroid carcinoma and have association with the clinical behavior of individual tumors. To test this, we determined TK expression by immunohistochemistry in 74 archival thyroid tissue blocks (48 papillary thyroid carcinoma, PTC; 9 follicular thyroid carcinoma, FTC; 17 benign thyroid diseases) from children and young adults. Mean TK expression was greater for PTC (2.1 +/- 0.11) than benign lesions (1.6 +/- 0.2, p = 0.027), and also tended to be greater in FTC (2.1 +/- 0.25, p = 0.12). Recurrence risk was three-fold greater for PTC with intense TK expression (4/15, 27%) than for PTC with minimal - moderate TK expression (3/33, 9.0%). However, this was not statistically significant (p = 0.10). In PTC, TK expression correlated with expression of the receptor for hepatocyte growth factor / scatter factor (cMET, r = 0.31, p = 0.044). In FTC, TK expression did not correlate with cMET, but tended to be greater in young patients (r = -0.59, p = 0.09). We conclude that TK expression is increased in PTC and possibly associated with an increased recurrence risk.

Solid variant of papillary thyroid carcinoma: incidence, clinical-pathologic characteristics, molecular analysis, and biologic behavior.

Solid variant is a rare and poorly characterized variant of papillary thyroid carcinoma. In this study we analyzed 20 primary cases of the solid variant of papillary carcinoma found in a series of 756 papillary carcinomas operated at the Mayo Clinic between 1962 and 1989. The criteria for classification included predominantly (>70%) solid growth pattern of primary tumor, retention of cytologic features typical of papillary carcinoma, and absence of tumor necrosis. For each case of the solid variant, a control case of classical papillary carcinoma matched by age, sex, tumor size, and length of follow-up was selected. The follow-up ranged from 6 to 32 years. Two patients with the solid variant of papillary carcinoma (10%) died from disease 7 and 10 years after initial surgery, while another two patients (10%) are alive with lung metastases. In contrast, the control group had no cases with distant metastases or death from disease. Molecular analyses showed a similar prevalence of RET /PTC rearrangements in both groups. In conclusion, the solid variant of papillary carcinoma is associated with a slightly higher frequency of distant metastases and less favorable prognosis than classical papillary carcinoma. However, it should be distinguished from poorly differentiated thyroid carcinoma, which has a reported lower survival rate compared with the solid variant of papillary carcinoma.

Thyroid transcription factor-1, thyroglobulin, cytokeratin 7, and cytokeratin 20 in thyroid neoplasms.

Thyroid transcription factor-1 (TTF-1), a member of the NKx2 family of homeodomain transcription factors, is a mediator of thyroid-specific transcription of the thyroglobulin (TG) gene. The combined immunohistochemical profile of TTF-1, TG, cytokeratin 7 (CK7), and cytokeratin 20 (CK20) in neoplasms of the thyroid gland and their metastases to other sites has not been defined previously. Formalin-fixed tissue of 43 thyroid tumors, including 31 carcinomas and 12 adenomas, and 16 metastasic lesions were immunostained using monoclonal antibodies to TTF-1, TG, CK7, and CK20. Immunoreactivity of the primary tumors (adenomas and carcinomas) for TTF-1 was seen in 32 cases (74%), TG 32 (74%), and CK7 34 (79%), whereas none (0%) showed positivity for CK20. The distribution of reactivity in the 31 carcinomas for TTF-1, TG, and CK7, respectively was papillary (8/8), (8/8), and (8/8); poorly differentiated (6/7), (4/7), and (6/7); oncocytic (Hürthle) cell (2/6), (6/6), and (4/6); follicular (4/4), (3/4), and (3/4); medullary (1/2), (0/2), and (1/2). One of four anaplastic carcinomas was focally immunoreactive showing positivity for TTF-1 only. Of the six follicular adenomas, five were positive for TTF-1, six for TG, and six for CK7. Among the six oncocytic cell adenomas, five were reactive for TTF-1, five for TG, and all six for CK7. Twelve (75%) of the 16 metastatic tumors were positive for TTF-1, 10 (63%) for TG, 15 (94%) for CK7, and none (0%) for CK20. In summary, TTF-1 and TG are demonstrable by immunohistochemistry in the majority of thyroid neoplasms. Compared with TG, an antibody to TTF-I is a similarly sensitive marker for thyroid tumors. Moreover, TTF-1 is a more sensitive marker for poorly differentiated carcinomas and metastasis. In most cases, its nuclear pattern of immunoreactivity facilitates interpretation. Thyroid tumors are CK7+/CK20-. The panel of antibodies for TG, TTF-1, CK7, and CK20 is useful when the thyroid origin of a metastatic tumor is a consideration.

ret rearrangements in Japanese pediatric and adult papillary thyroid cancers.

Oncogenic rearrangements of the ret proto-oncogene (ret/PTC) are found uniquely in papillary thyroid carcinomas. The prevalence of ret/PTC in these tumors varies widely, from 0% to 87%, among patient series from different geographical regions. The differences in the prevalence of ret rearrangement have been ascribed to age, genetic, and/or environmental factors. The very high prevalence of ret/PTC in tumors arising in children after the Chernobyl nuclear accident has generated speculation that this oncogene may be an indicator of overt or inadvertent radiation exposure. In Japan, the prevalence of ret activation is reportedly quite low (0% to 9%). Here we examined the frequency of ret rearrangements in papillary carcinomas from Japanese adults and children by means of reverse transcription polymerase chain reaction (RT-PCR) followed by Southern hybridization. Ret rearrangements were detected in 4 of 11 (36%) tumors from the adult population, and in 3 of 10 (30%) pediatric tumors. One child with a solid variant papillary carcinoma had a ret-PTC3 rearrangement, further supporting the association between the solid variant histotype and this particular rearrangement of ret. The present data do not support a major geographic difference in the prevalence of ret/PTC rearrangements in papillary carcinomas between Japan, the United States, and Italy.

Validation of a new method to measure cardiac output during extracorporeal detoxification.

Cardiac output was measured in 11 patients during extra-corporeal detoxification after open heart surgery. All patients were mechanically ventilated and had pulmonary artery catheters for cardiac output (COT) measured by thermodilution. A sensor on the arterial side of the extracorporeal circulation measured flow and sound velocity transients. Injections of 2-5 ml 0.9% saline at 37 degrees C into the arterial line upstream of the sensor permitted its calibration; 10-20 ml of the same solution was injected intravenously or into the venous dialysis injection port, and cardiac output (COUD) was calculated by the ultrasound velocity dilution technique. COT was measured within 5 min of the ultrasound dilution measurement. CO was in the range of 2-8 L/m. The regression equation was COUD = 1.09 x COT-0.32 (r = 0.97, n = 31). These data suggest agreement between the ultrasound dilution technique and thermodilution. Ultrasound dilution is preferable in patients undergoing extracorporeal detoxification when pulmonary artery catheterization is not required or dangerous.

Spatial positioning of RET and H4 following radiation exposure leads to tumor development.

Exposure to ionizing radiation is a well-known risk factor for a number of human cancers, including leukemia, thyroid cancer, soft tissue sarcomas, and many others. Although it has been known for a long time that radiation exposure to the cell results in extensive DNA damage, including double strand DNA breaks, the exact mechanisms of radiation-induced carcinogenesis remain unknown. Recently, a large increase in incidence of thyroid cancer was observed in children exposed to radiation after the Chernobyl nuclear accident. A high prevalence of chromosomal rearrangements involving the RET gene was found among these radiation-induced thyroid tumors. As a result of such rearrangement, a portion of the RET gene is fused with another gene, typically with the H4 or ELE1 . However, since the DNA targets of ionizing radiation are randomly distributed throughout the cell nucleus, the reason for predilection for the RET rearrangements in thyroid cells was unclear.

Progression of BRAF-induced thyroid cancer is associated with epithelial-mesenchymal transition requiring concomitant MAP kinase and TGFß signaling.

Mice with thyroid-specific expression of oncogenic BRAF (Tg-Braf) develop papillary thyroid cancers (PTCs) that are locally invasive and have well-defined foci of poorly differentiated thyroid carcinoma (PDTC). To investigate the PTC-PDTC progression, we performed a microarray analysis using RNA from paired samples of PDTC and PTC collected from the same animals by laser capture microdissection. Analysis of eight paired samples revealed a profound deregulation of genes involved in cell adhesion and intracellular junctions, with changes consistent with an epithelial-mesenchymal transition (EMT). This was confirmed by immunohistochemistry, as vimentin expression was increased and E-cadherin lost in PDTC compared with adjacent PTC. Moreover, PDTC stained positively for phospho-Smad2, suggesting a role for transforming growth factor (TGF)ß in mediating this process. Accordingly, TGFß-induced EMT in primary cultures of thyroid cells from Tg-Braf mice, whereas wild-type thyroid cells retained their epithelial features. TGFß-induced Smad2 phosphorylation, transcriptional activity and induction of EMT required mitogen-activated protein kinase (MAPK) pathway activation in Tg-Braf thyrocytes. Hence, tumor initiation by oncogenic BRAF renders thyroid cells susceptible to TGFß-induced EMT, through a MAPK-dependent process.

DNA breaks at fragile sites generate oncogenic RET/PTC rearrangements in human thyroid cells.

Human chromosomal fragile sites are regions of the genome that are prone to DNA breakage, and are classified as common or rare, depending on their frequency in the population. Common fragile sites frequently coincide with the location of genes involved in carcinogenic chromosomal translocations, suggesting their role in cancer formation. However, there has been no direct evidence linking breakage at fragile sites to the formation of a cancer-specific translocation. Here, we studied the involvement of fragile sites in the formation of RET/PTC rearrangements, which are frequently found in papillary thyroid carcinoma (PTC). These rearrangements are commonly associated with radiation exposure; however, most of the tumors found in adults are not linked to radiation. In this study, we provide structural and biochemical evidence that the RET, CCDC6 and NCOA4 genes participating in two major types of RET/PTC rearrangements, are located in common fragile sites FRA10C and FRA10G, and undergo DNA breakage after exposure to fragile site-inducing chemicals. Moreover, exposure of human thyroid cells to these chemicals results in the formation of cancer-specific RET/PTC rearrangements. These results provide the direct evidence for the involvement of chromosomal fragile sites in the generation of cancer-specific rearrangements in human cells.

Pediatric thyroid cancer after the Chernobyl disaster. Pathomorphologic study of 84 cases (1991-1992) from the Republic of Belarus.

BACKGROUND: During the initial period after the Chernobyl accident, large amounts of radioactive iodine were released in fallout, resulting in serious exposure to the thyroid gland in the residents of areas around the nuclear power station. Beginning in 1990, a definite increase in the incidence of thyroid cancer was noted in children of the Republic of Belarus. METHODS: Morphologic and clinical features of 84 cases of post-Chernobyl thyroid carcinoma in Belarussian children from 5 to 14 years of age are reported. The latent period for tumor development was 4-6 years, with a mean of 5.8 years. RESULTS: Papillary carcinoma was found in 83 patients and medullary carcinoma in one. Besides typical papillary carcinoma (14%), solid (34%), follicular (33%), mixed (10%), and diffuse sclerosing (9%) variants were observed. The follow-up period ranged from 8 months to 2.5 years. One patient died, local recurrence developed in 2, and cervical lymph node metastases developed in 10. To date, the incidence of local recurrence or metastatic disease after surgery was significantly higher in patients 5-8 years of age and in residents of areas nearest to the Chernobyl station. CONCLUSIONS: Post-Chernobyl pediatric thyroid carcinoma is characterized by a short latency, a higher proportion of tumors arising in young children, and an almost equal sex ratio. Microscopically, these tumors were usually aggressive, often demonstrating intraglandular tumor dissemination (92%), thyroid capsular and adjacent soft tissue invasion (89%), and cervical lymph node metastases (88%). Papillary carcinoma was diagnosed in 99% of cases, with an unusually high frequency of solid growth patterns. Morphologic changes in nonneoplastic thyroid tissue were present in 90% of the glands, and the most specific findings were vascular changes and perifollicular fibrosis.