hREC2, a RAD51-like gene, is disrupted by t(12;14) (q15;q24.1) in a uterine leiomyoma

Uterine leiomyoma with RAD51B::NUDT3 fusion: a report of 2 cases

Three main uterine leiomyoma molecular subtypes include tumors with MED12 mutation, molecular aberrations leading to HMGA2 overexpression, and biallelic loss of FH. These aberrations are mutually exclusive and can be found in approximately 80-90% of uterine leiomyoma, in which they seem to be a driver event. Approximately 10% of uterine leiomyoma, however, does not belong to any of these categories. Uterine leiomyoma with HMGA2 overexpression is the most common subtype in cellular and second most common category of usual leiomyoma. In some of these tumors, rearrangement of HMGA2 gene is present. The most common fusion partner of HMGA2 gene is RAD51B. Limited data suggests that RAD51B fusions with other genes may be present in uterine leiomyoma. In our study, we described two cases of uterine leiomyoma with RAD51B::NUDT3 fusion, which occur in one case of usual and one case of highly cellular leiomyoma. In both cases, no other driver molecular aberrations were found. The results of our study showed that RAD51::NUDT3 fusion can occur in both usual and cellular leiomyoma. RAD51B may be a fusion partner of multiple genes other than HMGA2 and HMGA1. In these cases, RAD51B fusion seems to be mutually exclusive with other driver aberrations defining molecular leiomyoma subtypes. RAD51B::NUDT3 fusion should be added to the spectrum of fusions which may occur in uterine leiomyoma, which can be of value especially in cellular leiomyoma in the context of differential diagnosis against endometrial stromal tumors.

A View on Uterine Leiomyoma Genesis through the Prism of Genetic, Epigenetic and Cellular Heterogeneity

Uterine leiomyomas (ULs), frequent benign tumours of the female reproductive tract, are associated with a range of symptoms and significant morbidity. Despite extensive research, there is no consensus on essential points of UL initiation and development. The main reason for this is a pronounced inter- and intratumoral heterogeneity resulting from diverse and complicated mechanisms underlying UL pathobiology. In this review, we comprehensively analyse risk and protective factors for UL development, UL cellular composition, hormonal and paracrine signalling, epigenetic regulation and genetic abnormalities. We conclude the need to carefully update the concept of UL genesis in light of the current data. Staying within the framework of the existing hypotheses, we introduce a possible timeline for UL development and the associated key events—from potential prerequisites to the beginning of UL formation and the onset of driver and passenger changes.

Pathogenesis, diagnosis and treatment of uterine lipoleiomyoma: A review

Uterine lipoleiomyomas are variants of uterine leiomyomas and are characterized by progressive enlargement that can occur even after menopause. These tumors can produce serious clinical symptoms and are difficult to diagnosis preoperatively. The growth rate of uterine lipoleiomyomas after menopause is comparatively higher than that of conventional uterine leiomyomas, and lipoleiomyosarcomas as well as tumor-to-tumor metastasis associated with lipoleiomyomas have been reported. However, detailed histogenic mechanisms of the tumor remain unclear. Surgical treatments are the current choice for the management of lipoleiomyomas. The purpose of this review is to promote greater awareness of lipoleiomyoma characteristics with a focus on histogenesis, diagnosis, and treatment. We performed an exhaustive literature review and have summarized the available data. We assessed the interpretation of auxiliary examinations to help physicians in making an early accurate diagnosis of the disease and to help with treatment decision-making, particularly regarding whether surgery should be performed or avoided.

MED12 mutations and FH inactivation are mutually exclusive in uterine leiomyomas

Background:

Uterine leiomyomas from hereditary leiomyomatosis and renal cell cancer (HLRCC) patients are driven by fumarate hydratase (FH) inactivation or occasionally by mediator complex subunit 12 (MED12) mutations. The aim of this study was to analyse whether MED12 mutations and FH inactivation are mutually exclusive and to determine the contribution of MED12 mutations on HLRCC patients' myomagenesis.

Methods:

MED12 exons 1 and 2 mutation screening and 2SC immunohistochemistry indicative for FH deficiency was performed on a comprehensive series of HLRCC patients' (122 specimens) and sporadic (66 specimens) tumours. Gene expression analysis was performed using Affymetrix GeneChip Human Exon Arrays (Affymetrix, Santa Clara, CA, USA).

Results:

Nine tumours from HLRCC patients harboured a somatic MED12 mutation and were negative for 2SC immunohistochemistry. All remaining successfully analysed lesions (107/116) were deficient for FH. Of sporadic tumours, 35/64 were MED12 mutation positive and none displayed a FH defect. In global gene expression analysis FH-deficient tumours clustered together, whereas HLRCC patients' MED12 mutation-positive tumours clustered together with sporadic MED12 mutation-positive tumours.

Conclusions:

Somatic MED12 mutations and biallelic FH inactivation are mutually exclusive in both HLRCC syndrome-associated and sporadic uterine leiomyomas. The great majority of HLRCC patients' uterine leiomyomas are caused by FH inactivation, but incidental tumours driven by somatic MED12 mutations also occur. These MED12 mutation-positive tumours display similar expressional profiles with their sporadic counterparts and are clearly separate from FH-deficient tumours.

Detection and screening of chromosomal rearrangements in uterine leiomyomas by long-distance inverse PCR

Genome instability is a hallmark of many tumors and recently, next-generation sequencing methods have enabled analyses of tumor genomes at an unprecedented level. Studying rearrangement-prone chromosomal regions (putative "breakpoint hotspots") in detail, however, necessitates molecular assays that can detect de novo DNA fusions arising from these hotspots. Here we demonstrate the utility of a long-distance inverse PCR-based method for the detection and screening of de novo DNA rearrangements in uterine leiomyomas, one of the most common types of human neoplasm. This assay allows in principle any genomic region suspected of instability to be queried for DNA rearrangements originating there. No prior knowledge of the identity of the fusion partner chromosome is needed. We used this method to screen uterine leiomyomas for rearrangements at genomic locations known to be rearrangement-prone in this tumor type: upstream HMGA2 and within RAD51B. We identified a novel DNA rearrangement upstream of HMGA2 that had gone undetected in an earlier whole-genome sequencing study. In more than 30 additional uterine leiomyoma samples, not analyzed by whole-genome sequencing previously, no rearrangements were observed within the 1,107 bp and 1,996 bp assayed in the RAD51B and HMGA2 rearrangement hotspots. Our findings show that long-distance inverse PCR is a robust, sensitive, and cost-effective method for the detection and screening of DNA rearrangements from solid tumors that should be useful for many diagnostic applications.

Epidemiological and genetic clues for molecular mechanisms involved in uterine leiomyoma development and growth

BACKGROUND

Uterine leiomyomas (fibroids) are highly prevalent benign smooth muscle tumors of the uterus. In the USA, the lifetime risk for women developing uterine leiomyomas is estimated as up to 75%. Except for hysterectomy, most therapies or treatments often provide only partial or temporary relief and are not successful in every patient. There is a clear racial disparity in the disease; African-American women are estimated to be three times more likely to develop uterine leiomyomas and generally develop more severe symptoms. There is also familial clustering between first-degree relatives and twins, and multiple inherited syndromes in which fibroid development occurs. Leiomyomas have been described as clonal and hormonally regulated, but despite the healthcare burden imposed by the disease, the etiology of uterine leiomyomas remains largely unknown. The mechanisms involved in their growth are also essentially unknown, which has contributed to the slow progress in development of effective treatment options.

METHODS

A comprehensive PubMed search for and critical assessment of articles related to the epidemiological, biological and genetic clues for uterine leiomyoma development was performed. The individual functions of some of the best candidate genes are explained to provide more insight into their biological function and to interconnect and organize genes and pathways in one overarching figure that represents the current state of knowledge about uterine leiomyoma development and growth.

RESULTS

In this review, the widely recognized roles of estrogen and progesterone in uterine leiomyoma pathobiology on the basis of clinical and experimental data are presented. This is followed by fundamental aspects and concepts including the possible cellular origin of uterine fibroids. The central themes in the subsequent parts are cytogenetic aberrations in leiomyomas and the racial/ethnic disparities in uterine fibroid biology. Then, the attributes of various in vitro and in vivo, human syndrome, rodent xenograft, naturally mutant, and genetically modified models used to study possible molecular mechanisms of leiomyoma development and growth are described. Particular emphasis is placed on known links to fibrosis, hypertrophy, and hyperplasia and genes that are potentially important in these processes.

CONCLUSIONS

Menstrual cycle-related injury and repair and coinciding hormonal cycling appears to affect myometrial stem cells that, at a certain stage of fibroid development, often obtain cytogenetic aberrations and mutations of Mediator complex subunit 12 (MED12). Mammalian target of rapamycin (mTOR), a master regulator of proliferation, is activated in many of these tumors, possibly by mechanisms that are similar to some human fibrosis syndromes and/or by mutation of upstream tumor suppressor genes. Animal models of the disease support some of these dysregulated pathways in fibroid etiology or pathogenesis, but none are definitive. All of this suggests that there are likely several key mechanisms involved in the disease that, in addition to increasing the complexity of uterine fibroid pathobiology, offer possible approaches for patient-specific therapies. A final model that incorporates many of these reported mechanisms is presented with a discussion of their implications for leiomyoma clinical practice.

Homologous recombination and human health: the roles of BRCA1, BRCA2, and associated proteins

Homologous recombination (HR) is a major pathway for the repair of DNA double-strand breaks in mammalian cells, the defining step of which is homologous strand exchange directed by the RAD51 protein. The physiological importance of HR is underscored by the observation of genomic instability in HR-deficient cells and, importantly, the association of cancer predisposition and developmental defects with mutations in HR genes. The tumor suppressors BRCA1 and BRCA2, key players at different stages of HR, are frequently mutated in familial breast and ovarian cancers. Other HR proteins, including PALB2 and RAD51 paralogs, have also been identified as tumor suppressors. This review summarizes recent findings on BRCA1, BRCA2, and associated proteins involved in human disease with an emphasis on their molecular roles and interactions.

Novel MED12 gene somatic mutations in women from the Southern United States with symptomatic uterine fibroids

Although somatic mutations in exon 2 of the mediator complex subunit 12 (MED12) gene have been reported previously in uterine fibroids in women from Finland, South Africa, and North America, the status of these mutations was not reported in the Southern United States women. The aim of this study is to determine the MED12 somatic mutations in uterine fibroids of women from Southern Unites States, which will help to better understand the contribution of MED12 mutations in fibroid tumor biology. Herein, we determined the frequency of MED12 gene exon 2 somatic mutations in 143 fibroid tumors from a total of 135 women from the Southern United States and in 50 samples of the adjacent myometrium using PCR amplification and Sanger sequencing. We observed that the MED12 gene is mutated in 64.33 % (92/143) of uterine fibroid cases in the exon 2 (including deletion mutations). These mutations include 107T > G (4.3 %), 130G > C (2.8 %), 130G > A (7.0 %), 130G > T (2.8 %), 131G > C (2.1 %), 131G > A (20.2 %), and 131G > T (2.1 %). Interestingly, we identified four novel mutations in these patients: 107 T > C (12.8 %), 105A > T (2.1 %), 122T > A (2.1 %), and 92T > A (2.1 %). As expected, we did not observe any mutations in the normal myometrium. Moreover, we found a higher rate of deletion mutations (17.5 %, 25/143) in the above fibroid tumors. Our results clearly demonstrate that the MED12 gene exon 2 is frequently mutated in human uterine fibroids in Southern United States women. These results highlight the molecular pathogenesis of human uterine fibroids with the central role of MED12 somatic mutations.

Current understanding of somatic stem cells in leiomyoma formation

OBJECTIVE

To provide a detailed summary of current scientific knowledge of somatic stem cells (SSCs) in murine and human myometrium and their putative implication in leiomyoma formation, as well as to establish new therapeutic options.

DESIGN

Pubmed and Scholar One manuscripts were used to identify the most relevant studies on SSCs and their implications in human myometrium and leiomyomas.

SETTING

University research laboratory-affiliated infertility clinic.

PATIENT(S)

Not applicable.

INTERVENTION(S)

Not applicable.

MAIN OUTCOME MEASURE(S)

Not applicable.

RESULT(S)

Despite numerous publications on SSCs, it was not until 2007 that scientific evidence based on the use of 5-bromo-2'-deoxyuridine (BrdU) and side population (SP) methods in murine and human myometrium were first published. Recently, it has been reported that SP cells are present in human leiomyomas; however, to date the pathogenesis of this benign tumor remains unclear. Besides many genetic/epigenetic alterations, changes to steroid hormones and growth factors may also be associated with the impaired function, proliferation, and differentiation of a subset of putative SSCs in human myometrium.

CONCLUSION(S)

These findings open up new possibilities for understanding the origin of this benign tumor and help to develop new nonsurgical approaches for their management.

Exomic landscape of MED12 mutation-negative and -positive uterine leiomyomas

Uterine leiomyomas are extremely common tumors originating from the smooth muscle cells of myometrium. We recently reported recurrent somatic mutations in mediator complex subunit 12 (MED12) in the majority of these lesions, and analyzed chromosomal abnormalities in leiomyomas by whole-genome sequencing. The aim of our study was to examine in detail uterine leiomyoma exomes, to search for driver mutations in MED12 mutation-negative leiomyomas and to scrutinize MED12 mutation-positive leimyomas for additional contributing mutations. We analyzed whole exome sequencing data of 27 uterine leiomyomas (12 MED12 mutation-negative and 15 MED12 mutation-positive) and their paired normal myometrium. We searched for genes, which would be recurrently mutated. No such genes were identified in MED12 mutation-negative uterine leiomyomas. Similarly, MED12 mutation-positive leiomyomas displayed no additional recurrent changes. The complete lack of novel driver point mutations in the examined series highlights the unique role of MED12 mutations in genesis of uterine leiomyomas, and suggests that these mutations alone may be sufficient for tumor development. Additional factors that cannot be detected by exome sequencing, such as somatic structural rearrangements, epigenetic events and intronic variants, are likely to have a particular impact to the development of MED12 wild-type lesions.

Characterization of uterine leiomyomas by whole-genome sequencing

BACKGROUND

Uterine leiomyomas are benign but affect the health of millions of women. A better understanding of the molecular mechanisms involved may provide clues to the prevention and treatment of these lesions.

METHODS

We performed whole-genome sequencing and gene-expression profiling of 38 uterine leiomyomas and the corresponding myometrium from 30 women.

RESULTS

Identical variants observed in some separate tumor nodules suggested that these nodules have a common origin. Complex chromosomal rearrangements resembling chromothripsis were a common feature of leiomyomas. These rearrangements are best explained by a single event of multiple chromosomal breaks and random reassembly. The rearrangements created tissue-specific changes consistent with a role in the initiation of leiomyoma, such as translocations of the HMGA2 and RAD51B loci and aberrations at the COL4A5-COL4A6 locus, and occurred in the presence of normal TP53 alleles. In some cases, separate events had occurred more than once in single tumor-cell lineages.

CONCLUSIONS

Chromosome shattering and reassembly resembling chromothripsis (a single genomic event that results in focal losses and rearrangements in multiple genomic regions) is a major cause of chromosomal abnormalities in uterine leiomyomas; we propose that tumorigenesis occurs when tissue-specific tumor-promoting changes are formed through these events. Chromothripsis has previously been associated with aggressive cancer; its common occurrence in leiomyomas suggests that it also has a role in the genesis and progression of benign tumors. We observed that multiple separate tumors could be seeded from a single lineage of uterine leiomyoma cells. (Funded by the Academy of Finland Center of Excellence program and others.).

The HsRAD51B-HsRAD51C stabilizes the HsRAD51 nucleoprotein filament

There are six human RAD51 related proteins (HsRAD51 paralogs), HsRAD51B, HsRAD51C, HsRAD51D, HsXRCC2, HsXRCC3 and HsDMC1, that appear to enhance HsRAD51 mediated homologous recombinational (HR) repair of DNA double strand breaks (DSBs). Here we model the structures of HsRAD51, HsRAD51B and HsRAD51C and show similar domain orientations within a hypothetical nucleoprotein filament (NPF). We then demonstrate that HsRAD51B-HsRAD51C heterodimer forms stable complex on ssDNA and partially stabilizes the HsRAD51 NPF against the anti-recombinogenic activity of BLM. Moreover, HsRAD51B-HsRAD51C stimulates HsRAD51 mediated D-loop formation in the presence of RPA. However, HsRAD51B-HsRAD51C does not facilitate HsRAD51 nucleation on a RPA coated ssDNA. These results suggest that the HsRAD51B-HsRAD51C complex plays a role in stabilizing the HsRAD51 NPF during the presynaptic phase of HR, which appears downstream of BRCA2-mediated HsRAD51 NPF formation.

Recognition, signaling, and repair of DNA double-strand breaks produced by ionizing radiation in mammalian cells: the molecular choreography

The faithful maintenance of chromosome continuity in human cells during DNA replication and repair is critical for preventing the conversion of normal diploid cells to an oncogenic state. The evolution of higher eukaryotic cells endowed them with a large genetic investment in the molecular machinery that ensures chromosome stability. In mammalian and other vertebrate cells, the elimination of double-strand breaks with minimal nucleotide sequence change involves the spatiotemporal orchestration of a seemingly endless number of proteins ranging in their action from the nucleotide level to nucleosome organization and chromosome architecture. DNA DSBs trigger a myriad of post-translational modifications that alter catalytic activities and the specificity of protein interactions: phosphorylation, acetylation, methylation, ubiquitylation, and SUMOylation, followed by the reversal of these changes as repair is completed. "Superfluous" protein recruitment to damage sites, functional redundancy, and alternative pathways ensure that DSB repair is extremely efficient, both quantitatively and qualitatively. This review strives to integrate the information about the molecular mechanisms of DSB repair that has emerged over the last two decades with a focus on DSBs produced by the prototype agent ionizing radiation (IR). The exponential growth of molecular studies, heavily driven by RNA knockdown technology, now reveals an outline of how many key protein players in genome stability and cancer biology perform their interwoven tasks, e.g. ATM, ATR, DNA-PK, Chk1, Chk2, PARP1/2/3, 53BP1, BRCA1, BRCA2, BLM, RAD51, and the MRE11-RAD50-NBS1 complex. Thus, the nature of the intricate coordination of repair processes with cell cycle progression is becoming apparent. This review also links molecular abnormalities to cellular pathology as much a possible and provides a framework of temporal relationships.

Omental lipoblastoma

A large intra-abdominal mass was discovered in a 6-month-old boy during a routine well-child examination. Imaging studies revealed a solid mass which appeared to arise from the left lobe of the liver, extending caudally and filling the entire pelvis. At the time of surgical excision, the mass was found to be unassociated with the liver, but was instead localized to the omentum. Histologic examination revealed adipocytes of varying stages of maturation arranged in a lobular architecture, consistent with a lipoblastoma. This unusual tumor is only the eighth reported omental lipoblastoma [J. Hicks, A. Dilley, D. Patel, J. Barrish, S. Zhu, M. Brandt, Lipoblastoma and lipoblastomatosis in infancy and childhood: histologic, ultrastructural, and cytogenetic features. Ultrastruct. Pathol. 25 (2001) 321-333; J. Harrer, G. Hammon, T. Wagner, M. Bolkenius, Lipoblastoma and lipoblastomatosis: a report of two cases and review of the literature. Eur. J. Pediatr. Surg. 11 (2001) 342-349; S. Weiss and J. Goldblum, Enzinger and Weiss's Soft Tissue Tumors, fourth ed., Mosby, St. Louis, MO, 2001, pp. 601-605, 670-686; S. Soin, S. Andronikou, R. Lisle, K. Platt, K. Lakhoo, Omental lipoblastoma in a child; diagnosis based in CT density measurements. J. Pediatr. Hematol. Oncol. 28(1) (2006) 57-58; A. Prando, S. Wallace, J.L. Marins, R.M. Pereira, E.R. de Oliveira, M. Alvarenga, Sonographic features of benign intraperitoneal lipomatous tumors in children-report of 4 cases. Pediatr. Radiol. 20(8) (1990) 571-574; C. Blank, E. Schoenmakers, P. Rogalla, E. Huys, A. Van Rijk, N. Drieschner, J. Bullerdiek, Intragenic breakpoint within RAD51L1 in a t(6;14)(p21.3;q24) of a pulmonary chondroid hamartoma. Cytogenet. Cell Genet. 95 (2001) 17-19; S. Ingraham, R. Lynch, S. Kathiresan, A. Buckler, A. Menon, hREC2, a RAD51-like gene, is disrupted by t(12;14)(q15;q24.1) in a uterine leiomyoma. Cancer Genet. Cytogenet. 115 (1999) 56-61]. Cytogenetics revealed a karyotype of 46,XY,t(8;14)(q13;q24). While lipoblastomas characteristically involve 8q, only one prior case has been reported with 14q24 as its fusion partner [M. He, K. Das, M. Blacksin, J. Benevenia, M. Hameed, A translocation involving the placental growth factor gene is identified in an epithelioid hemangioendothelioma. Cancer Genet. Cytogenet. 168 (2006) 150-154]. We report this unique case of an omental lipoblastoma with a focus on its unusual karyotype, as well as its differentiation from myxoid liposarcoma.

A translocation involving the placental growth factor gene is identified in an epithelioid hemangioendothelioma

Hemangioendothelioma is a relatively rare vascular tumor that is considered a low- to intermediate-grade malignant neoplasm. Cytogenetic reports of hemangioendothelioma are rare. Two reports with translocations involving chromosomes 1 and 3 have been described. Here we report a case of an epithelioid hemangioendothelioma arising in the foot of a 56-year-old female, with a 46,XX,-6,t(10;14)(p13;q24),+r. The regions in 10p13 and 14q24 encompass genes that participate in growth regulation. One of the genes at 14q24 encodes for placental growth factor [PlGF, also called vascular endothelial growth factor (VEGF)-related protein]. Placental growth factor is a member of the VEGF growth factor family. Placenta growth factor binds only to VEGF receptor-1 (FLT-1). It has been suggested that PlGF may modulate VEGF-induced angiogenesis by the formation of PlGF/VEGF heterodimers in cells producing both factors. It has been postulated that PlGF is involved in intra- and intermolecular cross-talk between VEGF receptor-1 (FLT) and receptor-2 (FLK-1/KDR). Since expression of VEGF and its receptor, FLK-1, is seen in several cases of epithelioid hemangioendothelioma and plasma VEGF level is also used to follow-up this tumor, we performed immunohistochemical analysis for PlGF and VEGF in our case. The strong positivity for both PlGF and VEGF observed in our case implies that the t(10;14)(p13;q24) most likely involves PlGF, which may be one of the genes driving oncogenesis in these tumors.

Haploinsufficiency of RAD51B causes centrosome fragmentation and aneuploidy in human cells

The Rad51-like proteins, Rad51B, Rad51C, Rad51D, XRCC2, and XRCC3, have been shown to form two distinct complexes and seem to assist Rad51 in the early stages of homologous recombination. Although these proteins share sequence similarity with Rad51, they do not show functional redundancy. Among them, Rad51B is unique in that the gene maps to the human chromosome 14q23-24, the region frequently involved in balanced chromosome translocations in benign tumors particularly in uterine leiomyomas. Despite accumulating descriptive evidence of altered Rad51B function in these tumors, the biological significance of this aberration is still unknown. To assess the significance of reduced Rad51B function, we deleted the gene in the human colon cancer cell line HCT116 by gene targeting. Here, we show that haploinsufficiency of RAD51B causes mild hypersensitivity to DNA-damaging agents, a mild reduction in sister chromatid exchange, impaired Rad51 focus formation, and an increase in chromosome aberrations. Remarkably, haploinsufficiency of RAD51B leads to centrosome fragmentation and aneuploidy. In addition, an approximately 50% reduction in RAD51B mRNA levels by RNA interference also leads to centrosome fragmentation in the human fibrosarcoma cell line HT1080. These findings suggest that the proper biallelic expression of RAD51B is required for the maintenance of chromosome integrity in human cells.

Familial occurrence of thymoma and autoimmune diseases with the constitutional translocation t(14;20)(q24.1;p12.3)

Thymomas are low-grade epithelial cancers of the thymus whose prevalence varies between 0.1/100,000 and 0.4/100,000. Familial occurrence of thymoma is very rare. We studied a family bearing the constitutional chromosome translocation t(14;20)(q24;p12), 3 of whose members had a thymoma. In this family, among 27 patients, 11 had the translocation: 3 had thymoma and 4 others had 5 different autoimmune diseases: type 1 diabetes mellitus, Graves' disease, pernicious anemia, primitive Sjögren disease, and autoimmune pancytopenia. FISH studies allowed us to be more specific about the translocation breakpoints. The 14q24 breakpoint was in intron 5 of RAD51L1, and the 20p12 breakpoint was 100 kb telomeric to BMP2. RAD51L1 is a tumor-suppressor gene belonging to the RAD51 family, already implicated in many tumors (uterine leiomyomas, pseudo-Meigs syndromes, pulmonary chondroid hamartomas) and involved in recombinational repair of DNA double-strand breaks. BMP2 belongs to the TGFbeta superfamily, and the BMP2-BMP4 genes are involved in thymocyte differentiation by blocking progression from CD4-CD8- to CD4+CD8+ while maintaining a sufficient pool of immature precursors. Dysregulation of RAD51L1 and/or BMP2 may explain this familial occurrence of thymomas and autoimmune diseases. Using QRT-PCR, we studied the expression of BMP2 in 20 sporadic thymomas and found various levels of expression that may be associated with autoimmune diseases.

The RAD51 gene family, genetic instability and cancer

Inefficient repair or mis-repair of DNA damage can cause genetic instability, and defects in some DNA repair genes are associated with rare human cancer-prone disorders. In the last few years, homologous recombination has been found to be a key pathway in human cells for the repair of severe DNA damage such as double-strand breaks. The RAD51 family of genes, including RAD51 and the five RAD51-like genes (XRCC2, XRCC3, RAD51L1, RAD51L2, RAD51L3) are known to have crucial non-redundant roles in this pathway. Current knowledge of the functions of the RAD51 gene family is reviewed, as well as the evidence for extensive genetic instability arising from loss of their activity. Reports of potential associations between variants of RAD51 family genes and specific forms of cancer are summarized, but it is seen that many of these studies have relatively low statistical power. As yet these data provide only tantalizing suggestions of modified cancer risks arising from polymorphisms, mutations, or changes in expression of the RAD51 gene family, and there is still a lot to learn before firm conclusions can be made.

Disruption and aberrant expression of HMGA2 as a consequence of diverse chromosomal translocations in myeloid malignancies

Chromosomal translocations that target HMGA2 at chromosome band 12q14 are seen in a variety of malignancies, notably lipoma, pleomorphic salivary adenoma and uterine leiomyoma. Although some HMGA2 fusion genes have been reported, several lines of evidence suggest that the critical pathogenic event is the expression of truncated HMGA2 isoforms. We report here the involvement of HMGA2 in six patients with myeloid neoplasia, dysplastic features and translocations or an inversion involving chromosome bands 12q13-15 and either 7p12, 8q22, 11q23, 12p11, 14q31 or 20q11. Breaks within or very close to HMGA2 were found in all six cases by molecular cytogenetic analysis, leading to overexpression of this gene as assessed by RT-PCR. Truncated transcripts consisting of HMGA2 exons 1-2 or exons 1-3 spliced to intron-derived sequences were identified in two patients, but were not seen in controls. These findings suggest that abnormalities of HMGA2 play an important and previously unsuspected role in myelodysplasia.

Etiology and pathogenesis of uterine leiomyomas: a review

Uterine leiomyomas, or fibroids, represent a major public health problem. It is believed that these tumors develop in the majority of American women and become symptomatic in one-third of these women. They are the most frequent indication for hysterectomy in the United States. Although the initiator or initiators of fibroids are unknown, several predisposing factors have been identified, including age (late reproductive years), African-American ethnicity, nulliparity, and obesity. Nonrandom cytogenetic abnormalities have been found in about 40% of tumors examined. Estrogen and progesterone are recognized as promoters of tumor growth, and the potential role of environmental estrogens has only recently been explored. Growth factors with mitogenic activity, such as transforming growth factor- (subscript)3(/subscript), basic fibroblast growth factor, epidermal growth factor, and insulin-like growth factor-I, are elevated in fibroids and may be the effectors of estrogen and progesterone promotion. These data offer clues to the etiology and pathogenesis of this common condition, which we have analyzed and summarized in this review.

Current status of human chromosome 14

Over the past three decades, extensive genetic, physical, transcript, and sequence maps have assisted in the mapping of over 30 genetic diseases and in the identification of over 550 genes on human chromosome 14. Additional genetic disorders were assigned to chromosome 14 by studying either constitutional or acquired chromosome aberrations of affected subjects. Studies of benign and malignant tumours by karyotype analyses and by allelotyping with a panel of polymorphic genetic markers have further suggested the presence of several tumour suppressor loci on chromosome 14. The search for disease genes on human chromosome 14 has also been achieved by exploiting the human-mouse comparative maps. Research on uniparental disomy and on the search for imprinted genes has supported evidence of epigenetic inheritance as a result of imprinting on human chromosome 14. This review focuses on the current developments on human chromosome 14 with respect to genetic maps, physical maps, transcript maps, sequence maps, genes, diseases, mouse-human comparative maps, and imprinting.

Complex formation by the human RAD51C and XRCC3 recombination repair proteins

In vertebrates, the RAD51 protein is required for genetic recombination, DNA repair, and cellular proliferation. Five paralogs of RAD51, known as RAD51B, RAD51C, RAD51D, XRCC2, and XRCC3, have been identified and also shown to be required for recombination and genome stability. At the present time, however, very little is known about their biochemical properties or precise biological functions. As a first step toward understanding the roles of the RAD51 paralogs in recombination, the human RAD51C and XRCC3 proteins were overexpressed and purified from baculovirus-infected insect cells. The two proteins copurify as a complex, a property that reflects their endogenous association observed in HeLa cells. Purified RAD51C--XRCC3 complex binds single-stranded, but not duplex DNA, to form protein--DNA networks that have been visualized by electron microscopy.

Homologous recombinational repair of DNA ensures mammalian chromosome stability

The process of homologous recombinational repair (HRR) is a major DNA repair pathway that acts on double-strand breaks and interstrand crosslinks, and probably to a lesser extent on other kinds of DNA damage. HRR provides a mechanism for the error-free removal of damage present in DNA that has replicated (S and G2 phases). Thus, HRR acts in a critical way, in coordination with the S and G2 checkpoint machinery, to eliminate chromosomal breaks before the cell division occurs. Many of the human HRR genes, including five Rad51 paralogs, have been identified, and knockout mutants for most of these genes are available in chicken DT40 cells. In the mouse, most of the knockout mutations cause embryonic lethality. The Brca1 and Brca2 breast cancer susceptibility genes appear to be intimately involved in HRR, but the mechanistic basis is unknown. Biochemical studies with purified proteins and cell extracts, combined with cytological studies of nuclear foci, have begun to establish an outline of the steps in mammalian HRR. This pathway is subject to complex regulatory controls from the checkpoint machinery and other processes, and there is increasing evidence that loss of HRR gene function can contribute to tumor development. This review article is meant to be an update of our previous review [Biochimie 81 (1999) 87].

Chromosome instability and defective recombinational repair in knockout mutants of the five Rad51 paralogs

The Rad51 protein, a eukaryotic homologue of Escherichia coli RecA, plays a central role in both mitotic and meiotic homologous DNA recombination (HR) in Saccharomyces cerevisiae and is essential for the proliferation of vertebrate cells. Five vertebrate genes, RAD51B, -C, and -D and XRCC2 and -3, are implicated in HR on the basis of their sequence similarity to Rad51 (Rad51 paralogs). We generated mutants deficient in each of these proteins in the chicken B-lymphocyte DT40 cell line and report here the comparison of four new mutants and their complemented derivatives with our previously reported rad51b mutant. The Rad51 paralog mutations all impair HR, as measured by targeted integration and sister chromatid exchange. Remarkably, the mutant cell lines all exhibit very similar phenotypes: spontaneous chromosomal aberrations, high sensitivity to killing by cross-linking agents (mitomycin C and cisplatin), mild sensitivity to gamma rays, and significantly attenuated Rad51 focus formation during recombinational repair after exposure to gamma rays. Moreover, all mutants show partial correction of resistance to DNA damage by overexpression of human Rad51. We conclude that the Rad51 paralogs participate in repair as a functional unit that facilitates the action of Rad51 in HR.

Evidence for RAD51L1/HMGIC fusion in the pathogenesis of uterine leiomyoma

Chromosome rearrangements involving 12q15 are frequently observed in a variety of human mesenchymal tumors. The high mobility group protein gene HMGIC has been identified as the target involved in these rearrangements. Uterine leiomyomas frequently use chromosome band 14q24 as a translocation partner to HMGIC. Recent studies within the chromosome 14 breakpoint region in cell lines carrying t(12;14)(q15;q23-24) revealed that RAD51L1, a member of the RAD51 recombination gene family, is the HMGIC partner. Using RT-PCR, we screened a panel of 81 uterine leiomyomas removed from 30 women for rearrangement between RAD51L1 and HMGIC. This is the first molecular analysis in which primary tumors have been examined for the RAD51L1/HMGIC transcripts. The chimeric transcripts were identified from two cases in which exon 7 of the RAD51L1 gene is fused in frame to either exon 2 or exon 3 of the HMGIC gene. These transcripts encode fusion proteins containing RAD51L1 nucleotide binding domains and the HMGIC protein lacking the N-terminal AT hook motifs. The detection of the RAD51L1/HMGIC fusion in primary tumors adds to the accumulating evidence implicating this fusion in a proportion of uterine leiomyoma patients.

Role of BRCA2 in control of the RAD51 recombination and DNA repair protein

Individuals carrying BRCA2 mutations are predisposed to breast and ovarian cancers. Here, we show that BRCA2 plays a dual role in regulating the actions of RAD51, a protein essential for homologous recombination and DNA repair. First, interactions between RAD51 and the BRC3 or BRC4 regions of BRCA2 block nucleoprotein filament formation by RAD51. Alterations to the BRC3 region that mimic cancer-associated BRCA2 mutations fail to exhibit this effect. Second, transport of RAD51 to the nucleus is defective in cells carrying a cancer-associated BRCA2 truncation. Thus, BRCA2 regulates both the intracellular localization and DNA binding ability of RAD51. Loss of these controls following BRCA2 inactivation may be a key event leading to genomic instability and tumorigenesis.

Molecular cytogenetic analysis of uterine leiomyoma and leiomyosarcoma by comparative genomic hybridization

Uterine leiomyomata are among the most common of human neoplasms and are associated with abnormal uterine bleeding, infertility, and abdominal pain. Uterine leiomyosarcomata are presumed to be the malignant counterpart to uterine leiomyomata and are very rare. Transformation of uterine leiomyoma (ULM) into uterine leiomyosarcoma (ULMS) is yet to be conclusively confirmed, and each type of tumor may represent a distinct genetic entity. We used comparative genomic hybridization (CGH) to evaluate DNA sequence copy-number changes in 12 specimens of ULM and 8 of ULMS. CGH analysis of ULM demonstrated chromosomal imbalances in 8 of 12 (66. 7%) specimens. The most frequent ULM gains were observed at 9q34 (a novel finding) and on chromosome 19. Other ULM imbalances included gains and losses of chromosome 1p, losses on 7q, and gains on 12q. All ULMS specimens demonstrated chromosomal aberrations. Chromosome 1 imbalances were very prominent. The most frequent losses were detected on 14q and 22q. Losses on 14q are rarely seen in other types of leiomyo-sarcoma and may be a distinctive feature of ULMS. Gains on chromosomes 8, 17, and X were observed in half the cases and were accompanied by high-level amplification. Other chromosome arms overrepresented included 12q and 19p. The absence of specific anomalies common to all ULM and ULMS argues against their being benign-malignant counterparts.