Constitutional RB1-gene mutations in patients with isolated unilateral retinoblastoma.

In most patients with isolated unilateral retinoblastoma, tumor development is initiated by somatic inactivation of both alleles of the RB1 gene. However, some of these patients can transmit retinoblastoma predisposition to their offspring. To determine the frequency and nature of constitutional RB1-gene mutations in patients with isolated unilateral retinoblastoma, we analyzed DNA from peripheral blood and from tumor tissue. The analysis of tumors from 54 (71%) of 76 informative patients showed loss of constitutional heterozygosity (LOH) at intragenic loci. Three of 13 uninformative patients had constitutional deletions. For 39 randomly selected tumors, SSCP, hetero-duplex analysis, sequencing, and Southern blot analysis were used to identify mutations. Mutations were detected in 21 (91%) of 23 tumors with LOH. In 6 (38%) of 16 tumors without LOH, one mutation was detected, and in 9 (56%) of the tumors without LOH, both mutations were found. Thus, a total of 45 mutations were identified in tumors of 36 patients. Thirty-nine of the mutations-including 34 small mutations, 2 large structural alterations, and hypermethylation in 3 tumors-were not detected in the corresponding peripheral blood DNA. In 6 (17%) of the 36 patients, a mutation was detected in constitutional DNA, and 1 of these mutations is known to be associated with reduced expressivity. The presence of a constitutional mutation was not associated with an early age at treatment. In 1 patient, somatic mosaicism was demonstrated by molecular analysis of DNA and RNA from peripheral blood. In 2 patients without a detectable mutation in peripheral blood, mosaicism was suggested because 1 of the patients showed multifocal tumors and the other later developed bilateral retinoblastoma. In conclusion, our results emphasize that the manifestation and transmissibility of retinoblastoma depend on the nature of the first mutation, its time in development, and the number and types of cells that are affected.     

An 800-kb region of deletion at 13q14 in human prostate and other carcinomas

Deletions of regions at 13q14 have been detected by various genetic approaches in human cancers including prostate cancer. Several studies have defined one region of loss of heterozygosity (LOH) at 13q14 that seems to reside in a DNA segment of 7.1 cM between genetic markers D13S263 and D13S153. To define the smallest region of overlap (SRO) for deletion at 13q14, we first applied tissue microdissection and multiplex PCR to detect homozygous deletion and/or hemizygous deletion at 13q14 in 134 prostate cancer specimens from 114 patients. We detected deletions at markers D13S1227, D13S1272, and A005O48 in 13 (10%) of these tumor specimens. Of the 13 tumors with deletions, 12 were either poorly differentiated primary tumors or metastases of prostate cancer. To fine-map the deletion region, we then constructed a high-resolution YAC/BAC/STS/EST physical map based on experimental and database analyses. Several markers encompassing the deletion region were analyzed for homozygous deletion and/or hemizygous deletion in 61 cell lines/xenografts derived from human cancers of the prostate, breast, ovary, endometrium, cervix, and bladder, and a region of deletion was defined by duplex PCR assay between markers A005X38 and WI-7773. We also analyzed LOH at 13q14 in the 61 cell lines/xenografts using the homozygosity mapping of deletion approach and 26 microsatellite markers. We found 24 (39%) of the cell lines/xenografts to show LOH at 13q14 and defined a region of LOH by markers M1 and M5. Combination of homozygous or hemizygous deletion and LOH results defined the SRO for deletion to be an 800-kb DNA interval between A005X38 and M5. There are six known genes located in or close to the SRO for deletion. This region of deletion is at least 2 Mb centromeric to the RB1 tumor-suppressor gene and the leukemia-associated genes 1 and 2, each of which is located at 13q14. These data suggest that the 800-kb DNA segment with deletion contains a gene whose deletion may be important for the development of prostate and other cancers. This study also provides a framework for the fine-mapping, cloning, and identification of a novel tumor-suppressor gene at 13q14.     

Altered structure and deregulated expression of the tumor suppressor gene retinoblastoma (RB1) in human brain tumors

A total of 40 human brain tumor samples were analyzed for tumor-specific alterations at the RB1 gene locus. Gliomas were more prevalent in younger males and meningiomas in older females. Southern blot analysis revealed loss of heterozygosity (LOH) at the intron 1 locus of RB1 gene in 19.4% of informative cases and this is the first report showing LOH at this locus in human brain tumors. Levels of RB1 mRNA and protein, pRb, and the percentage of hyperphosphorylated form of pRb were also analyzed in these tumors. Normal human fibroblast cell line WI38 was used as control in northern and western analysis. Normal sized RB1 mRNA and protein were present in all the tumor samples. Majority of the gliomas had 2.0-fold or higher levels of RB1 mRNA and most meningiomas had less than 2.0-fold of RB1 mRNA compared to control WI38 cells. The total pRb levels were 2.0-fold or higher in all the tumor samples compared to control. More than 50% of pRb existed in hyperphosphorylated form in all gliomas except two. However, six out of 13 meningiomas had less than 50% of total pRb in the hyperphosphorylated form. These results indicate that the increased percentage of hyperphosphorylated form of pRb in gliomas could provide growth advantage to these tumors. Presence of LOH at the RB1 gene locus and the increased levels of RB1 RNA and protein and increased percentage of hyperphosphorylated form of pRb are indicative of an overall deregulation of pRb pathway in human brain tumors.     

Constitutional and somatic RB1 mutation spectrum in nonfamilial unilateral and bilateral retinoblastoma in India

An epidemiologic survey has indicated a comparatively high prevalence of retinoblastoma (Rb) in Asian countries. Recently, the development of preventive strategies in nonfamilial Rb has become a major goal. The present studies were designed for identification and characterization of constitutional and somatic RB1 gene mutations by conventional cytogenetics, fluorescent in situ hybridization (FISH) and polymerase chain reaction-single strand conformation polymorphism (PCR-SSCP)-DNA sequencing. Of 34 patients 32 were nonfamilial and 2 were familial Rb. Maternal inheritance of del (13q14) was common. FISH was sensitive in detecting monoallelic RB1 deletion/deletion mosaicism as a first genetic hit in 20% of cases. Somatic and germline RB1 point mutations affected exons 3, 17, 20, and 21 and these were identified as novel mutations. Involvement of exon 20 as a predisposing mutation in sporadic unilateral retinoblastoma (URB) probably suggests the susceptibility of exon 20 to unknown etiologic factors in our population. A de novo RB1 deletion along with transmitted RB1 point mutation from an asymptomatic parent was identified as a unique predisposing RB1 mutation chimerism in a URB case that later evolved to bilateral retinoblastoma (BRB). The predisposing mutations such as del (13q), RB1 mono-allelic deletion and RB1 point mutation in sporadic Rb were de novo as well as transmitted mutations from asymptomatic/symptomatic parents. The RB1 mutation incidence was comparatively higher (25%) in nonfamilial Rb with emphasis on high prevalence in sporadic URB (18% versus 0%-9% in the literature series). The present studies demonstrated the efficacy of a multitechnique approach to detect various types of constitutional RB1 mutations such as RB1 deletion, deletion mosaicism, point mutation, mutation chimerism in patients of symptomatic/asymptomatic parents.     

Allelic loss is frequent in tuberous sclerosis kidney lesions but rare in brain lesions.

Tuberous sclerosis (TSC) is an autosomal dominant disorder characterized by seizures, mental retardation, and hamartomatous lesions. Although hamartomas can occur in almost any organ, they are most common in the brain, kidney, heart, and skin. Allelic loss or loss of heterozygosity (LOH) in TSC lesions has previously been reported on chromosomes 16p13 and 9q34, the locations of the TSC2 and TSC1 genes, respectively, suggesting that the TSC genes act as tumor-suppressor genes. In our study, 87 lesions from 47 TSC patients were analyzed for LOH in the TSC1 and TSC2 chromosomal regions. Three findings resulted from this analysis. First, we confirmed that the TSC1 critical region is distal to D9S149. Second, we found LOH more frequently on chromosome 16p13 than on 9q34. Of the 28 patients with angiomyolipomas or rhabdomyomas, 16p13 LOH was detected in lesions from 12 (57%) of 21 informative patients, while 9q34 LOH was detected in lesions from only 1 patient (4%). This could indicate that TSC2 tumors are more likely than TSC1 tumors to require surgical resection or that TSC2 is more common than TSC1 in our patient population. It is also possible that small regions of 9q34 LOH were missed. Lastly, LOH was found in 56% of renal angiomyolipomas and cardiac rhabdomyormas but in only 4% of TSC brain lesions. This suggests that brain lesions can result from different pathogenic mechanisms than kidney and heart lesions.     

At least four different chromosomal regions are involved in loss of heterozygosity in human breast carcinoma

Three chromosome regions, i.e., 11p15, 13q, and 17p, were previously reported by three independent groups to be specifically reduced to hemizygosity in human primary breast cancer. We examined the DNA of 64 mammary tumors for loss of heterozygosity (LOH) with 28 polymorphic DNA markers dispersed on 10 arms of 8 different chromosomes. Complete or near-complete absence of LOH was observed on 5 arms (5 chromosomes). LOH at all three previously invoked regions was confirmed, and the highest frequency was found on 17p (67% of heterozygous patients). Allele loss of a marker from chromosome 3 (region p14-p21) was found in 7 of 15 informative cases. Concurrent LOH at 2 to 4 loci was noted in 20 of the 43 tumors showing LOH. Allele losses did not correlate with any of the six clinico-histopathological variables investigated, but in a group of patients in which we were unable to demonstrate LOH, the absence of distant metastases was statistically significant (P less than 0.05). These results suggest that some of the observed allele losses reflect random events, possibly as a result of genetic instability, but are not without biological significance for the progression of particular subclasses of breast tumors.     

Genetic and Epigenetic Alterations in Primary–Progressive Paired Oligodendroglial Tumors

The aim of the present study was to identify genetic and epigenetic alterations involved in the progression of oligodendroglial tumors. We characterized 21 paired, World Health Organization (WHO) grade II and III oligodendroglial tumors from patients who received craniotomies for the partial or complete resection of primary and secondary oligodendroglial tumors. Tumor DNA was analyzed for alterations in selected genetic loci (1p36, 9p22, 10q23–24, 17p13, 19q13, 22q12), isocitrate dehydrogenase 1 (IDH1), isocitrate dehydrogenase 2 (IDH2) and the CpG island methylation status of critical tumor-related genes (MGMT, P16, DAPK, PTEN, RASSF1A, Rb1). Alterations of these markers were common early in the tumorigenesis. In the primary tumors we identified 12 patients (57.1%) with 1p36 deletions, 17 (81.0%) with 19q13 deletions, 9 (42.9%) with 1p36/19q13 codeletions, 11 (52.3%) with 9p22 deletions, and 12 (57.1%) with IDH1 mutation. Epigenetic analysis detected promoter methylation of the MGMT, P16, DAPK, PTEN, RASSF1A, and Rb1 genes in 38.1%, 19.0%, 38.1%, 33.3%, 66.7%, and 14.3% of primary tumors, respectively. After progression, additional losses of 1p, 9p, 10q, 17p, 19q and 22q were observed in 3 (14.3%), 1 (4.8%), 3 (14.3%), 2 (9.5%), 1 (4.8%) and 3 (14.3%) cases, respectively. Additional methylations of the MGMT, P16, DAPK, PTEN, RASSF1A, and RB1 promoters was observed in 4 (19.0%), 2 (9.5%), 0 (0%), 6 (28.6%), 2(9.5%) and 3 (14.3%) cases, respectively. The status of IDH1 mutation remained unchanged in all tumors after progression. The primary tumors of three patients with subsequent progression to high-grade astrocytomas, all had 9p deletion, intact 1p, intact 10q and unmethylated MGMT. Whether this may represent a molecular signature of patients at-risk for the development of aggressive astrocytomas needs further investigation.     

中国人卵巢上皮性肿瘤nm23H1基因遗传不稳定性的研究

The aim of this study was to examine microsatellite instability (MSI) and loss of heterozygosity (LOH) of locus D17S396 on chromosome 17 and their influence on the expression of nm23H1 in the epithelial ovarian tumors, which may provide experimental basis for the mechanism of nm23H1 gene and tumor metastasis. Techniques such as DNA extraction from formalin-fixed paraffin-embedded tissues, polymerase chain reaction-single strand conformation polymorphism (PCR-SSCP), ordinary silver stain were used to study MSI and LOH of locus D17S396. Envision immunohistochemistry and Leica-Qwin computer imaging techniques were used to assess the expression of nm23H1 gene. In our experiments, the frequency of heredity instability of malignant ovarian tumors was 40%, which is higher than that of borderline ovarian tumors, while there were no heredity instability occurred in benign ovarian tumors and normal ovarian tissue. Among 25 epithelial ovarian carcinomas, the frequency of LOH in lymph node metastasis cases (66.67%) was significantly higher than those without metastasis (10.53%). Moreover, the frequency of LOH was higher in FIGO stage III and IV than in stage I and II. However, the frequency of MSI showed no correlation with any clinicopathologic characteristics. The positive frequency of nm23H1 protein in the ovarian epithelial carcinoma and borderline tumors were 56.00% and 57.14%, respectively. They were both higher than those of the benign tumors and normal ovarian tissue. In the epithelial ovarian carcinomas, the positive frequency of nm23H1 protein in lymph node metastasis case was significantly lower than those without metastasis. FIGO stage III and IV also exhibited lower positive frequency of nm23H1 protein compared with stage I and II. Furthermore, there was no difference in nm23H1 protein expression intensity analyzed by computer imaging. In the epithelial ovarian carcinomas, the positive frequency of nm23H1 protein in LOH positive group was 0.00%, which is lower than that of LOH negative group (P < 0.01). The results indicated that the heredity instability of nm23H1 gene might be implicated in pathogenesis and progression of epithelial ovarian tumor. The occurrence of LOH might be the molecule marker of the deteriorism of ovarian tissue. Both MSI and LOH of nm23H1 gene controlled development of the epithelial ovarian tumor independently in different paths. LOH could inhibit the expression of nm23H1 in local tissue of the epithelial ovarian carcinoma, which endowed it with high aggressive and poor prognosis. Increasing the amount of nm23H1 protein expression could effectively restrain metastasis of the ovarian epithelial carcinoma and improve prognosis of patients.     

Collaborative software for traditional and translational research

Neurofibromatosis type-1 (NF1), resulting from NF1 gene loss of function, is characterized by an increased risk of developing benign and malignant peripheral nerve sheath tumors (MPNSTs). Whereas the cellular heterogeneity of NF1-associated tumors has been well studied, the molecular heterogeneity of MPNSTs is still poorly understood. Mutational heterogeneity within these malignant tumors greatly complicates the study of the underlying mechanisms of tumorigenesis. We have explored this molecular heterogeneity by performing loss of heterozygosity (LOH) analysis of the NF1, TP53, RB1, PTEN, and CDKN2A genes on sections of 10 MPNSTs derived from 10 unrelated NF1 patients. LOH data for the TP53 gene was found to correlate with the results of p53 immunohistochemical analysis in the same tumor sections. Further, approximately 70% of MPNSTs were found to display intra-tumoral molecular heterogeneity as evidenced by differences in the level of LOH between different sections of the same tumor samples. This study constitutes the first systematic analysis of molecular heterogeneity within MPNSTs derived from NF1 patients. Appreciation of the existence of molecular heterogeneity in NF1-associated tumors is important not only for optimizing somatic mutation detection, but also for understanding the mechanisms of NF1 tumorigenesis, a prerequisite for the development of specifically targeted cancer therapeutics.     

Loss of heterozygosity on chromosome 11 in sporadic gastrinomas

Summary Gastrinomas are pancreatic endocrine neoplasms that arise either sporadically or are inherited as part of the multiple endocrine neoplasia type I syndrome (MEN I). Loss of heterozygosity (LOH) in the region flanking the MEN I gene at chromosome 11q13 has been documented in a few sporadic and familial pancreatic endocrine tumors, but not previously in sporadic gastrinomas. It has therefore been suggested that gastrinomas develop by a mechanism different from other tumors associated with the MENI syndsrome. We report LOH on chromosome 11 in 5 of 11 sporadic gastrinomas. Four of these tumors have LOH for markers flanking the MEN I region. Molecular evaluation of segments of chromosomes 3, 13, and 17 known to contain cloned or putative tumor suppressor genes fail to show LOH except at one locus in one tumor. These data suggest that a tumor suppressor DNA segment exists at 11q13 that may be involved in the development of sporadic gastrinomas.     

A Common Region of Deletion on Chromosome 17q in Both Sporadic and Familial Epithelial Ovarian Tumors Distal to BRCA1

Linkage analysis in familial breast and ovarian cancer and studies of allelic deletion in sporadic ovarian tumors have identified a region on chromosome 17q containing a candidate tumor-suppressor gene (referred to as BRCA1) of likely importance in ovarian carcinogenesis. We have examined normal and tumor DNA samples from 32 patients with sporadic and 8 patients with familial forms of the disease, for loss of heterozygosity (LOH) at 21 loci on chromosome 17 (7 on 17p and 14 on 17q). LOH on 17p was 55% (22/40) for informative 17pl3.1 and 17pl3.3 markers. When six polymorphic markers flanking the familial breast/ovarian cancer susceptibility locus on 17ql2-q21 were used, LOH was 58% (23/40), with one tumor showing telomeric retention. Evaluation of a set of markers positioned telomeric to BRCA1 resulted in the highest degree of LOH, 73% (29/40), indicating that a candidate locus involved in ovarian cancer may reside distal to BRCA1. Five of the tumors demonstrating allelic loss for 17q markers were from individuals with a strong family history of breast and ovarian cancer. More important, two of these tumors (unique patient number [UPN] 57 and UPN 79) retained heterozygosity for all informative markers spanning the BRCA1 locus but showed LOH at loci distal to but not including the anonymous markers CMM86 (D17S74) and 42D6 (D17S588), respectively. Deletion mapping of seven cases (two familial and five sporadic) showing limited LOH on 17q revealed a common region of deletion, distal to GH and proximal to D17S4, that spans −25 cM. These results suggest that a potential tumor-suppressor gene involved in both sporadic and familial ovarian cancer may reside on the distal portion of chromosome 17q and is distinct from the BRCA1 gene.     

Detection of mutations of the RB1 gene in retinoblastoma patients by using exon-by-exon PCR-SSCP analysis.

Most sporadic cases of retinoblastoma, malignant eye tumor of children, may require the identification of a mutation of the retinoblastoma gene (RB1 gene) for precise genetic counseling. We established a mutation detection system of and screened for the RB1 gene mutation in 24 patients with retinoblastoma--12 bilateral patients and 12 unilateral patients. Mutation analysis was performed by PCR-mediated SSCP analysis in the entire coding region and promoter region, as an initial screening method, followed by direct genomic sequencing. Possible oncogenic mutations were identified in 14 (58%) of 24 tumors, of which 6 were single base substitutions, 4 were small deletions, 3 were small insertions, and 1 was a complex alteration due to deletion-insertion. A constitutional somatic mosaicism was suggested in one bilateral patient. A majority (57%) of mutations were found in E1A binding domains, and all were presumed to truncate the normal gene products. The mutation analysis presented here may provide a basis for the screening system of RB1 gene mutations in retinoblastoma patients.     

Abnormal methylation of several tumor suppressor genes in sporadic breast cancer

Multiplex methylation-sensitive PCR was employed in studying the methylation of CpG islands in the RB1, p16/CDKN2A, p15/CDKN2B, p14/ARF, CDH1, MGMT, HIC1, and N33 promoter regions in breast carcinoma (105 tumors). Methylation was often observed for the two major suppressor genes involved in cell-cycle control through the Cdk-Rb-E2F signaling pathway, RB1 (18/105, 17%) and p16 (59/105, 56%); both genes were methylated in 13 tumors. Methylation involved p15 in two (2%) tumors; CDH1, in 83 (79%) tumors; MGMT, in eight (8%) tumors, and N33, in nine (9%) tumors. The p14 promoter was not methylated in the tumors examined.     

Microsatellite Alterations and Protein Expression of 5 Major Tumor Suppressor Genes in Gastric Adenocarcinomas

PURPOSE: In gastric adenocarcinoma (GC), the major tumor suppressor genes (TSGs) such as p16, PTEN, Rb, E-cadherin, and p53, may play important roles in various regulatory pathways and in tumor suppression. This study evaluated the loss of heterozygosity (LOH) of microsatellite and protein expression of 5 TSGs and the results were examined for their correlation with clinicopathological factors. METHODS: LOH analysis was carried out using polymerase chain reactions with 15 polymorphic microsatellite markers of 5 chromosomes containing TSGs in 100 surgically resected tumors. Protein expression was evaluated by immunohistochemistry (IHC). RESULTS: LOH was detected in 83% of GCs. LOH of 9p21, 10q23, 13q14, 16q22, and 17p13 were detected in 26%, 31%, 24%, 22%, and 35% of cases, respectively. Protein expression of p16, PTEN, Rb, E-cadherin, and p53 were found to be 31%, 39%, 28%, 32%, and 46% of cases. Advanced GCs showed significantly higher rates of 17p13 LOH and p53 expression. 9p21 LOH and E-cadherin IHC were correlated with higher tumor grade. Lymph node metastasis was correlated with the LOH of 9p21, 16q22, and 17p13 and IHC of the Rb and p53. A higher stage was correlated with 10q23 and 17p13 in LOH and p53 for IHC. CONCLUSION: These results suggest that LOH and protein expression of various TSGs are important in carcinogenesis and tumor invasion. Additionally, LOH and IHC may be useful clinical indicators for determining the prognosis of patients with GCs. In particular, the 17p13 LOH and p53 for IHC can be applied as simple evaluations in the clinic.     

Differential loss of heterozygosity in familial, sporadic, and uremic hyperparathyroidism

Various genetic loci harboring oncogenes, tumor suppressor genes, and genes for calcium receptors have been implicated in the development of parathyroid tumors. We have carried out loss of heterozygosity (LOH) studies in chromosomes 1p, 1q, 3q, 6q, 11q, 13q, 15q, and X in a total of 89 benign parathyroid tumors. Of these, 28 were sporadic parathyroid adenomas from patients with no family history of the disease, 41 were secondary parathyroid tumors, 5 were from patients with a history of previous irradiation to the neck, 12 were from patients with a family history of hyperparathyroidism, and 3 were parathyroid tumors related to multiple endocrine neoplasia type 1 (MEN1). In addition, we determined the chromosomal localization of a second putative calcium-sensing receptor, CaS, for inclusion in the LOH studies. Based on analysis of somatic cell hybrids and fluorescent in situ hybridization to metaphase chromsomes, the gene for CaS was mapped to chromosomal region 2q21-q22. The following results were obtained from the LOH studies: (1) out of the 24 tumors that showed LOH, only 4 had more than one chromosomal region involved, (2) in the tumours from uremic patients, LOH of chromosome 3q was detected in a subset of the tumors, (3) LOH of the MEN1 region at 11q13 was the most common abnormality found in both MEN1-related and sporadic parathyroid tumours but was not a feature of the other forms of parathyroid tumors, (4) LOH in 1p and 6q was not as frequent as previously reported, and (5) tumor suppressor genes in 1q and X might have played a role, particularly on the X chromosome, in the case of familial parathyroid adenomas. We therefore conclude that the tumorigenesis of familial, sporadic, and uremic hyperparathyroidism involves different genetic triggers in a non-progressive pattern. Received: 28 October 1996 / Revised: 16 November 1996     

Different capacities for recombination in closely related human lymphoblastoid cell lines with different mutational responses to X-irradiation.

WIL2-NS and TK6 are two distinct human lymphoblast cell lines derived from a single male donor. WIL2-NS cells are significantly more resistant to the cytotoxic effects of X-irradiation but considerably more sensitive to induced mutation. In an effort to determine the mechanistic basis for these differences, we analyzed the physical structures of thymidine kinase (tk)-deficient mutants isolated after X-ray treatment of tk heterozygotes derived from TK6 and the more mutable WIL2-NS. Southern analysis showed that while 84% of TK6-derived mutants had arisen by loss of heterozygosity (LOH), all 106 mutants from WIL2-NS derivatives arose with LOH at tk and all but one showed LOH at other linked loci on chromosome 17. We adapted a fluorescence in situ hybridization technique to distinguish between LOH due to deletion, which results in retention of only one tk allele, and LOH due to a mechanism involving the homologous chromosome (e.g., recombination), which results in the retention of two alleles. Among the LOH mutants derived that were analyzed in this way, 9 of 26 from WIL2-NS and 11 of 17 from TK6 cell lines arose by deletion. The remaining mutants retained two copies of the tk gene and thus arose by a mechanism involving the homologous allele. Since many of these mutants arising by a homologous mechanism retained partial heterozygosity of chromosome 17, they must have arisen by recombination or gene conversion, and not chromosome loss and reduplication. Finally, the recombinational capacities of WIL2-NS and TK6 were compared in transfection assays with plasmid recombination substrates. Intermolecular recombination frequencies were greater in WIL2-NS than in TK6. These data are consistent with a model suggesting that a recombinational repair system is functioning at a higher level in WIL2-NS than in TK6; the greater mutability of the tk locus in WIL2-NS results from more frequent inter- and intramolecular recombination events.     

Visualization of mosaicism in tissues of normal and mismatch-repair-deficient mice carrying a microsatellite-containing transgene

We tested for azoospermia factor (AZF) deletions 17 loci corresponding to AZF subintervals a-d in 17 cases of testicular tumors occurring in Finns. While DNA samples from 48 CEPH and 32 Finnish males showed no deletions, patients with testicular cancer displayed AZF deletion mosaicisms in various non-tumor tissues (13 cases) and specific deletion haplotypes in tumor tissues (10 cases). Two of the cases with AZF deletions were testicular non-Hodgkin lymphomas indicating that Y-microdeletions appear also in malignancies other than seminoma and non-seminoma tumors. In good agreement with this assumption, we detected one AZF deletion in normal cells from 1 of 5 HNPCC cases, heterozygous for an MLH1 mutation. We propose that AZF deletions occur in early embryogenesis due to mutations of TSPY, mismatch repair (MMR), or X-specific genes. Since fathers of testicular, tumor cases did not exhibit AZF deletions, we assumed they were not carriers of the mutation inducing AZF deletion-mosaicisms. Therefore, tumor cases should have received the MMR gene or X mutations via the maternal lineage, or for the case of TSPY and MMR genes via a sperm carrying a mutation occurred in the paternal germ-cell line. We consider AZF microdeletions in non-tumor cells to be part of a broader pattern of chromosome instability producing susceptibility to testicular tumors. Clonal transformation and expansion of one of these tumor-susceptible cell lineages give rise to testicular tumors showing genome anomalies characteristic of testicular cancers (i12p, LOH and genetic imbalance for various autosomal regions, Y- and autosomal MSI, specific AZF deletion haplotypes).     

Marked differences in unilateral isolated retinoblastomas from young and older children studied by comparative genomic hybridization

Although it is established that the loss of function of both alleles of the RB1 gene is a prerequisite for the development of retinoblastoma, little is known about the genetic events that are required for tumor progression. We used comparative genomic hybridization (CGH) to search for DNA copy number changes in isolated unilateral retinoblastomas. From a series of 66 patients with retinoblastomas with somatic mutations in both RB1 alleles, tumor samples from 13 children with the youngest (2.0-9.8 months) and 13 with the oldest (36.2-84.1 months) age at operation were studied. Loss at 13q14, the location of RB1, was demonstrated in two tumors only. Recurring chromosome imbalances included gains at 6p (11/26), 1q (10/26), 2p (4/26), and 17q (4/26), gains of the entire chromosome 19 (3/26), and losses at 16q (9/26). A commonly gained region at 1q32 was identified. Increased dosage of GAC1, a candidate oncogene located in 1q32, was found in two of four tumors by Southern blot analysis. Comparison of the CGH findings revealed that retinoblastomas from children with an older age at operation showed significantly more frequent (13/13 cases vs 4/13 cases; P = 0.0005) and more complex genetic abnormalities (median, 5 changes/abnormal tumor vs median, 1.5 changes/abnormal tumor; P = 0.003) than retinoblastomas from children with a young age at operation. Gains at 1q, 2p, 17q, of the entire chromosome 19 and losses of 16q were restricted to the older age group. Our results suggest that the progression of retinoblastomas from older patients follows mutational pathways different from those of younger patients.