Infrequent genomic rearrangement and normal expression of the putative RB1 gene in retinoblastoma tumors

Conservation of theRB1gene in human and primates

Mutations in the RB1 gene are associated with retinoblastoma, which has served as an important model for understanding hereditary predisposition to cancer. Despite the great scrutiny that RB1 has enjoyed as the prototypical tumor suppressor gene, it has never been the object of a comprehensive survey of sequence variation in diverse human populations and primates. Therefore, we analyzed the coding (2,787 bp) and adjacent intronic and untranslated (7,313 bp) sequences of RB1 in 137 individuals from a wide range of ethnicities, including 19 Asian Indian hereditary retinoblastoma cases, and five primate species. Aside from nine apparently disease-associated mutations, 52 variants were identified. They included six singleton, coding variants that comprised five amino acid replacements and one silent site. Nucleotide diversity of the coding region (pi=0.0763+/-1.35 x 10(-4)) was 52 times lower than that of the noncoding regions (pi=3.93+/-5.26 x 10(-4)), indicative of significant sequence conservation. The occurrence of purifying selection was corroborated by phylogeny-based maximum likelihood analysis of the RB1 sequences of human and five primates, which yielded an estimated ratio of replacement to silent substitutions (omega) of 0.095 across all lineages. RB1 displayed extensive linkage disequilibrium over 174 kb, and only four unique recombination events, two in Africa and one each in Europe and Southwest Asia, were observed. Using a parsimony approach, 15 haplotypes could be inferred. Ten were found in Africa, though only 12.4% of the 274 chromosomes screened were of African origin. In non-Africans, a single haplotype accounted for from 63 to 84% of all chromosomes, most likely the consequence of natural selection and a significant bottleneck in effective population size during the colonization of the non-African continents.

Malignancy: Tumor Suppressor Gene Aberrations in Acute Myelogenous Leukemia

Acute myelogenous leukemia is a heterogeneous disease that appears to evade the normal regulatory controls of tumor suppressor genes. Studies in AML have documented mutations in both p53 and Retinoblastoma (Rb) genes, but these mutations are relatively uncommon, especially compared to their mutational frequency in solid tumors. In addition, expression abnormalities have now been documented in several tumor suppressor genes or related genes including MDM2, p73, Rb, p14(ARF), p15(INK4B), and p16(INK4A). We review the current literature regarding tumor suppressor genes in AML and suggest how these genes may be involved in the development of the disease.

Microsatellite instability in sarcomas

BACKGROUND

Microsatellite instability (MIN) has been studied in a variety of carcinomas and gynecologic sarcomas, but never in musculoskeletal sarcomas.

METHODS

We evaluated 16 skeletal and soft tissue sarcomas at nine genetic loci from chromosomal regions 1q, 5q, 7q, 12p, 13q, 17p, 19q, and two at 11p--all potential regions of interest regarding musculoskeletal sarcomas.

RESULTS

MIN was identified at one or more loci in seven of the cancers studied (44%). Three tumors had more than one locus with MIN and one tumor, a high-grade osteogenic sarcoma, had five of nine loci positive for MIN.

CONCLUSION

These results indicate that musculoskeletal sarcomas show instability in areas inside and outside the loci of known oncogenes. Areas of mismatch repair, as heralded by MIN, may contribute to the vast heterogeneity of these neoplasms.

Clinical, cytogenetic, and molecular testing of Argentine patients with retinoblastoma

PURPOSE

The purpose of this study is to determine the clinical, chromosomal, and molecular characteristics of Argentine patients with unilateral and bilateral retinoblastoma.

STUDY DESIGN

Eighty-six patients belonging to 82 families were studied; 59% of them were examined during the first year of life. Leukocoria was the most common reason for consultation. Other presenting signs were strabismus and glaucoma. Enucleation of the affected eye was performed in 85% of the cases and the complication rate was 13%.

RESULTS

An appropriate therapy allowed the survival of 84 of the 86 patients. Two children with malformations and growth retardation had an abnormal karyotype with a deletion in 13q14. Segregation analysis of polymorphic sites within the retinoblastoma gene and the parental origin of the allele lost in the tumor were analyzed in 30 of the 82 families. Five mutant alleles transmitted through the germline and six de novo germline mutant alleles were identified in 12 patients with hereditary retinoblastoma. Most de novo germline mutant alleles were paternally derived. Molecular analysis of nonhereditary retinoblastoma showed loss of heterozygosity in three of eight cases. From these, two maternal alleles and one paternal allele were lost, thus not indicating a significant difference in the parental origin for the lost allele.

CONCLUSIONS

These data are useful for deoxyribonucleic acid diagnosis of susceptibility to retinoblastoma in relatives of hereditary patients, even if mutations have not been identified.

Involvement of p53 and WAF1/CIP1 in gamma-irradiation-induced apoptosis of retinoblastoma cells

Retinoblastoma is an intraocular malignancy of childhood, which is very radiosensitive. gamma-irradiation, however, induces side effects, and the precise mechanisms of tumor cell death after the treatment remain unknown. In this study, we demonstrated that gamma-irradiation induced apoptosis (programmed cell death) in human retinoblastoma cell lines Y79 and WERI-Rb-1. The expression levels of p53, tumor suppressor gene product, and its-associated protein, WAF1/CIP1, were both up-regulated, and function of p53 was remarkably activated after the treatment. Moreover, apoptosis was induced in the absence of gamma-irradiation by overexpression of the WAF1/CIP1 gene in both retinoblastoma cells. These results indicate that the transfer of the WAF1/CIP1 gene may have potential for the treatment of human retinoblastomas instead of gamma-irradiation.

Deletion of RB exons 24 and 25 causes low-penetrance retinoblastoma

A deletion in the tumor-suppressor gene, RB, discovered by quantitative multiplex PCR, shows low penetrance (LP), since only 39% of eyes at risk in this family develop retinoblastoma. The 4-kb deletion spanning exons 24 and 25 (delta24-25) is the largest ever observed in an LP retinoblastoma family. Unlike the usual RB mutations, which cause retinoblastoma in 95% of at-risk eyes and yield no detectable protein, the delta24-25 allele transcribed a message splicing exon 23 to exon 26, resulting in a detectable protein (pRBdelta24-25) that lacks 58 amino acids from the C-terminal domain, proving that this domain is essential for suppression of retinoblastoma. Two functions were partially impaired by delta24-25-nuclear localization and repression of E2F-consistent with the idea that LP mutations generate "weak alleles" by reducing but not eliminating essential activities. However, delta24-25 ablated interaction of pRB with MDM2. Since a homozygous LP allele is considered nontumorigenic, the pRB/MDM2 interaction may be semi- or nonessential for suppressing retinoblastoma. Alternatively, some homozygous LP alleles may not cause tumorigenesis because an additional event is required (the "three-hit hypothesis"), or the resulting imbalance in pRB function may cause apoptosis (the "death allele hypothesis"). pRBdelta24-25 was also completely defective in suppressing growth of Saos-2 osteosarcoma cells. Targeting pRBdelta24-25 to the nucleus did not improve Saos-2 growth suppression, suggesting that C-terminal domain functions other than nuclear localization are essential for blocking proliferation in these cells. Since delta24-25 behaves like a null allele in these cells but like an LP allele in the retina, pRB may use different mechanisms to control growth in different cell types.

The retinoblastoma protein and cell cycle regulation

Although the precise function of the retinoblastoma gene product, p110RB1, remains unknown, recent data suggest that it plays a role in the control of cellular proliferation by regulating transcription of genes required for a cell to enter or stay in a quiescent or G0 state, or for progression through the G1 phase of the cell cycle. However, it is difficult to rationalize the expression of p110RB1 in a wide range of tissues with the fact that mutations in the RB1 gene initiate cancers in a limited number of tissues.

Alterations of the retinoblastoma a susceptibility gene in chronic lymphocytic leukemia

Research in recent years has shown that malignant transformation is a genetic multistep process. This holds true not only for in-vitro model systems, but has also been elegantly shown in-vivo, as in colorectal cancer. Chronic lymphocytic leukemia (CLL) is the most frequent leukemia in Western countries and occurs mainly in elderly patients, suggesting that in this form of leukemia, cumulative molecular lesions may be necessary for transformation. However, the molecular background is unknown in most cases. Cytogenetic aberrations may be used as markers for genes involved in the process of malignant transformation. In CLL, the most frequently observed structural cytogenetic lesion is a deletion/translocation involving the long arm of chromosome 13, a region where the retinoblastoma susceptibility gene (Rb-gene) has been mapped (13q14). Many groups have studied the question as to whether alterations of the Rb-gene play a causal role in the pathogenesis of CLL. This review deals with recent data indicating that i) the Rb-gene may be altered in a minority of CLL cases, and ii) there may be another gene localized on chromosome 13q14 that may be important in the molecular biology of CLL.

Parental origin of germ-line and somatic mutations in the retinoblastoma gene

Segregation analysis of polymorphic sites within the retinoblastoma (RB) gene and on chromosome 13, as well as the parental origin of the lost allele in the tumor, were analyzed in 24 families with RB patients. Four mutant alleles transmitted through the germ-line and seven de novo germ-line mutant alleles were identified in 11 patients with hereditary RB. Segregation analysis within the RB gene and on chromosome 13 was useful for DNA diagnosis of susceptibility to RB in relatives of hereditary patients, even if mutations were not identified. All seven de novo germ-line mutant alleles were paternally derived. The bias toward the paternal allele for de novo germ-line mutations of the RB gene was statistically significant. Seven paternal alleles and six maternal alleles were lost in 13 non-hereditary RB tumors with no bias in the parental origin of the somatic allele loss. These results suggest that the physical environment or a deficiency in DNA repair during spermatogenesis may be associated with significant risk factors for de novo germ-line mutations.

Mutations in the retinoblastoma gene and their expression in somatic and tumor cells of patients with hereditary retinoblastoma

Two intragenic deletions (exon 18-19 and exon 24) and two point mutations (one missense mutation in exon 21 and one mutation at splice-donor site for exon 13) were detected in the retinoblastoma gene in somatic and tumor cells of patients with hereditary retinoblastoma. Three mutations were located in a domain essential for binding to oncoproteins encoded by DNA tumor viruses (Hu et al., 1990; Huang et al., 1990). One mutation (deletion of exon 24) was outside this domain but it is in the region essential for binding to transcriptional factor E2F, and for suppression of malignant phenotypes (Qian et al., 1992; Qin et al., 1992). A minisatellite-like sequence and short repeated sequences were located at the breakpoint of the deletion of exon 24, suggesting that two deletions on both sides of the minisatellite-like sequence may be generated by a "DNA slippage and misalignment" mechanism. Upon amplification of cDNA by the polymerase chain reaction, no transcript of gene with frameshift mutation (deletion of exon 24) was detected in skin fibroblasts, while transcripts of genes with missense mutations were detected. The results, in combination with previous reports (Dunn et al., 1989; Hashimoto et al., 1991), suggest the instability of transcripts with a premature stop codon or the suppressed expression of alleles with a premature stop codon in the retinoblastoma gene in somatic cells of hereditary patients.

Molecular characterization of the retinoblastoma susceptibility gene

Retinoblastoma is recognized as a hereditary cancer. Genetic and epidemiological analysis of the disease has been incorporated into a two-hit mutational inactivation hypothesis of the origin of retinoblastoma. The molecular cloning and characterization of the retinoblastoma gene and gene product has allowed a critical testing of this two-hit hypothesis. All the predications of the model have been born out by experiment so far. These include inheritance of one mutated RB allele as the origin of hereditary retinoblastoma, subsequent loss of the remaining allele upon tumorigenesis, the involvement of the same RB gene in both sporadic and hereditary retinoblastoma, the somatic mutation of both RB alleles in sporadic retinoblastoma, the lack of RB expression in any retinoblastoma yet examined, and the recessiveness of mutated RB alleles. The RB gene exhibits functional properties consistent with its role as a suppressor of tumor formation. For example, re-expression of RB in tumor cells lacking endogenous RB leads to a loss of tumorigenic properties. RB protein can also inhibit progression through the cell division cycle, and it physically and/or functionally interacts with important cell cycle regulatory molecules. Although confirmation of the two-hit hypothesis seems complete, we can not rule out the possibility that other genes are involved in the genesis of this tumor. For example, there seems to be variable resistance to tumor development even in patients inheriting retinoblastoma susceptibility. Further, heterozygous RB null mice do not develop retinoblastoma, but develop a characteristic brain tumor instead. The molecular isolation of the RB gene is an important achievement in research on cancer. For the first time, it has become possible to examine, at the molecular level, genes that inhibit the growth of tumor cells. The precise mechanism of action of RB is unknown, but a broad outline is beginning to emerge. RB seems to negatively influence tumor cell growth by participating in regulation of the cell division cycle. RB has also been implicated in differentiation; its effect on the cell division cycle and its effects on differentiation may be different manifestations of the same function. Since RB is involved in oncogenesis, gene regulation, and cellular differentiation, it is obviously an attractive gene for intense study; understanding the function and mechanism of action of RB will impact the understanding of many, important cell processes.

Molecular mechanisms in the evolution of chronic myelocytic leukemia

Chronic myelocytic or Ph1-positive acute lymphoblastic leukemias have been analyzed for alterations in a variety of proto-oncogenes and anti-oncogenes implicated in the progression of chronic myeloid leukemia (CML) from its chronic phase to blast crisis. The most frequent genetic change found in disease evolution is an alteration of the p53 gene involving a point mutation, a rearrangement or a deletion. These gene changes are common in myeloid and undifferentiated variants of blast crisis but are usually undetectable in lymphoid leukemic transformants. Other molecular changes also occur in the clonal evolution of CML. The retinoblastoma-susceptibility (Rb) gene is an anti-oncogene. Structural abnormalities of Rb are frequent in all types of human acute leukemia, but are particularly common in Ph1-positive leukemia of lymphoid phenotype including both Ph1-positive ALL and lymphoid blast crisis of CML. Changes in Rb occur early in the transition to blast crisis with loss of Rb protein being the common factor. Mutations in the N-RAS gene also occur, but are rare in typical blast crisis. They are sometimes seen in Ph1-negative myeloid blast crisis. Since changes in the p53 gene are generally associated with progression of disease of a myeloid phenotype and changes in the Rb gene occur more often with a lymphoid phenotype, a particular molecular alteration may influence the character of disease evolution in CML.

Detection of small RB1 gene deletions in retinoblastoma by multiplex PCR and high-resolution gel electrophoresis

Loss of function of both copies of the RB1 gene is a causal event in the development of retinoblastoma. The predisposition to this tumor can be inherited as an autosomal dominant trait. Direct detection of the genetic defect is important for presymptomatic DNA diagnosis and genetic counseling in families with hereditary retinoblastoma. We have used multiplex polymerase chain reaction and high-resolution polyacrylamide gel electrophoresis to detect RB1 gene deletions as small as one base pair. By using three independent sets of amplification reactions, which cover 26% of the RB1 gene coding region, we identified RB1 gene deletions in the DNA of peripheral blood cells in 3 out of 24 (12.5%) unrelated patients with hereditary retinoblastoma. In one case, formalin-fixed paraffin-embedded tumor material was also used to detect the mutation. Sequencing of the mutated alleles revealed deletions of 1, 3 and 10 base pairs. Each deleted region was flanked by direct repeats.

Studies of the RB1 gene and the p53 gene in human osteosarcomas

We analyzed 14 native osteosarcoma tissue samples for alterations of the tumor suppressor genes RB1 and p53 on the DNA level, and as far as possible, the RNA level. Southern blot analyses concerning both tumor suppressor genes were carried out in all osteosarcomas. In two cases we could demonstrate a deletion within the RB1 gene. DNA analysis of a third osteosarcoma patient revealed a rearrangement of the p53 gene. We had the opportunity of performing corresponding northern analyses in eight native osteosarcoma specimens. The RB1 gene expression was significantly decreased or completely absent in six tumor samples. In two of these tissue probes the expression of both tumor suppressor genes was missing. We determined coexistence of decreased expression of both tumor suppressor genes in one additional case. In summary, 7/14 or 6/8 cases of osteosarcomas (including only those cases which allowed both analyses) showed RB1 gene alteration. In 3/14 or 3/8 osteosarcomas we could determine p53 gene abnormalities. This may indicate that either loss of p53 function is etiologically important only for the development of some osteosarcomas, or a major part of p53 gene mutations are subtle ones and their detection requires more sophisticated techniques, which are currently under development.

Malignant transformation of NIH 3T3 fibroblasts by human c-sis is dependent upon the level of oncogene expression

High-level expression of the c-sis oncogene, which encodes the beta chain of platelet-derived growth factor, transforms immortalized rodent fibroblasts in vitro to a malignant phenotype. c-sis gene expression has been demonstrated in a variety of human tumors, although generally at levels much lower than those shown to transform cells in vitro. We examined the effect of lower levels of c-sis expression on the phenotype of NIH 3T3 fibroblasts. Clones with various levels of c-sis expression were generated by transfecting NIH 3T3 cells with a plasmid that expressed the human c-sis cDNA and the TN5 neomycin-resistance gene. G418-resistant clones, which expressed the c-sis cDNA, were selected and characterized. Alterations in the phenotype of the clones that expressed c-sis ranged from increased growth in soft agar to malignant tumor formation in nude and syngeneic mice. Increased levels of c-sis cDNA expression correlated with the acquisition of features of transformation in a dose-dependent manner and altered the cellular phenotype in a manner consistent with the progression of cells towards malignancy. These data support a model in which low levels of sis gene expression in tumors contribute to the acquisition of some features of transformation but require complementation by other genes or factors to produce a fully malignant phenotype.

Inactivation of the retinoblastoma susceptibility gene in a human high grade non-Hodgkin's lymphoma cell line

We studied the expression of the retinoblastoma (RB) gene product (p105) in a B-cell line established from a patient with non-Hodgkin's lymphoma (large cell type). The karyotype of this cell line, named Ri-1, showed amongst other changes an apparent deletion of one chromosome 13 on band q14. No p105 could be detected by immunoprecipitation analysis and Western blotting in Ri-1 cells. Northern blotting revealed that RB mRNA is not expressed in Ri-1. Southern blotting confirmed the loss of one RB allele but showed a normal gross structure of the remaining allele. This suggests that the inactivation of the RB gene in Ri-1 cells is due to deletion of one allele and point mutations or small deletions in the other, as is often the case in retinoblastomas. Our findings imply that inactivation of the RB gene may play a role in the pathogenesis of high grade malignant lymphomas and that studies of RB in primary lymphoma samples would be of interest.

Molecular detection of constitutional deletions in patients with retinoblastoma

The recent cloning of the retinoblastoma (RB) gene as well as the identification of intragenic polymorphisms afford the necessary tools for the analysis of rearrangements using molecular hybridization. We searched for constitutional deletions by Southern blotting in 67 independent patients with normal karyotype comprising 15 familial and 52 sporadic cases. Among the latter, 33 were bilaterally and 19 unilaterally affected. We detected 6 deletions using cDNA probes covering almost all of the RB gene, as well as a genomic probe of the 5' part of the gene. With this approach, the incidence of detectable deletions was around 10%. No hot spots for deletion breakpoints were found. Asymptomatic carriers were detected in 2 families. The effectiveness of genetic counselling was largely improved by this approach.

Characterization of deletions at the retinoblastoma locus in patients with bilateral retinoblastoma

DNA samples from 92 unrelated patients with bilateral retinoblastoma were analyzed by Southern blot hybridization with cDNA and genomic clones of the retinoblastoma (RB-1) gene. Qualitative and quantitative evaluation of the Southern blot patterns showed a deletion of all or part of the RB-1 gene in 15 patients. Deletion hot spots were not detected. The study shows that 16% of germ cell mutations are detectable by Southern blot hybridization, but that densitometric analysis is required in most cases.

The genetics of retinoblastoma. Relevance to the patient

The understanding of the molecular biology of human cancer has advanced rapidly in the last decade, in part due to discoveries in the rare, pediatric ocular tumor, retinoblastoma. RB studies have led to recognition of a class of human genes, the tumor suppressor genes, that are critical in the initiation and progression of the malignant process. Mutations in the RB1 gene initiate RB and other specific tumors. They may also contribute to progressive stages of many other malignancies. The protein product of RB1 (p110RB1) is a basic regulator of the cell cycle. In the absence of normal protein, the cell proceeds to the next cell division without the potential to become quiescent. Understanding the genetics of RB has benefited the patients, as the precise identification of the RB1 mutations in families has led to accurate prediction of individuals at risk for RB tumors. It seems unlikely, in the foreseeable future, that direct genetic manipulation of mutant RB1 genes will play a role in therapy, but complete understanding of the function of p110RB1 may eventually allow exploitation of its powerful antiproliferative effect. Other molecular genetic events in addition to RB1 mutations are documented in RB tumors, and may play a critical role in the full malignant phenotype. The oncogene, N-myc, is amplified in some RB tumors and is expressed in normal fetal retina. The cytogenetic abnormality, i(6p), is almost unique to RB tumors. The molecular and tissue-specific roles of these abnormalities are not yet known. Many RB tumors also acquire excessive expression of the cell surface membrane glycoprotein, p170, linked to multidrug resistance, whether or not the RB tumor has been exposed to chemotherapy. We anticipate that ways to avoid or counteract the drug resistance of excessive p170 expression will be developed for other pediatric tumors and eventually will be applied to chemotherapy for RB patients.

A very large spontaneous deletion at aprt locus in CHO cells: sequence similarities with small aprt deletions

Spontaneous deletion mutants can be isolated from CHO cell lines heterozygous at the adenine phosphoribosyltransferase locus at frequencies up to 10(-4), i.e., about 100-fold higher than spontaneous point mutations. Indirect evidence has suggested that most deletions were in the megabase range. We have fully characterized one such mutant, Del 155, and shown that it resulted from an illegitimate recombination that took place between overlapping tetranucleotides. Comparisons with sequences of other deletions at various loci revealed a number of similarities, most striking of which was a CHI-like motif found within 6 bp of the upstream breakpoint of Del 155 and breakpoints of 8/21 previously described short deletions at the CHO aprt locus. Homology also existed between the downstream breakpoint of Del 155 and breakpoints of retinoblastoma gene deletions (3/6 cases) and also a 20-bp stretch of an Alu sequence in which breakpoints at the low-density lipoprotein receptor locus have been shown to cluster. The magnitude of the deletion event in Del 155 was assessed by pulsed field (PF) gel analysis and found to be at least 2100 kb long. PF analysis also indicated that the downstream breakpoint was near a region of structural differences between the two chromosomes carrying apart. These structural differences were probably not implicated in the mechanism of the high frequency event, since no indication of breakpoint clustering among a large collection of mutants was found either by conventional or PF electrophoretic analyses.

The candidate Wilms' tumour gene is involved in genitourinary development

Wilms' tumour is an embryonic kidney tumour thought to arise through aberrant mesenchymal stem cell differentiation and to result from loss of function of a 'tumour suppressor' gene(s). Both sporadic and syndrome-associated Wilms' tumours are accompanied by an increased frequency of abnormalities of the urinary tract and genitalia. Deletional analysis of individuals with the WAGR syndrome (for, Wilms' tumour, aniridia, genitourinary abnormalities and mental retardation) showed that a Wilms' tumour gene lies at chromosomal position 11p13. This led to the isolation of a candidate Wilms' tumour gene, encoding a zinc-finger protein which is likely to be a transcription factor. To gain insight into the role of this candidate gene in normal development and tumorigenesis, we have now performed in situ messenger RNA hybridization on sections of human embryos and Wilms' tumours. The candidate Wilms' tumour gene is expressed specifically in the condensed mesenchyme, renal vesicle and glomerular epithelium of the developing kidney, in the related mesonephric glomeruli and in cells approximating these structures in tumours. The other main sites of expression are the genital ridge, fetal gonad and mesothelium. These data suggest that (1) this candidate is indeed a Wilms' tumour gene, (2) the associated genital abnormalities are pleiotropic effects of mutation in the Wilms' tumour gene itself, in support of recent genetic analysis, and (3) this gene has a specific role in kidney development and a wider role in mesenchymal-epithelial transitions.

An internal deletion within an 11p13 zinc finger gene contributes to the development of Wilms' tumor

We have recently described the isolation of a candidate for the Wilms' tumor susceptibility gene mapping to band p13 of human chromosome 11. This gene, primarily expressed in fetal kidney, appears to encode a DNA binding protein. We now describe a sporadic, unilateral Wilms' tumor in which one allele of this gene contains a 25 bp deletion spanning an exon-intron junction and leading to aberrant mRNA splicing and loss of one of the four zinc finger consensus domains in the protein. The mutation is absent in the affected individual's germline, consistent with the somatic inactivation of a tumor suppressor gene. In addition to this intragenic deletion affecting one allele, loss of heterozygosity at loci along the entire chromosome 11 points to an earlier chromosomal nondisjunction and reduplication. We conclude that inactivation of this gene, which we call WT1, is part of a series of events leading to the development of Wilms' tumor.

Somatic mosaicism in a patient with bilateral retinoblastoma

We describe two cell lines with different deletions of the retinoblastoma gene in a patient with bilateral retinoblastoma. This patient has transmitted the mutation less frequent in his lymphocytes to two affected children. We cloned, mapped, and sequenced the junction fragments of the two deletions and found that they share one breakpoint but extend into opposite directions. An insertion of 4 bp of unknown origin is present between the breakpoints in one of the deletions. The second deletion shows a more complex rearrangement, including an inversion at the 5' end. Short regions of homology were found at the breakpoints and flanking the inversion. These results support the notion that bilateral retinoblastoma may not only be due to a germ-line mutation but also to a postzygotic mutation leading to somatic mosaicism.

Homozygous deletion in Wilms tumours of a zinc-finger gene identified by chromosome jumping

Cytogenetic analysis has identified chromosome 11p13 as the smallest overlap region for deletions found in individuals with WAGR syndrome, which includes Wilms tumour (a recessive childhood nephroblastoma), aniridia, genito-urinary abnormalities and mental retardation. The underlying loci have since been resolved into an aniridia (AN2) locus at a telomeric position, and a locus of closely spaced genes or a single pleiotropic gene involved in genito-urinary tract abnormalities and Wilms tumour at a more centromeric position. Pulsed-field gel analysis of the 11p13 region has revealed the presence of several putative CpG islands, structures which are frequently associated with the 5' ends of expressed sequences, mainly housekeeping genes and some tissue-specific genes. Starting from a CpG island, we have now isolated four neighbouring CpG islands, all within 650 kilobases (kb), by means of two consecutive bidirectional jumps in rare-cutting restriction-enzyme jumping libraries. In two instances, flanking sequences were conserved in other species and RNA transcripts were identified. A complementary DNA clone isolated for one of them derives from an RNA highly expressed in fetal kidney, and is predicted to encode a Krüppel-like zinc-finger protein that is probably a transcription factor. The entire cDNA region is included in two partially overlapping homozygous deletions found in Wilms tumour DNA samples. Cloning of the breakpoints in one tumour revealed a deletion size of 170 kb, one-third of which is covered by the cDNA. The expression pattern and sequence of this cDNA could point to an important role for its corresponding gene in the normal development of the renal system as well as in Wilms tumour.

A physical map around the retinoblastoma gene

The gene predisposing to retinoblastoma, RB1, has been mapped to 13q14 and a cDNA clone has been isolated. Alterations of this chromosomal region are found not only in retinoblastoma, but in other tumor types including bone and soft tissue sarcomas, gastric tumors, small cell lung cancer, hematologic malignancies, rhabdomyosarcoma, and breast cancer. Genetic alterations implicating RB1 in some of these cancers have been observed. A long-range, overlapping restriction map around RB1 has been derived to provide a basis for study of rearrangements in tumors. Putative CpG islands closely linked to RB1 were identified, the effect of methylation was investigated, and RB1 transcriptional direction was determined. Using data in the literature, the map was oriented with respect to the centromere and it was determined that the distance between esterase D, a nearby gene, and RB1 was greater than 200 kb.

Genetic and cellular basis of multistep carcinogenesis

Experimental evidence indicates that cancer development is a multistep process, and that multiple genetic changes are required before a normal cell becomes fully neoplastic. These genetic changes involve oncogenes, tumor suppressor genes, and possibly senescence genes. From studies in vivo using several different animal models, the stages are broadly defined as initiation, progression, and clearly involve both genetic and epigenetic events. Studies in vitro using cell culture systems have allowed the multistep process to be dissected in greater detail at both the cellular and molecular genetic level. In the Syrian hamster embryo cell culture model, neoplastic progression requires four heritable changes, involving activation of two oncogenes and loss of two tumor suppressor genes. Like the experimental systems, a limited number of studies of human tumors suggest that the multistep paradigm is also applicable, and that similar genetic events are involved in the development of cancer in humans.

Oncogenic point mutations in the human retinoblastoma gene: their application to genetic counseling

Mutations of the retinoblastoma gene, most of which cannot be detected by conventional Southern blotting, are known to cause both the nonhereditary and hereditary forms of retinoblastoma and have been implicated in the development of other cancers. Nonhereditary retinoblastoma is caused by a somatic mutation. Hereditary retinoblastoma is caused by a germ-cell mutation, most often a new one, and thus there is usually no family history of the disease. Unlike patients with the nonhereditary disease, those with the hereditary form are at risk for additional retinoblastomas, and their progeny are at risk for the tumors. We used a sensitive technique of primer-directed enzymatic amplification, followed by DNA sequence analysis, to identify mutations as small as a single nucleotide change in tumors from seven patients with simplex retinoblastoma (with no family history of the disease). In four patients the mutation involved only the tumor cells, and in three it involved normal somatic cells as well as tumor cells but was not found in either parent; thus, these mutations appeared to be new, germ-cell mutations. In addition, we found point mutations in cells from a bladder carcinoma, a small-cell carcinoma of the lung, and another retinoblastoma. We conclude that the technique that we have described can distinguish hereditary from nonhereditary retinoblastoma and that it is useful in risk estimation and genetic counseling.

Cancer genes by illegitimate recombination

Retroviral onc genes are as yet the only proven cancer genes. They are generated by rare illegitimate recombinations between retroviruses and cellular genes, termed proto-onc genes. The claims that these proto-onc genes cause virus-free cancers upon "activation" by mechanisms that do not alter their germline structure are challenged. Instead, it is proposed that retroviral onc genes and cellular cancer genes are generated de novo by illegitimate recombinations that alter the germline structure of normal genes.

Preferential germline mutation of the paternal allele in retinoblastoma

The event triggering malignant proliferation in 70% of retinoblastoma tumours is loss of heterozygosity for chromosome 13q14, whereby the normal retinoblastoma gene (RB1) allele is lost and an already mutated RB1 allele remains in the tumour. The first allele suffers a mutational event--deletion, duplication or point mutation (manuscript in preparation)--either in the germ line (all bilateral patients) or in a somatic retinal cell (most unilateral patients). Most bilateral patients have no family history of retinoblastoma and are presumed to have new germline mutations which arose in the egg, sperm or early embryo. We have determined the parental origin of the retained allele in nine retinoblastoma tumours from eight unrelated non-familial cases by using RB1-linked genetic markers. Six tumours retained the paternal allele and three retained the maternal allele. Of the three unilateral tumours, only one retained the paternal RB1 allele. Thus, there is no evidence that the paternal RB1 allele is preferentially retained in retinoblastoma, as has been suggested to be the case in osteosarcoma. By contrast, tumours from four of the five bilateral patients retained the paternal RB1 allele. This suggests either that new germline RB1 mutations arise more frequently during spermatogenesis than during oogenesis, or that imprinting in the early embryo affects chromosomal susceptibility to mutation.

Oncogene expression in small-cell lung cancer cell lines and a testicular germ-cell tumor: activation of the N-myc gene and decreased RB mRNA

N-myc and L-myc proto-oncogenes are expressed in many developing embryonic tissues. In contrast to expression of the closely related c-myc gene, N-myc and L-myc expression is very restricted in adult tissues. We show that small amounts of the L-myc RNA can be detected in normal adult testicular tissue. A high level of N-myc expression from a single-copy N-myc gene was found in a malignant tumor of the testis, histopathologically defined as a seminoma. This tumor also showed a decrease of mRNA from the retinoblastoma gene (RB), which is ubiquitously expressed in all normal tissues. Strikingly similar results on elevated expression of the L-myc and N-myc genes and a lack of RB mRNA have been reported in small-cell lung cancer cell lines, and are confirmed in our study.

The murine retinoblastoma homolog maps to chromosome 14 near Es-10

Restriction fragment length variants have been exploited to map genetically Rb-1, the murine homolog of the human retinoblastoma gene. Rb-1 localized to mouse chromosome 14 on the basis of results from analysis of somatic cell hybrids. In an interspecific backcross involving Mus spretus, Rb-1 and the murine homolog of the human esterase D gene (ESD), which we refer to here as Esd, were inseparable. Furthermore, the strain distribution patterns of Rb-1 and Es-10 are the same in 31 of 32 recombinant inbred strains. Close linkage of the chromosome 14 morphological marker hairless (hr) to Rb-1 is also implied. These results localize Rb-1 on the mouse linkage map and provide close genetic markers to follow Rb-1 in somatic as well as in germline genetic experiments. Additionally, the results suggest that Es-10 is the murine homolog of ESD and provide further evidence for linkage conservation during mammalian evolution.

One hundred years of retinoblastoma research. From the clinic to the gene and back again

Retinoblastoma (RB) has provided the prototype for the study of hereditary predisposition to cancer. An intraocular tumour of young children, it has both hereditary and sporadic forms. The familial form of RB is inherited as an autosomal dominant with high penetrance. In a small group of patients the identification of a constitutional deletion on the long arm of chromosome 13 indicated the location of the critical gene in region 13q14 Close linkage between the hereditary, non-deletion form of RB and the esterase-D gene, which is also located in 13q14, demonstrated that all hereditary forms of RB are due to defects in a gene at this locus. Analysis of the development of homozygosity for region 13q14 in sporadic tumours subsequently confirmed that probably all RB tumors are due to mutations in a single gene, RB1. Using molecular biological techniques a candidate gene has been isolated which maps to region 13q14 and which shows structural re-arrangements within tumour cells. Predisposing, hereditary mutations have also been shown to involve this gene thereby providing strong evidence for its authenticity. The isolation of DNA sequences from within this gene, which identify restriction fragment length polymorphisms, means that it is now possible to use them to carry out pre-natal diagnosis and identify individuals at high risk to tumour development.

Physical mapping of the von Recklinghausen neurofibromatosis region on chromosome 17

The von Recklinghausen neurofibromatosis (NF1) locus has been linked to chromosome 17, and recent linkage analyses place the gene on the proximal long arm. NF1 probably resides in 17q11.2, since two unrelated NF1 patients have been identified who possess constitutional reciprocal translocations involving 17q11.2 with chromosomes 1 and 22. We have used a somatic-cell hybrid from the t(17;22) individual, along with other hybrid cell lines, to order probes around the NF1 locus. An additional probe, 17L1, has been isolated from a NotI linking library made from flow-sorted chromosome 17 material and has been mapped to a region immediately proximal to the translocation breakpoint. While neither NF1 translocation breakpoint has yet been identified by pulse-field gel analysis, an overlap between two probes, EW206 and EW207, has been detected. Furthermore, we have identified the breakpoint in a non-NF1 translocation, SP-3, on the proximal side of the NF1 locus. This breakpoint has been helpful in creating a 1,000-kb pulsed-field map, which includes the closely linked NF1 probes HHH202 and TH17.19. The combined somatic-cell hybrid and pulsed-field gel analysis we report here favors the probe order D17Z1-HHH202-TH17.19-CRYB1-17L1-NF1- (EW206, EW207, EW203, L581, L946)-(ERBB2, ERBA1). The agreement in probe ordering between linkage analysis and physical mapping is excellent, and the availability of translocation breakpoints in NF1 should now greatly assist the cloning of this locus.