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

Age at diagnosis to discriminate those patients for whom constitutional DNA sequencing is appropriate in sporadic unilateral retinoblastoma

RB1 gene constitutional mutations were studied using 'exon-by-exon' sequencing in a series of 17 patients with sporadic unilateral retinoblastomas. Constitutional de novo germline mutations were detected in 4 patients. The age at diagnosis of retinoblastoma in all these cases was lower (mean 10.8 months; range 5-18) than in cases in which constitutional mutations were not found (mean 31.7 months; range 19-42). These results strongly indicate that age at retinoblastoma diagnosis may be a major factor for discriminating patients for whom a search for RB1 gene constitutional mutations could be justifiable in sporadic unilateral retinoblastomas.

Molecular analysis of the retinoblastoma (RB1) gene in acute myeloid leukemia patients

The pathogenesis of acute leukemia is still poorly understood. In the past few years several groups have reported deletion of the RB1 gene or altered pRB expression in certain hematologic malignancies, suggesting a possible role of RB1 gene inactivation in the process of leukemogenesis. Most studies regarding structural abnormalities of the RB1 gene indicate that gross deletions or rearrangements are present in a small percentage of patients with acute myeloid leukemia (AML), as is the case with retinoblastoma, where the majority of RB1 gene abnormalities are attributed to point mutations. To investigate if such point mutations in the RB1 gene may have a role in leukemogenesis in AML, we screened the RB1 gene of 36 AML patients using conformation-sensitive gel electrophoresis (CSGE). No point mutations were found in the 27 exons, their flanking intron regions or in the promoter region in any of the 36 patients. Thus, according to our findings, the susceptibility in these patients for developing AML does not appear to be related to point mutations in the RB1 gene. While screening for point mutations, we identified a number of new and previously noted neutral sequence variations indicating the efficiency and sensitivity of CSGE in identifying small changes in the RB1 gene.

Trilateral retinoblastoma: insights into histogenesis and management

Trilateral retinoblastoma (TRb) is a syndrome involving midline intracranial malignancies in children with the heritable form of retinoblastoma. All cases of TRb reported from 1971 to 1997 were reviewed. The histopathologic findings, clinical features, treatment modalities, and survival rates from 80 cases were evaluated. Histopathologic findings from intracranial malignancies demonstrated primitive neuroectodermal tumors in 61.5% of cases. Various degrees of neuronal or photoreceptor differentiation were seen in the other 38.5% of cases. Autopsy, histopathologic, and radiologic examinations did not show a more definitive site of origin of these intracranial tumors, although "pinealoblastoma" was often the diagnosis reported. These findings, together with analysis of the histopathologic similarities among human primitive neuroectodermal tumors, pinealoblastoma, retinoblastoma, and ependymoblastoma, suggest that TRb more likely arises from a germinal layer of predisposed primitive subependymal neuroblasts that are not necessarily destined for pineal or photoreceptor differentiation. Trilateral tumors have also been found in transgenic mice expressing the simian virus 40 T-antigen. Transgenic murine intracranial tumors are primitive neuroectodermal tumors arising from the subependymal layer. Transgenic mice with the murine interphotoreceptor cell binding protein promoter and simian virus 40 T-antigen also develop pineal tumors. Trilateral retinoblastoma is usually fatal, with an average survival time of 11.2 months. Therapies include radiation, systemic chemotherapy, intrathecal chemotherapy, and surgical resection/craniotomy in combination with radiation and/or chemotherapy. Survival may be prolonged with combination chemotherapy (24.6 months) and if neuroradiologic screening identifies TRb before symptoms are present (23.5 months). Recent success with platinum-based chemoreduction of intraocular retinoblastoma may indicate a similar role for platinum-based chemotherapy in the treatment of TRb. Routine central nervous system imaging should be considered in the management of TRb.

Nitric oxide-induced deamination of cytosine and guanine in deoxynucleosides and oligonucleotides

The autoxidation of nitric oxide (NO.) forms the nitrosating agent N2O3, which can directly damage DNA by deamination of DNA bases following nitrosation of their primary amine functionalities. Within the G:C base pair, deamination results in the formation of xanthine and uracil, respectively. To determine the effect of DNA structure on the deamination of guanine and cytosine, the NO.-induced deamination rate constants for deoxynucleosides, single- and double-stranded oligonucleotides, and a G-quartet oligonucleotide were measured. Deamination rate constants were determined relative to morpholine using a Silastic membrane to deliver NO. at a rate of approximately 10-20 nmol/ml/min for 60 min, yielding a final concentration of approximately 600-1200 microM NO2-. GC/MS analysis revealed formation of nanomolar levels of deamination products from millimolar concentrations of deoxynucleosides and oligomers. Deamination rate constants for cytosine and guanine in all types of DNA were lower than the morpholine nitrosation rate constant by a factor of approximately 10(3)-10(4). Xanthine was formed at twice the rate of uracil, and this may have important consequences for mechanisms of NO.-induced mutations. Single-stranded oligomers were 5 times more reactive than deoxynucleosides toward N2O3. Double-stranded oligomers were 10-fold less reactive than single-stranded oligomers, suggesting that Watson-Crick base pairing protects DNA from deamination. G-quartet structures were also protective, presumably because of hydrogen bonding. These results demonstrate that DNA structure is an important factor in determining the reactivity of DNA bases with NO.-derived species.

Frequency of somatic and germ-line mosaicism in retinoblastoma: implications for genetic counseling

Although mosaicism can have important implications for genetic counseling of families with hereditary disorders, information regarding the incidence of mosaicism is available for only a few genetic diseases. Here we describe an evaluation of 156 families with retinoblastoma; the initial oncogenic mutation in the retinoblastoma gene had been identified in these families. In 15 ( approximately 10%) families, we were able to document mosaicism for the initial mutation in the retinoblastoma gene, either in the proband or in one of the proband's parents. The true incidence of mosaicism in this group of 156 families is probably higher than our findings indicate; in some additional families beyond the 15 we identified, mosaicism was likely but could not be proven, because somatic or germ-line DNA from key family members was unavailable. Germ-line DNA from two mosaic fathers was analyzed: in one of these, the mutation was detected in both sperm and leukocyte DNA; in the other, the mutation was detected only in sperm DNA. Our data suggest that mosaicism is more common than is generally appreciated, especially in disorders such as retinoblastoma, in which a high proportion of cases represent new mutations. The possibility of mosaicism should always be considered during the genetic counseling of newly identified families with retinoblastoma. As demonstrated here, genetic tests of germ-line DNA can provide valuable information that is not available through analysis of somatic (leukocyte) DNA.

Structure of the retinoblastoma tumour-suppressor pocket domain bound to a peptide from HPV E7

The pocket domain of the retinoblastoma (Rb) tumour suppressor is central to Rb function, and is frequently inactivated by the binding of the human papilloma virus E7 oncoprotein in cervical cancer. The crystal structure of the Rb pocket bound to a nine-residue E7 peptide containing the LxCxE motif, shared by other Rb-binding viral and cellular proteins, shows that the LxCxE peptide binds a highly conserved groove on the B-box portion of the pocket; the A-box portion appears to be required for the stable folding of the B box. Also highly conserved is the extensive A-B interface, suggesting that it may be an additional protein-binding site. The A and B boxes each contain the cyclin-fold structural motif, with the LxCxE-binding site on the B-box cyclin fold being similar to a Cdk2-binding site of cyclin A and to a TBP-binding site of TFIIB.

Mechanism of retinoblastoma gene inactivation in the spectrum of neuroendocrine lung tumors

The retinoblastoma (RB) gene plays a key role in cell cycle control by regulation of G1 growth arrest. This gene is inactivated in some human cancers and in most small-cell lung carcinoma (SCLC) cell lines. The aim of this study was to analyze the mechanisms of RB silencing in freshly excised neuroendocrine (NE) tumors embracing the entire spectrum of NE lung neoplasms (typical and atypical carcinoids, large-cell neuroendocrine carcinomas [LCNECs], and SCLCs). To study the role and mechanism of RB inactivation in tumor differentiation and malignant potential, the status of the Rb protein was analyzed in 37 NE lung tumors, using immunohistochemistry with five Rb antibodies. Loss or altered expression of Rb protein was more frequently observed in high-grade NE lung carcinoma (23 of 28, 82%) than in typical and atypical carcinoids (1 of 9, 11%) (P < 0.001). Of 24 tumors with abnormal Rb staining, Southern blotting showed 1 to have undergone rearrangement, SSCP (single-strand conformation polymorphism) and sequencing showed that 6 (25%) exhibited mutations in exons 13-18 or 20-24 of the RB gene, and RT-PCR (reverse transcriptase-polymerase chain reaction) revealed that 14 (58%) showed a low level of or entirely absent RB mRNA (messenger RNA) expression, whereas hypermethylation of the CpG-rich island at the 5' end of the RB gene was not observed. Abnormal Rb protein expression was always associated with one of these three alternative mechanisms in the SCLCs analyzed, but in only 50% of LCNECs. These results indicate that inactivation of the RB gene is highly frequent in freshly excised high-grade NE lung tumors through distinct mechanisms including point mutations and frequent abnormal mRNA expression. Different modes of RB inactivation seem to be implicated along the spectrum of NE lung carcinomas, depending on differentiation state, phenotype, and malignancy grade.

Stable binding to E2F is not required for the retinoblastoma protein to activate transcription, promote differentiation, and suppress tumor cell growth

The retinoblastoma tumor suppressor protein (pRB) can inhibit cell cycle progression and promote differentiation. pRB interacts with a variety of transcription factors, including members of the E2F and C-EBP protein families and MyoD, and can either repress or activate transcription depending on the promoter under study. These biological and biochemical activities of pRB have been mapped previously to a core domain, referred to as the pRB pocket. Using a panel of synthetic pRB pocket mutants, we found that the acute induction of a G1/S block by pRB is linked to its ability to both bind to E2F and to repress transcription. In contrast, these functions were not required for pRB to promote differentiation, which correlated with its ability to activate transcription in concert with fate-determining proteins such as MyoD. All tumor-derived pRB mutants tested to date failed to bind to E2F and did not repress transcription. Despite an inability to bind to E2F, pRB mutants associated with a low risk of retinoblastoma, unlike high-risk mutants, retained the ability to activate transcription and promote differentiation. Thus, the pRB pocket participates in dual tumor suppressor functions, one linked to cell cycle progression and the other to differentiation control, and these functions can be genetically and mechanistically dissociated.

Hypermethylation in the retinoblastoma gene is associated with unilateral, sporadic retinoblastoma

We previously reported 9 unilateral, sporadic retinoblastomas with hypermethylation in the 5' region of the RB gene, and we found that CpG methylation in the RB promoter inhibits the binding of the retinoblastoma binding factor 1 (RBF-1) and the activating transcription factor (ATF)-like factors, thereby resulting in a considerable reduction in RB promoter activity. In this study, we screened for hypermethylation in 121 additional cases of retinoblastoma, and found 5 tumors with hypermethylation, including 4 unilateral, sporadic tumors, and one hereditary tumor. The hereditary tumor had a germline deletion of one allele, and the hypermethylation was an acquired, epigenetic change in the other allele. Another tumor had hypermethylation restricted to approximately 800 base pairs in the RB promoter region including the essential RBF-1 and ATF sites. The frequency of hypermethylation in unilateral, sporadic tumors was 9.3% combining our previous and present examinations (13 among 140), whereas the frequency was 1.0% in bilateral hereditary tumors (one among 101). The statistical analyses using the chi-square test indicated significant correlation between hypermethylation and unilateral, sporadic tumors (p < 0.05). These results suggest that hypermethylation in the RB gene is always an acquired, epigenetic change and causes about 9% of unilateral, sporadic tumors.

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.

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.

Genetics of retinoblastoma: a study

We have analyzed 43 families with either familial retinoblastoma (RB) (four kindreds), bilateral sporadic RB (10 individuals), or unilateral sporadic RB (29 individuals). Genetic studies focused on karyotype analysis, loss of heterozygosity of intragenic polymorphisms, and search for point mutations. We have been able to identify the genetic defect underlying the disease in eight cases. Deletions have been found in three patients with sporadic RB, two bilateral in one of which karyotyping had previously detected an interstitial deletion of chromosome 13 affecting (q13-q31) and one unilateral. Five different point mutations were responsible for three cases of bilateral sporadic RB, one case of bilateral sporadic RB, and one case of bilateral familial RB. The low frequency of constitutional mutations found in our study has led us to review and evaluate the possibilities and limitations of the present genetic analyses on RB and to access the different factors influencing the detection of mutations causing the disease, because genetic counseling is mainly based on mutation identification.

Biphenotypic blast crisis of chronic myelogenous leukemia: abnormalities of p53 and retinoblastoma genes

The molecular mechanisms responsible for progression of chronic myelogenous leukemia (CML) to blast crisis have not been well defined. Blast crisis may be partially related to inactivation of tumor suppressor genes/such as p53 or retinoblastoma (Rb) gene. There is evidence for an association of blast cell phenotypes in CML with alterations of these genes: a strong association of myeloid phenotypes with abnormalities of the p53 gene and a weaker association of lymphoid phenotypes with abnormalities of the Rb system. We found a marked decrease in Rb gene product and rearrangements of the p53 gene simultaneously in two cases of biphenotypic blast crisis of CML (myeloid and B-lymphoid). These results support the association of blast cell phenotypes with alterations in tumor suppressor genes in CML blast crisis.

Influence of oxygen radical injury on DNA methylation

One of the most prevalent products of oxygen radical injury in DNA is 8-hydroxyguanosine. Cells must be able to withstand damage by oxygen radicals and possess specific repair mechanisms that correct this oxidative lesion. However, when these defenses are oversaturated, such as under conditions of high oxidative stress, or when repair is inefficient, the miscoding potential of this lesion can result in mutations in the mammalian genome. In addition to causing genetic changes, active oxygen species can lead to epigenetic alterations in DNA methylation, without changing the DNA base sequence. Such changes in DNA methylation patterns can strongly affect the regulation of expression of many genes. Although DNA methylation patterns have been found to be altered during carcinogenesis, little is known about the mechanism(s) that produce this loss of epigenetic controls of gene expression in tumors. Replacement of guanine with the oxygen radical adduct 8-hydroxyguanine profoundly alters methylation of adjacent cytosines, suggesting a role for oxidative injury in the formation of aberrant DNA methylation patterns during carcinogenesis. In this paper, we review both the genetic and epigenetic mechanisms of oxidative DNA damage and its association with the carcinogenic process, with special emphasis on the influence of free radical injury on DNA methylation.

Musculoskeletal oncology--advances in cytogenetics and molecular genetics and their clinical implications

Although musculoskeletal malignancies comprise a small group of cancers, a vast number of histological subtypes have been identified attesting to the heterogeneity of this class of tumours and the growing interest in their development. The mode of management for both bone and soft tissue sarcomas has been examined extensively and treatment guidelines have been proposed. Despite the intensive study and multidisciplinary treatment, a substantial proportion of tumours remain recalcitrant to therapy and recur locally and systemically. Improved methods of characterising these tumours may help in understanding their biology. Cytogenetic and molecular genetic techniques allow a subcellular dissection of these malignancies which may aid the identification of mechanisms that are important in tumorigenesis. Already candidate genes have been isolated which may play an important role in the deregulation of proliferation and or the adoption of a malignant phenotype, features which are fundamental in tumour development. By studying the molecular biology and cytogenetics of tumours it may be possible to improve diagnostic and prognostic accuracy thereby minimising over and under treatment.

The RB1 gene mutation in a child with ectopic intracranial retinoblastoma

The RB1 gene mutation was investigated in a child with ectopic intracranial retinoblastoma using DNA obtained from both the pineal and retinal tumours of the patient. A nonsense mutation in exon 17 (codon 556) of the RB1 gene was found to be present homozygously in both the retinal and the pineal tumours. The same mutation was present heterozygously in the DNA from the constitutional cells of the patient, proving it to be of germline origin. The initial mutation was shown to have occurred in the paternally derived RB1 allele. The mutation is in an area of the gene that encodes the protein-binding region known as the 'pocket' region and has been detected in other cases of retinoblastoma.

Molecular genetic and immunohistochemical analysis of the tumor suppressor genes Rb and p53 in palmar and aggressive fibromatosis

This pilot project analyzed the tumor suppressor genes p53 and Rb in 13 cases of aggressive fibromatoses and six cases of palmar fibromatoses (Dupuytren contracture). Immunohistochemistry, reverse transcription polymerase chain reaction, polymerase chain reaction followed by single-strand confirmation polymorphism analysis, and Southern blot to detect gene rearrangements were used. No abnormalities were detected in p53. The aggressive fibromatoses demonstrated a lack of Rb immunohistochemical staining and decreased mRNA for Rb. No structural mutation in the coding sequence of the Rb gene was detected. The decreased level of Rb gene expression, despite a normal coding sequence, may indicate increased proliferation and may suggest potential treatment schemes.

Constitutional nonsense germline mutations in the RB1 gene detected in patients with early onset unilateral retinoblastoma

The 'two-hit' hypothesis for the development of the childhood eye cancer, retinoblastoma (Rb), predicts that bilaterally affected individuals will carry germline mutations. The second suggestion is that patients with early presentation of unilateral tumours also carry predisposing mutations. We have used SSCP analysis to study the 27 individual exons of the RB1 gene in constitutional DNA from 3 patients whose tumours were treated under the age of 12 months. Bandshifts on SSCP gels were detected in 2 of these patients which, on sequencing, were shown to be a C-->T transition converting a CGAarg to a TGAstop codon in exon 17 and an 8 bp deletion in exon 20 resulting in a downstream stop codon. The mutations seen in these patients are reminiscent of those seen in patients with hereditary Rb and confirms that at least some early onset unilateral cases carry constitutional mutations, which has important implications for genetic screening and counselling of these individuals.

A maize cDNA encoding a member of the retinoblastoma protein family: involvement in endoreduplication

Retinoblastoma (RB-1) is a tumor suppressor gene that encodes a 105-kDa nuclear phosphoprotein. To date, RB genes have been isolated only from metazoans. We have isolated a cDNA from maize endosperm whose predicted protein product (ZmRb) shows homology to the "pocket" A and B domains of the Rb protein family. We found ZmRb behaves as a pocket protein based on its ability to specifically interact with oncoproteins encoded by DNA tumor viruses (E7, T-Ag, E1A). ZmRb can interact in vitro and in vivo with the replication-associated protein, RepA, encoded by the wheat dwarf virus. The maize Rb-related protein undergoes changes in level and phosphorylation state concomitant with endoreduplication, and it is phosphorylated in vitro by an S-phase kinase from endoreduplicating endosperm cells. Together, our results suggest that ZmRb is a representative of the pocket protein family and may play a role in cell cycle progression. Moreover, certain plant monopartite geminiviruses may operate similarly to mammalian DNA viruses, by targeting and inactivating the retinoblastoma protein, which otherwise induces G1 arrest.

Activity of the retinoblastoma family proteins, pRB, p107, and p130, during cellular proliferation and differentiation

Genetic evidence from retinoblastoma patients and experiments describing the mechanism of cellular transformation by the DNA tumor viruses have defined a central role for the retinoblastoma protein (pRB) family of tumor suppressors in the normal regulation of the eukaryotic cell cycle. These proteins, pRB, p107, and p130, act in a cell cycle-dependent manner to regulate the activity of a number of important cellular transcription factors, such as the E2F-family, which in turn regulate expression of genes whose products are important for cell cycle progression. In addition, inhibition of E2F activity by the pRB family proteins is required for cell cycle exit after terminal differentiation or nutrient depletion. The loss of functional pRB, due to mutation of both RB1 alleles, results in deregulated E2F activity and a predisposition to specific malignancies. Similarly, inactivation of the pRB family by the transforming proteins of the DNA tumor viruses overcomes cellular quiescence and prevents terminal differentiation by blocking the interaction of pRB, p107, and p130 with the E2F proteins, leading to cell cycle progression and, ultimately, cellular transformation. Together these two lines of evidence implicate the pRB family of negative cell cycle regulators and the E2F family of transcription factors as central components in the cell cycle machinery.

Retinoblastoma referral patterns in Saudi Arabia

The purpose of this study was to review the patterns of referral and presentation of patients with retinoblastoma in Saudi Arabia from 1983-1994. Retinoblastoma represents a particularly high volume at the King Khaled Eye Specialist Hospital and the Hospital's Retinoblastoma Registry provided relevant data. Records of 257 registered patients were analyzed. Of these, 112 were bilateral whereas 145 were unilateral, resulting in a total of 369 eyes. Referral diagnosis accuracy was high, 75% of referring physicians ranking retinoblastoma at the top of their differential diagnosis. Delay in referral was found to average 9.4 weeks. However, 49.4% of eyes were staged at Reese-Ellsworth Stage V at presentation. Significantly, the number of patients presenting with extraocular disease fell from 22.8% (1983-1988) to 12.3% (1989-1994), highlighting increasing awareness of retinoblastoma and availability of services in Saudi Arabia.

CDKN2A (p16INK4A) somatic and germline mutations

The cell cycle is composed of a series of steps that can be negatively or positively regulated by various factors. A group of low-molecular-weight proteins have recently been identified that specifically inhibit the function of cyclin-dependent kinases in mammalian cells. Inactivation of the CDKN2A gene (also known as p16INK4A and MTS1) attracted considerable interest after it was mapped to 9p21, a locus for familial melanoma. In an effort to standardize the information regarding human CDKN2A mutations detected in cancers, a database with information of 146 point mutations has been created. Cancer type, origin of cells, specific mutation, amino acid change, literature citation, and other data are provided for each mutation entry. Studies of biochemical and biological functions of both wild-type and mutant proteins are central to our understanding of the role of p16INK4a mutations in tumorigenesis, a summary of these studies is also included in the present update.

Retinoblastoma gene mutations in primary human prostate cancer

Structural alterations in the entire coding regions (exons 1 to 27) of the retinoblastoma (RB) gene in primary human prostate cancers were investigated, using polymerase chain reaction and single strand conformational polymorphism analysis of RNA. Of 25 samples obtained from patients, four (16.4%) were found to have RB alterations. DNA sequencing of the PCR products revealed point mutations resulting in single amino-acid substitutions of exons 6 and 19 in two cases, and base deletions of exons 8 and 17 in two cases. Two of four cases with RB mutations were moderately differentiated localized tumors and other two with RB mutations were poorly differentiated tumors with metastases. Our results suggest that RB gene mutation is involved in progression steps of prostate carcinogenesis.

Germline mutations in the RB1 gene in patients with hereditary retinoblastoma

We have analyzed the 27 exons and the promoter region of the RB1 gene in familial or sporadic bilateral retinoblastoma by using single-strand conformation polymorphism analysis. For improvement over previous studies, a new set of primers has been designed, which allow for amplification of the coding and splicing sequences only. The positioning of the polymerase chain reaction (PCR) primers was such that the resulting PCR products were of different sizes, which enabled us to analyze two different exons simultaneously and still distinguish between the banding profiles for both (biplex analysis). By using this approach, we were able to identify mutation in 22 new patients, but the overall efficiency of the procedure when we used a single-pass regimen was only 48%. The mutations were small insertions and deletions and point mutations in roughly equal proportions.

Genetic analysis of chromosome 13 deletions in BCR/ABL negative chronic myeloproliferative disorders

Chromosomal deletions of band 13q14 occur recurrently in BCR/ABL negative chronic myeloproliferative disorders (CMPD), including myelosclerosis with myeloid metaplasia (MMM), polycythemia vera (PV), essential thrombocythemia (ET), juvenile chronic myeloid leukemia (JCML), and the so-called BCR/ABL- chronic myeloid leukemia (CML). The RBI tumor suppressor locus, mapping to 13q14, has long since been hypothesized as the important gene. In this report, we have determined the frequency of 13q14 deletions at the molecular level in a large panel of BCR/ABL- CMPD at different disease stages and performed a detailed genetic analysis of gross rearrangements/deletions and point mutations of the RBI gene in these disorders. Our data show that molecular deletions of 13q14 are detected in a relatively large fraction of BCR/ABL- CMPD (38%), that they appear to be more frequent in MMM than in other BCR/ABL- CMPD, and that they may be present at diagnosis or occur during blastic evolution of the neoplasia. The RBI gene displayed a germline configuration in all BCR/ABL- CMPD tested, suggesting that 13q14 deletions in these disorders affect a tumor suppressor locus distinct from RBI.

The adenomatous polyposis coli gene and human cancers

The APC (adenomatous polyposis coli) gene was isolated as a gene responsible for familial polyposis coli, an autosomal-dominant disease, characterized by development of hundreds to thousands of adenomatous polyps in the colon and rectum. However, recent studies revealed that inactivation of the APC gene also plays a significant role in development of sporadic forms of colorectal adenoma and carcinoma. Furthermore, somatic mutations have also been detected in pancreatic carcinomas as well as some type of gastric carcinomas, suggesting that APC has a critical function in regulation of cell growth in digestive tissues.

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.

The retinoblastoma gene (RB-1) status in multiple myeloma: a report on 35 cases

We looked for abnormalities of the retinoblastoma (RB-1) gene and of RB protein expression in 35 patients with multiple myeloma (MM). Mutations in exons 20 to 24 of the RB-1 gene (exons where mutations predominate in retinoblastoma and other solid tumors) were analyzed by single stranded conformation polymorphism (SSCP). RB-1 protein was studied in bone marrow plasma cells by immunocytochemistry (ABC peroxidase technique) with a specific monoclonal antibody. Southern blot analysis of RB-1 gene was also performed in 20 of the patients. Twenty two patients analyzed had advanced disease (stage III or, in one case, plasma cell leukemia) and cytogenetic analysis (performed in 31 cases) found monosomy 13 in 9 patients. No rearrangement of the RB-1 gene was found by Southern analysis. Absent or greatly reduced RB-1 protein level was found in plasma cells in 4 of the patients (11%), whereas normal levels were seen in the remaining cases. No point mutation in exons 20 to 24 and their flanking introns were found in any of the 35 patients. Three of the 4 patients with absent or reduced RB-1 protein expression had advanced MM (stage III: 2 cases; plasma cell leukemia: 1 case); all 4 patients were resistant to treatment (as compared to 7 of the 31 patients with normal RB-1 protein levels); only one of them was subsequently found to have monosomy 13 (as compared to 9 of the 28 other karyotyped patients). Our findings suggest that abnormalities of the RB-1 gene and its expression are rare in MM. Absent or reduced expression of RB-1 protein was not significantly correlated to monosomy 13 and was not associated with gross rearrangements of the RB-1 gene by Southern analysis or point mutations in exons 20 to 24 of the gene. Reduced expression of RB-1 protein may be associated with advanced disease and poor response to treatment, although larger numbers of patients will be required for more adequate conclusions.

Chromosome evolution in retinoblastoma

An attempt to reconstruct the chromosome evolution in 19 primary retinoblastoma tumors was made, taking into consideration the frequencies of the rearrangements for each case. The rearrangements were classified as early or late according to their frequency. Early chromosome rearrangements were +1q, +6p, -13/del(13q), -16/del(16q), -17/del(17p), and late rearrangements were (in decreasing order) -8, -17/del(17p), -22, +3/+3q, -4, -19, +1q, +7/+7q, -14, +21. Chromosome evolution was reconstructed by the analysis of parsimony using programs of phylogenetic inference, which belong to the computer package PHYLIP. Results of the reconstruction of chromosome evolution suggest that there are two types of retinoblastoma tumors which can be distinguished based on chromosome imbalance, and correlate with the presence or absence of additional copies of the short arm (p) of chromosome 6. Tumors with additional 6p chromosome had marked heterogeneity, with a high frequency of early as well as late chromosome changes. By contrast, tumors with a normal 6p complement had the fewest chromosome changes and, in particular, a lower frequency of late changes.

A germline substitution in the human MSH2 gene is associated with high-grade dysplasia and cancer in ulcerative colitis

BACKGROUND & AIMS

The DNA mismatch repair gene human MSH2 shows a germline mutation in certain family members with hereditary nonpolyposis colorectal cancer. There is an increased risk of colorectal cancer in patients with ulcerative colitis (UC) with extensive disease of > 8 years' duration; however, specific constitutional predisposing genetic abnormalities have not yet been identified.

METHODS

A germline human MSH2 abnormality was sought in patients with UC with high-grade dysplasia or carcinoma.

RESULTS

After direct sequencing of exon 13 and flanking regions of human MSH2, a germline T to C substitution was shown at the -6 intronic splice acceptor site of exon 13. This substitution was found in 14 of 53 patients with UC with high-grade dysplasia or carcinoma (26%) compared with 4 of 36 high-risk patients with UC without dysplasia or cancer (11%) (P < or = 0.04) and in 7 of 80 healthy adult blood donors (9%) (P < or = 0.003). The patients with UC who had the substitution were three times more likely to develop neoplasia than patients with UC who did not carry it.

CONCLUSIONS

An intronic splice-site substitution in the human MSH2 gene is present in the general population but may predispose to cancer in the setting of UC.

Identification of RB1 germline mutations in Argentinian families with sporadic bilateral retinoblastoma

Hereditary predisposition to retinoblastoma is caused by germline mutations in the RB1 gene. Most of these mutations occur de novo and differ from one patient to another. DNA samples from 10 families with a child presenting sporadic bilateral retinoblastoma have been analysed for the causative mutation. Using intragenic DNA polymorphisms we detected large deletions in two patients. Heteroduplex and DNA sequence analysis of PCR products from each exon and the promoter region showed small mutations in four patients: a C to T transition in exon 18; 1 bp and 2 bp deletion in exons 20 and 19 respectively; and a 4 bp insertion in exon 7. All these mutations are likely to result in premature termination of transcription. In one of these families, an unaffected carrier was detected. This emphasises the importance of detection of the causative mutation for predictive diagnosis in families with sporadic bilateral retinoblastoma.

Retinoblastoma gene mutations in primary human bladder cancer

Inactivation of the retinoblastoma (RB) gene is known to be implicated in the pathogenesis of several types of human cancers. Since structural alterations of the RB gene have not been well examined in human bladder cancer, we looked for mutations in the entire coding region of this gene using polymerase chain reaction (PCR) and single-strand conformational polymorphism analysis of RNA. We also examined allelic loss of the RB gene using PCR-based restriction fragment length polymorphism analysis. Of 30 samples obtained from patients with bladder cancer, eight (27%) were found to have RB gene mutations. DNA sequencing of the PCR products revealed five cases with single point mutations and three cases with small deletions. These mutations included one (10%) of ten low-grade (grade 1) tumours, four (50%) of eight intermediate-grade (grade 2) tumours and three (25%) of 12 high-grade (grade 3) tumours. Likewise, mutations were found in four (21%) of 19 superficial (pTa and pT1) tumours and four (36%) of 11 invasive (pT2 or greater) tumours. In 15 informative cases, loss of heterozygosity at the RB locus was shown in five cases (33%), three cases with RB mutations and two without them. These results suggest that RB gene mutations are involved in low-grade and superficial bladder cancers as well as in high-grade and invasive cancers.

Central neuronal tumors in childhood: relationship to dysplasia

A survey of 1,500 brain tumors at The Hospital for Sick Children in Toronto reveals that about 20-25% of tumors demonstrate some form of neuronal differentiation. At one end of the spectrum are the well-defined ganglionic tumors, sometimes difficult to differentiate from cortical dysplasia. At the other extreme are primitive neuroectodermal tumors with neuronal differentiation often confined to immunohistochemical observations. Of the total number of tumors, approximately 5% have a definitive ganglionic component, the majority being ganglioglioma, and others include dysembryoplastic neuroepithelial tumor, infantile ganglioglioma, paraganglioma, central neurocytoma, and gangliocytoma. Some tumors such as subependymal giant cell tumor associated with tuberous sclerosis occasionally have evidence of neuronal differentiation with immunoreactivity with antisera to neuron-specific enolase and negativity with antisera to GFAP. In children with epilepsy, improved brain imaging has identified lesions which on examination following temporal lobectomy show varying degrees of cortical dysplasia. At this site, there is also a high incidence of gangliogliomas. Is there a relationship between cortical dysplasia and neuronal tumors? Following a primary induction event during development, embryonal dysplasia and/or neoplasia may occur. The lesion may be malformative as in unilateral megalencephaly, hamartomatous as in tuberous sclerosis, neoplastic as in congenital tumors, or a combination of malformative and neoplastic.(ABSTRACT TRUNCATED AT 250 WORDS)

Loss of the retinoblastoma susceptibility gene (RB1) is a frequent and early event in prostatic tumorigenesis

Loss of the RB1 gene is an important event in the initiation and progression of many tumours. Prostate tissue from 43 patients with prostate cancers and ten with benign prostatic hypertrophy (BPH) were studied for loss of heterozygosity of the RB1 gene. Four intragenic polymorphic loci were studied with two techniques. These were restriction fragment length polymorphism (RFLP). Southern blotting and hybridisation with the p123m1.8 and p68RS2.0 probes (to introns 1 and 17 respectively) and also the polymerase chain reaction (PCR) to amplify loci within introns 17 and 20. Protein product (pRB) expression was determined by immunohistochemistry using the NCL-RB antibody in nine patients with cancer and four patients with BPH. Loss of heterozygosity was found in 24 out of 40 (60%) informative patients with cancer. Loss of RB1 occurred with a similar frequency in early-stage and low-grade cancers as in more advanced cancers. Loss of RB1 was also found in one patient with BPH. Expression of pRB was completely absent from seven cancers and markedly reduced in the other two, while nuclear pRB staining was always present in areas of BPH, whether alongside cancer-containing tissue or with BPH alone. We conclude that loss of RB1 is an early event in prostatic tumorigenesis.

Distinct RB1 gene mutations with low penetrance in hereditary retinoblastoma

The interfamilial diversity in penetrance and expressivity of hereditary retinoblastoma was investigated in 29 families. By using a simple parameter for estimating the severity of the disease (diseased-eye-ratio), we were able to identify four families with a discrete low-penetrance phenotype. The underlying genetic defect was identified in three families. One family has a 3-bp deletion in exon 16 that results in the deletion of Asn480. In two further unrelated families, the identical missense mutation at codon 661 in exon 20 (CGG to TGG, Arg to Trp) was identified. These mutations are distinct from the majority of retinoblastoma gene alterations, as they do not result in the disruption of the gene product. We propose that reduced penetrance of retinoblastoma is the result of a residual function of these alleles in retinoblastoma precursor cells.

Loss of heterozygosity at the retinoblastoma locus in human pituitary tumors

BACKGROUND

Recent studies using knockouts of the Retinoblastoma (Rb) gene by homologous recombination in transgenic mice have revealed that a high frequency of heterozygous animals develop pituitary tumors associated with loss of heterozygosity (LOH) at the Rb locus. The authors have determined the frequency of LOH at the Rb locus in 42 benign human pituitary tumors.

METHODS

Polymerase chain reaction- (PCR) amplification of polymorphic regions in introns 17 and 20 of the human Rb gene was used to detect heterozygosity in pituitary tumor DNA and matched control DNA samples.

RESULTS

The PCR assay was informative in 42 of 48 pituitary tumors examined, and no allelic deletion of Rb was detected in any of the tumors.

CONCLUSIONS

These studies confirmed a recent report that LOH at the Rb locus is rare in benign human pituitary tumors.

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.

Unbiased transmission of mutant alleles at the human retinoblastoma locus

The preferential transmission of the mutant allele to offspring from fathers who carry a germline mutation in the retinoblastoma gene was examined by analyzing 46 consecutive pedigrees. Among 75 offspring from 29 fathers, the ratio of carriers to noncarriers was 49%. Among the 106 offspring from 55 mothers the ratio was 57%. Neither ratio differs statistically from the expected 50%. When the analysis was limited to only those families with low-penetrance retinoblastoma, we still did not observe a biased transmission of alleles from fathers, although mothers did have an excess of carrier offspring of borderline statistical significance (the P-value was approximately 0.03). While we cannot rule out a biased transmission of alleles from some parents, there appears to be no such bias overall.

The retinoblastoma gene and its significance

The first human tumour suppressor gene, the Retinoblastoma Susceptibility gene (RB1) was first demonstrated in retinoblastoma, a rare paediatric eye tumour which has been studied extensively over the last century. Genetic studies of retinoblastoma have yielded unique insights into familial cancer syndromes and the mechanisms of oncogenesis by tumour suppressor genes such as the RB1 gene. In this view, we will summarize past research into the genetics of retinoblastoma that led to the discovery of the RB1 gene and discuss the influence these results have had on the field of cancer research. In addition, we will discuss current research into RB1 as it relates to cancer and its potential for new therapies.