Oncogenic point mutations in exon 20 of the RB1 gene in families showing incomplete penetrance and mild expression of the retinoblastoma phenotype

An overview of RB1 transcript alterations detected during retinoblastoma genetic screening

Identification of pathogenic RB1 variants aids in the clinical management of families with retinoblastoma. We routinely screen DNA for RB1 variants, but transcript analysis can also be used for variant screening, and to help decide variant pathogenicity. DNA was screened by conformation analysis followed by Sanger sequencing. Large deletion/insertions were detected by polymorphism analysis, MLPA and quantitative-PCR. Methylation-specific PCR was used to detect hypermethylation. RNA screening was performed when a DNA pathogenic variant was missing, or to determine effects on splicing.Two hundred and thirteen small coding variants were predicted to affect splicing in 207 patients. Splice donor (sd) variants were nearly twice as frequent as splice acceptor (sa) with the most affected positions being sd + 1 and sa-1. Some missense and nonsense codons altered splicing, while some splice consensus variants did not. Large deletion/insertions can disrupt splicing, but RNA analysis showed that some of these are more complex than indicated by DNA testing. RNA screening found pathogenic variants in 53.8% of samples where DNA analysis did not. RB1 splicing is altered by changes at consensus splice sites, some missense and nonsense codons, deep intronic changes and large deletion/insertions. Common alternatively spliced transcripts may complicate analysis. An effective molecular screening strategy would include RNA analysis to help determine pathogenicity.

Novel insights into RB1 mutation

Since the discovery of the retinoblastoma susceptibility gene (RB1) decades ago, RB1 has been regarded as a prototype tumor suppressor gene providing a paradigm for tumor genetic research. Constant research has updated the understanding of RB1-related pathways and their impact on tumor and nontumor diseases. Mutation of RB1 gene has been observed in multiple types of malignant tumors including prostate cancer, lung cancer, breast cancer, and almost every familial and sporadic case of retinoblastoma. Even if well-known and long-investigated, the application potential of RB1 mutation has not been fully tapped. In this review, we focus on the mechanism underlying RB1 mutation during oncogenesis. Therapeutically, we have further discussed potential clinical strategies by targeting RB1-mutated cancers. The unsolved problems and prospects of RB1 mutation are also discussed.

The RB1 Mutation Spectrum and Genetic Management Consultation in Pediatric Patients with Retinoblastoma in Beijing, China

Objective

The present study screened the structural mutations of the retinoblastoma (RB1) gene using gene capture and a preliminary exploration of the correlation between the genotypes and phenotypes.

Methods

A total of 45 formalin-fixed paraffin-embedded (FFPE) tissue samples and 12 peripheral venous blood samples from patients with retinoblastoma (RB) confirmed by pathological examination at Beijing Tongren Hospital were collected between May 2019 and May 2021. DNA from the samples was extracted, sequenced, and analyzed to detect the mutations in the RB1 gene by designing the targeted capture probes for exons and the flanking sequences of the gene.

Results

Of the 45 FFPE tissue samples, 23 were from male patients and 22 were from female patients, all aged between 4 months and 10 years, with an average age of 2.5 ± 1.3 years. Two of these patients had bilateral RB and 43 had unilateral RB (23 in the right eye and 20 in the left eye). Of the 12 peripheral venous blood samples, 7 were from male patients and 5 were from female patients, all aged between 8 months and 4 years, with an average age of 1.3 ± 0.9 years. Two of these patients had bilateral RB and 10 had unilateral RB (8 in the right eye and 2 in the left eye). Three de novo pathogenic mutations were found in the FFPE tissues, along with one de novo potentially pathogenic mutation, while three de novo potentially pathogenic mutations were found in the blood samples.

Conclusion

Gene capture is a low-cost and efficient method for the gene sequencing of RB. A total of seven de novo mutations were identified through mutation testing of the pathogenic gene RB1 in 56 pediatric patients with RB. This complemented the mutation spectrum of the RB1 gene and helped to improve the molecular diagnosis of RB, thereby providing a basis for genetic counseling and prediction of the clinical phenotype, as well as for the genetic testing of the offspring of patients with RB.

Clinical Registration Number

ChiCTR-EPC-17013892.

Introduction of a Variant Classification System for Analysis of Genotype-Phenotype Relationships in Heritable Retinoblastoma

Simple Summary

Heritable retinoblastoma is a genetic disease that predisposes to develop multiple retinoblastomas in childhood and other extraocular tumors later in life. It is caused by genetic variants in the RB1 gene. Here we present a new classification for genetic variants in the RB1 gene (REC) that focuses on the variant’s effect. The different classes, REC-I to -V, correlate with different risks of tumor predisposition. REC correlated with different clinical courses when applied in our study cohort. REC aims to facilitate risk estimation for physicians, patients and their families, and researchers and to improve the definition of the necessity of screening examination.

Abstract

Constitutional haploinsufficiency of the RB1 gene causes heritable retinoblastoma, a tumor predisposition syndrome. Patients with heritable retinoblastoma develop multiple retinoblastomas early in childhood and other extraocular tumors later in life. Constitutional pathogenic variants in RB1 are heterogeneous, and a few genotype-phenotype correlations have been described. To identify further genotype-phenotype relationships, we developed the retinoblastoma variant effect classification (REC), which considers each variant’s predicted effects on the common causal mediator, RB1 protein pRB. For validation, the RB1 variants of 287 patients were grouped according to REC. Multiple aspects of phenotypic expression were analyzed, known genotype-phenotype associations were revised, and new relationships were explored. Phenotypic expression of patients with REC-I, -II, and -III was distinct. Remarkably, the phenotype of patients with variants causing residual amounts of truncated pRB (REC-I) was more severe than patients with complete loss of RB1 (REC-II). The age of diagnosis of REC-I variants appeared to be distinct depending on truncation’s localization relative to pRB structure domains. REC classes identify genotype-phenotype relationships and, therefore, this classification framework may serve as a tool to develop tailored tumor screening programs depending on the type of RB1 variant.

Implementation of computational approaches to explore the deleterious effects of non-synonymous SNPs on pRB protein

Retinoblastoma 1 (RB1) is the first discovered tumor suppressor gene and recognized as the simple model system whose encoded defective protein can cause a pediatric cancer retinoblastoma. It functions as a negative regulator of the cell cycle through the interactions with members of the E2F transcription factors family. The protein of the RB1 gene (pRB) is engaged in various cell cycle processes including apoptosis, cell cycle arrest and chromatin remodeling. Recent studies on Retinoblastoma also exhibited multiple sets of point mutation in the associated protein due to its large polymorphic information in the local database. In this study, we identified the list of disease associated non-synonymous single nucleotide polymorphisms (nsSNPs) in RB1 by incorporating different computational algorithms, web servers, modeling of the mutants and finally superimposing it. Out of 826 nsSNPs, W516G and W563G were predicted to be highly deleterious variants in the conserved regions and found to have an impact on protein structure and protein-protein interaction. Moreover, our study concludes the effect of W516G variant was more detrimental in destabilizing protein's nature as compared to W563G variant. We also found defective binding of pRB having W516G mutation with E2F2 protein. Findings of this study will aid in shortening of the expensive experimental cost of identifying disease associated SNPs in retinoblastoma for which specialized personalized treatment or therapy can be formulated.Communicated by Ramaswamy H. Sarma.

Whole-Genome Sequencing of Retinoblastoma Reveals the Diversity of Rearrangements Disrupting RB1 and Uncovers a Treatment-Related Mutational Signature

The development of retinoblastoma is thought to require pathological genetic changes in both alleles of the RB1 gene. However, cases exist where RB1 mutations are undetectable, suggesting alternative pathways to malignancy. We used whole-genome sequencing (WGS) and transcriptomics to investigate the landscape of sporadic retinoblastomas derived from twenty patients, sought RB1 and other driver mutations and investigated mutational signatures. At least one RB1 mutation was identified in all retinoblastomas, including new mutations in addition to those previously identified by clinical screening. Ten tumours carried structural rearrangements involving RB1 ranging from relatively simple to extremely complex rearrangement patterns, including a chromothripsis-like pattern in one tumour. Bilateral tumours obtained from one patient harboured conserved germline but divergent somatic RB1 mutations, indicating independent evolution. Mutational signature analysis showed predominance of signatures associated with cell division, an absence of ultraviolet-related DNA damage and a profound platinum-related mutational signature in a chemotherapy-exposed tumour. Most RB1 mutations are identifiable by clinical screening. However, the increased resolution and ability to detect otherwise elusive rearrangements by WGS have important repercussions on clinical management and advice on recurrence risks.

Penetrance and Expressivity in Inherited Cancer Predisposing Syndromes

Inherited diseases are not always expressed in the same way in every individual that carries the same variant in a disease-causing gene. This phenomenon is known as reduced or incomplete penetrance. Variable and incomplete penetrance may explain why inherited diseases are occasionally transmitted through unaffected parents, but also why clinically healthy individuals can carry potentially pathogenic variants without expressing features of the disease. Here, we will provide an overview of factors that play a fundamental role in the concept of penetrance and expressivity of cancer predisposing genes in children with malignancies. These findings are important to understand the complexity of inherited diseases and cancer development and to improve genetic counselling for the affected families.

Detection and reporting of RB1 promoter hypermethylation in diagnostic screening

BACKGROUND

RB1 gene screening aids clinical management and genetic counselling in retinoblastoma families. Here we present epigenetic changes identified during routine molecular RB1 screening of tumor and blood samples. Complications in interpreting RB1 methylation are discussed.

MATERIALS AND METHODS

Screening for RB1 promoter hypermethylation was carried out by Methylation Specific PCR (MS-PCR) after bisulphite modification of DNA. The cohort consisted of 315 tumors, and 204 blood samples, from 497 retinoblastoma patients (22 patients had both blood and tumor screened).

RESULTS

11.4% of retinoblastoma tumors had promoter hypermethylation. It was not routinely detected in blood samples, or in tumors with two other oncogenic RB1 changes. One blood sample had promoter hypermethylation due to an X;13 translocation. One tumor had low level methylation as well as two other oncogenic changes. Histopathological analysis of a small subset of age-matched tumors was similar regardless of promoter hypermethylation status.

CONCLUSIONS

Promoter hypermethylation was detected in 11.4% of the retinoblastoma tumors and should be tested for in routine RB1 screening programmes. Constitutional samples are not expected to display RB1 hypermethylation. In a small proportion of cases it may not be possible to use this somatic change in patient management.

Parent-of-origin effect of hypomorphic pathogenic variants and somatic mosaicism impact on phenotypic expression of retinoblastoma

Retinoblastoma is the most common eye cancer in children. Numerous families have been described displaying reduced penetrance and expressivity. An extensive molecular characterization of seven families led us to characterize the two main mechanisms impacting on phenotypic expression, as follows: (i) mosaicism of amorphic pathogenic variants; and (ii) parent-of-origin-effect of hypomorphic pathogenic variants. Somatic mosaicism for RB1 splicing variants (c.1960+5G>C and c.2106+2T>C), leading to a complete loss of function was demonstrated by high-depth NGS in two families. In both cases, the healthy carrier parent (one with retinoma) showed a variant frequency lower than that expected for a heterozygous individual, indicating a 56-60% mosaicism level. Previous evidences of a ~3-fold excess of RB1 maternal canonical transcript led us to hypothesize that this differential allelic expression could influence phenotypic outcome in families at risk for RB onset. Accordingly, in five families, we identified a higher tumor risk associated with paternally inherited hypomorphic pathogenic variants, namely a deletion resulting in the loss of 37 amino acids at the N-terminus (c.608-16_608del), an exonic substitution with a "leaky" splicing effect (c.1331A>G), a partially deleterious substitution (c.1981C>T) and a truncating C-terminal variant (c.2663+2T>C). The identification of these mechanisms changes the genetic/prenatal counseling and the clinical management of families, indicating a higher recurrence risk when the hypomorphic pathogenic variant is inherited from the father, and suggesting the need for second tumor surveillance in unaffected carriers at risk of developing adult-onset cancer such as osteosarcoma or leiomyosarcoma.

Mutation spectrum of RB1 mutations in retinoblastoma cases from Singapore with implications for genetic management and counselling

Retinoblastoma (RB) is a rare childhood malignant disorder caused by the biallelic inactivation of RB1 gene. Early diagnosis and identification of carriers of heritable RB1 mutations can improve disease outcome and management. In this study, mutational analysis was conducted on fifty-nine matched tumor and peripheral blood samples from 18 bilateral and 41 unilateral unrelated RB cases by a combinatorial approach of Multiplex Ligation-dependent Probe Amplification (MLPA) assay, deletion screening, direct sequencing, copy number gene dosage analysis and methylation assays. Screening of both blood and tumor samples yielded a mutation detection rate of 94.9% (56/59) while only 42.4% (25/59) of mutations were detected if blood samples alone were analyzed. Biallelic mutations were observed in 43/59 (72.9%) of tumors screened. There were 3 cases (5.1%) in which no mutations could be detected and germline mutations were detected in 19.5% (8/41) of unilateral cases. A total of 61 point mutations were identified, of which 10 were novel. There was a high incidence of previously reported recurrent mutations, occurring at 38.98% (23/59) of all cases. Of interest were three cases of mosaic RB1 mutations detected in the blood from patients with unilateral retinoblastoma. Additionally, two germline mutations previously reported to be associated with low-penetrance phenotypes: missense-c.1981C>T and splice variant-c.607+1G>T, were observed in a bilateral and a unilateral proband, respectively. These findings have implications for genetic counselling and risk prediction for the affected families. This is the first published report on the spectrum of mutations in RB patients from Singapore and shows that further improved mutation screening strategies are required in order to provide a definitive molecular diagnosis for every case of RB. Our findings also underscore the importance of genetic testing in supporting individualized disease management plans for patients and asymptomatic family members carrying low-penetrance, germline mosaicism or heritable unilateral mutational phenotypes.

De novo mutational profile in RB1 clarified using a mutation rate modeling algorithm

BACKGROUND

Studies of de novo mutations offer great promise to improve our understanding of human disease. After a causal gene has been identified, it is natural to hypothesize that disease relevant mutations accumulate within a sub-sequence of the gene - for example, an exon, a protein domain, or at CpG sites. These assessments are typically qualitative, because we lack methodology to assess the statistical significance of sub-gene mutational burden ultimately to infer disease-relevant biology.

METHODS

To address this issue, we present a generalized algorithm to grade the significance of de novo mutational burden within a gene ascertained from affected probands, based on our model for mutation rate informed by local sequence context.

RESULTS

We applied our approach to 268 newly identified de novo germline mutations by re-sequencing the coding exons and flanking intronic regions of RB1 in 642 sporadic, bilateral probands affected with retinoblastoma (RB). We confirm enrichment of loss-of-function mutations, but demonstrate that previously noted 'hotspots' of nonsense mutations in RB1 are compatible with the elevated mutation rates expected at CpG sites, refuting a RB specific pathogenic mechanism. Our approach demonstrates an enrichment of splice-site donor mutations of exon 6 and 12 but depletion at exon 5, indicative of previously unappreciated heterogeneity in penetrance within this class of substitution. We demonstrate the enrichment of missense mutations to the pocket domain of RB1, which contains the known Arg661Trp low-penetrance mutation.

CONCLUSION

Our approach is generalizable to any phenotype, and affirms the importance of statistical interpretation of de novo mutations found in human genomes.

APC/C and retinoblastoma interaction: cross-talk of retinoblastoma protein with the ubiquitin proteasome pathway

The ubiquitin (Ub) ligase anaphase promoting complex/cyclosome (APC/C) and the tumour suppressor retinoblastoma protein (pRB) play key roles in cell cycle regulation. APC/C is a critical regulator of mitosis and G₁-phase of the cell cycle whereas pRB keeps a check on proliferation by inhibiting transition to the S-phase. APC/C and pRB interact with each other via the co-activator of APC/C, FZR1, providing an alternative pathway of regulation of G₁ to S transition by pRB using a post-translational mechanism. Both pRB and FZR1 have complex roles and are implicated not only in regulation of cell proliferation but also in differentiation, quiescence, apoptosis, maintenance of chromosomal integrity and metabolism. Both are also targeted by transforming viruses. We discuss recent advances in our understanding of the involvement of APC/C and pRB in cell cycle based decisions and how these insights will be useful for development of anti-cancer and anti-viral drugs.

Genetic screening in patients with Retinoblastoma in Israel

Retinoblastoma (Rb) is a childhood tumor (~1 in 20,000 live births) developing in the retina due to mutations in the RB1 gene. Identification of the oncogenic mutations in the RB1 gene is important for the clinical management and for genetic counseling to families with a child or a parent affected with the tumor. Here we present our experience in detecting the pathogenic mutations in blood samples, from 150 unrelated Rb patients and highlight the relevant counseling issues. Mutation screening in the RB1 gene was based on Sanger sequencing, mosaicism of recurrent CpG transition mutations was detected by allele specific PCR and multiplex ligation dependent probe amplification for detecting of large deletions/duplications. The overall detection rate of mutations in our cohort was 55% (82/150). In the familial cases it was 100% (17/17), in bilateral and unilateral-multifocal sporadic cases 91% (50/55), and in the unilateral sporadic cases 19% (15/78). Nonsense mutations and small deletions or insertions that results in transcripts with premature termination codons that are subject to nonsense mediated decay were the most frequent, detected in 50/82 (61%) of the patients. The rest were large deletions detected in 14/82 (17%), splice site mutations detected in 11/82 (13%), missense mutations in four patients and mutations in the promoter sequence in three patients. Mutation mosaicism ranging from 10 to 30% was detected by allele specific PCR in ten patients, 9% (5/55) of patients with bilateral tumor and 33% (5/15) of the patients with unilateral tumor. In three patients rare variants were detected as the only finding which was also detected in other healthy family members. Allele specific amplification of recurrent mutations raises in our cohort the identification rate from 82 to 91% in the sporadic bilateral cases and from 13 to 19% in the unilateral sporadic cases. Most mosaic cases could not be identified by Sanger sequencing and therefore screening for recurrent CpG transition mutations by allele specific amplification is of utmost importance. Molecular screening is important for the genetic counseling regarding the risk for tumor development and the relevance for prenatal diagnosis but in several families is accompanied by detecting rare variants that might be rare polymorphisms or low penetrant mutations.

A Parent-of-Origin Effect Impacts the Phenotype in Low Penetrance Retinoblastoma Families Segregating the c.1981C>T/p.Arg661Trp Mutation of RB1

Retinoblastoma (Rb), the most common pediatric intraocular neoplasm, results from inactivation of both alleles of the RB1 tumor suppressor gene. The second allele is most commonly lost, as demonstrated by loss of heterozygosity studies. RB1 germline carriers usually develop bilateral tumors, but some Rb families display low penetrance and variable expressivity. In order to decipher the underlying mechanisms, 23 unrelated low penetrance pedigrees segregating the common c.1981C>T/p.Arg661Trp mutation and other low penetrance mutations were studied. In families segregating the c.1981C>T mutation, we demonstrated, for the first time, a correlation between the gender of the transmitting carrier and penetrance, as evidenced by Fisher's exact test: the probability of being unaffected is 90.3% and 32.5% when the mutation is inherited from the mother and the father, respectively (p-value = 7.10(-7). Interestingly, a similar correlation was observed in families segregating other low penetrance alleles. Consequently, we investigated the putative involvement of an imprinted, modifier gene in low penetrance Rb. We first ruled out a MED4-driven mechanism by MED4 methylation and expression analyses. We then focused on the differentially methylated CpG85 island located in intron 2 of RB1 and showing parent-of-origin-specific DNA methylation. This differential methylation promotes expression of the maternal c.1981C>T allele. We propose that the maternally inherited c.1981C>T/p.Arg661Trp allele retains sufficient tumor suppressor activity to prevent retinoblastoma development. In contrast, when the mutation is paternally transmitted, the low residual activity would mimic a null mutation and subsequently lead to retinoblastoma. This implies that the c.1981C>T mutation is not deleterious per se but needs to be destabilized in order to reach pRb haploinsufficiency and initiate tumorigenesis. We suggest that this phenomenon might be a general mechanism to explain phenotypic differences in low penetrance Rb families.

The other side of the coin: the tumor-suppressive aspect of oncogenes and the oncogenic aspect of tumor-suppressive genes, such as those along the CCND-CDK4/6-RB axis

Although cancer-regulatory genes are dichotomized to oncogenes and tumor-suppressor gene s, in reality they can be oncogenic in one situation but tumor-suppressive in another. This dual-function nature, which sometimes hampers our understanding of tumor biology, has several manifestations: (1) Most canonically defined genes have multiple mRNAs, regulatory RNAs, protein isoforms, and posttranslational modifications; (2) Genes may interact at different levels, such as by forming chimeric RNAs or by forming different protein complexes; (3) Increased levels of tumor-suppressive genes in normal cells drive proliferation of cancer progenitor cells in the same organ or tissue by imposing compensatory proliferation pressure, which presents the dual-function nature as a cell-cell interaction. All these manifestations of dual functions can find examples in the genes along the CCND-CDK4/6-RB axis. The dual-function nature also underlies the heterogeneity of cancer cells. Gene-targeting chemotherapies, including that targets CDK4, are effective to some cancer cells but in the meantime may promote growth or progression of some others in the same patient. Redefining "gene" by considering each mRNA, regulatory RNA, protein isoform, and posttranslational modification from the same genomic locus as a "gene" may help in better understanding tumor biology and better selecting targets for different sub-populations of cancer cells in individual patients for personalized therapy.

Genetic testing in Tunisian families with heritable retinoblastoma using a low cost approach permits accurate risk prediction in relatives and reveals incomplete penetrance in adults

Heritable retinoblastoma is caused by oncogenic mutations in the RB1 tumor suppressor gene. Identification of these mutations in patients is important for genetic counseling and clinical management of relatives at risk. In order to lower analytical efforts, we designed a stepwise mutation detection strategy that was adapted to the spectrum of oncogenic RB1 gene mutations. We applied this strategy on 20 unrelated patients with familial and/or de novo bilateral retinoblastoma from Tunisia. In 19 (95%) patients, we detected oncogenic mutations including base substitutions, small length mutations, and large deletions. Further analyses on the origin of the mutations showed mutational mosaicism in one unilaterally affected father of a bilateral proband and incomplete penetrance in two mothers. In a large family with several retinoblastoma patients, the mutation identified in the index patient was also detected in several non-penetrant relatives. RNA analyses showed that this mutation results in an in-frame loss of exon 9. In summary, our strategy can serve as a model for RB1 mutation identification with high analytical sensitivity. Our results point out that genetic testing is needed to reveal or exclude incomplete penetrance specifically in parents of patients with sporadic disease.

RB1 mutation spectrum in a comprehensive nationwide cohort of retinoblastoma patients

BACKGROUND

Retinoblastoma (Rb) is a childhood cancer of the retina, commonly initiated by biallelic inactivation of the RB1 gene. Knowledge of the presence of a heritable RB1 mutation can help in risk management and reproductive decision making. We report here on RB1 mutation scanning in a unique nationwide cohort of Rb patients from the Netherlands.

METHODS

From the 1173 Rb patients registered in the Dutch National Retinoblastoma Register until January 2013, 529 patients from 433 unrelated families could be included. RB1 mutation scanning was performed with different detection methods, depending on the time period.

RESULTS

Our mutation detection methods revealed RB1 mutations in 92% of bilateral and/or familial Rb patients and in 10% of non-familial unilateral cases. Overall an RB1 germline mutation was detected in 187 (43%) of 433 Rb families, including 33 novel mutations. The distribution of the type of mutation was 37% nonsense, 20% frameshift, 21% splice, 9% large indel, 5% missense, 7% chromosomal deletions and 1% promoter. Ten per cent of patients were mosaic for the RB1 mutation. Six three-generation families with incomplete penetrance RB1 mutations were found. We found evidence that two variants, previously described as pathogenic RB1 mutations, are likely to be neutral variants.

CONCLUSIONS

The frequency of the type of mutations in the RB1 gene in our unbiased national cohort is the same as the mutation spectrum described worldwide. Furthermore, our RB1 mutation detection regimen achieves a high scanning sensitivity.

Spectrum of RB1 mutations identified in 403 retinoblastoma patients

BACKGROUND

Retinoblastoma (RB) is a malignant, childhood tumour of the developing retina that occurs with an estimated frequency of 1 in 20 000. Identification of oncogenic mutations in the RB1 gene aids in the clinical management of families with a heritable predisposition to RB. Here we present the spectrum of genetic and epigenetic changes identified in 194 tumours and 209 blood samples, from 403 unrelated RB patients.

METHODS

Mutation screening was carried out across all 27 RB1 exons and their associated splice sites. Small coding sequence changes were detected using fluorescent conformation analysis followed by sequencing. Large exonic deletions were detected by quantitative fluorescent PCR. Methylation specific PCR of the RB1 promoter was performed to detect epigenetic alterations. Polymorphism analysis was used to determine loss of heterozygosity in tumour samples.

RESULTS

95% of the expected mutations were identified in the tumour samples, with 16 samples exhibiting only one mutation, while two samples had no detectable RB1 mutation. 96% of bilateral/familial RB blood samples and 9.5% of unilateral sporadic blood samples, yielded mutations. 111 were novel mutations.

CONCLUSIONS

The full range of screening techniques is required to achieve a high screening sensitivity in RB patients.

Where genotype is not predictive of phenotype: towards an understanding of the molecular basis of reduced penetrance in human inherited disease

Some individuals with a particular disease-causing mutation or genotype fail to express most if not all features of the disease in question, a phenomenon that is known as 'reduced (or incomplete) penetrance'. Reduced penetrance is not uncommon; indeed, there are many known examples of 'disease-causing mutations' that fail to cause disease in at least a proportion of the individuals who carry them. Reduced penetrance may therefore explain not only why genetic diseases are occasionally transmitted through unaffected parents, but also why healthy individuals can harbour quite large numbers of potentially disadvantageous variants in their genomes without suffering any obvious ill effects. Reduced penetrance can be a function of the specific mutation(s) involved or of allele dosage. It may also result from differential allelic expression, copy number variation or the modulating influence of additional genetic variants in cis or in trans. The penetrance of some pathogenic genotypes is known to be age- and/or sex-dependent. Variable penetrance may also reflect the action of unlinked modifier genes, epigenetic changes or environmental factors. At least in some cases, complete penetrance appears to require the presence of one or more genetic variants at other loci. In this review, we summarize the evidence for reduced penetrance being a widespread phenomenon in human genetics and explore some of the molecular mechanisms that may help to explain this enigmatic characteristic of human inherited disease.

Prediction of protein-destabilizing polymorphisms by manual curation with protein structure

The relationship between sequence polymorphisms and human disease has been studied mostly in terms of effects of single nucleotide polymorphisms (SNPs) leading to single amino acid substitutions that change protein structure and function. However, less attention has been paid to more drastic sequence polymorphisms which cause premature termination of a protein’s sequence or large changes, insertions, or deletions in the sequence. We have analyzed a large set (n = 512) of insertions and deletions (indels) and single nucleotide polymorphisms causing premature termination of translation in disease-related genes. Prediction of protein-destabilization effects was performed by graphical presentation of the locations of polymorphisms in the protein structure, using the Genomes TO Protein (GTOP) database, and manual annotation with a set of specific criteria. Protein-destabilization was predicted for 44.4% of the nonsense SNPs, 32.4% of the frameshifting indels, and 9.1% of the non-frameshifting indels. A prediction of nonsense-mediated decay allowed to infer which truncated proteins would actually be translated as defective proteins. These cases included the proteins linked to diseases inherited dominantly, suggesting a relation between these diseases and toxic aggregation. Our approach would be useful in identifying potentially aggregation-inducing polymorphisms that may have pathological effects.

Epigenetic and copy number variation analysis in retinoblastoma by MS-MLPA

Retinoblastoma is the most common primary intraocular malignancy in children. Two step inactivation of RB1 (M1-M2) represents the key event in the pathogenesis of retinoblastoma but additional genetic and epigenetic events (M3-Mn) are required for tumor development. In the present study, we employed Methylation Specific Multiplex Ligation Probe Assay to investigate methylation status and copy number changes of 25 and 39 oncosuppressor genes, respectively. This technique was applied to analyse 12 retinoblastomas (5 bilateral and 7 unilateral) and results were compared to corresponding normal retina. We identified hypermethylation in seven new genes: MSH6 (50%), CD44 (42%), PAX5 (42%), GATA5 (25%), TP53 (8%), VHL (8%) and GSTP1 (8%) and we confirmed the previously reported hypermethylation of MGMT (58%), RB1 (17%) and CDKN2 (8%). These genes belong to key pathways including DNA repair, pRB and p53 signalling, transcriptional regulation, protein degradation, cell-cell interaction, cellular adhesion and migration. In the same group of retinoblastomas, a total of 29 copy number changes (19 duplications and 10 deletions) have been identified. Interestingly, we found deletions of the following oncosuppressor genes that might contribute to drive retinoblastoma tumorigenesis: TP53, CDH13, GATA5, CHFR, TP73 and IGSF4. The present data highlight the importance of epigenetic changes in retinoblastoma and indicate seven hypermethylated oncosuppressors never associated before to retinoblastoma pathogenesis. This study also confirms the presence of copy number variations in retinoblastoma, expecially in unilateral cases (mean 3 ± 1.3) where these changes were found more frequently respect to bilateral cases (mean 1.4 ± 1.1).

Low penetrance of retinoblastoma for p.V654L mutation of the RB1 gene

Background

Retinoblastoma is caused by compound heterozygosity or homozygosity of retinoblastoma gene (RB1) mutations. In germline retinoblastoma, mutations in the RB1 gene predispose individuals to increased cancer risks during development. These mutations segregate as autosomal dominant traits with high penetrance (90%).

Methods

We screened 30 family members from one family using high resolution melting assay and DNA direct sequencing for mutations in the RB1 gene. We evaluate the phenotype and penetrance of germline mutations of the RB1 gene in a large Taiwanese family.

Results

The molecular analysis and clinical details of this family showed phenotypic variability associated with the p.V654L mutation in exon 19 of the RB1 gene in 11 family members. The phenotype varied from asymptomatic to presence of a unilateral tumor. Only four individuals (2 males and 2 females) developed unilateral retinoblastoma, which resulted in calculated low penetrance of 36% (4/11). The four individuals with retinoblastoma were diagnosed before the age of three years. None of their relatives exhibited variable severity or bilateral retinoblastoma.

Conclusions

The diseased-eye ratio for this family was 0.36, which is lower than current estimates. This suggests that the RB1 p.V654L mutation is a typical mutation associated with low penetrance.

Germline mutations in retinoma patients: relevance to low-penetrance and low-expressivity molecular basis

PURPOSE

To study phenotype-genotype correlation in patients who have retinoma, which is a benign tumor resembling the post irradiation regression pattern of retinoblastoma (RB).

METHODS

We selected patients who had retinoma and positive family history for RB and patients who had retinoma in one eye and either retinoma or RB in the other eye. The study included 22 patients with available DNA: 18 from 11 families and four sporadic cases. DNA was extracted from peripheral blood leukocytes. The RB1 gene was screened by DHPLC and direct sequencing of the promoter and all the exons.

RESULTS

We identified 17 occurrences of 11 distinct germline mutations in two sporadic and in 15 familial cases (nine families). The 11 identified mutations were located in exons 1, 10,11,13,14, and 19 to 23. Four of the identified mutations were not previously reported, including g.64407delT, g.153236A>T, g.156743delTCTG, and g.162078delA. Eight out the 11 mutations were truncating and three were nontruncating (missense). There was no correlation between the type of mutation and the number of tumor foci per eye (RB or retinomas). Highly heterogeneous intrafamilial expressivity was observed.

CONCLUSIONS

To our knowledge, this study is the largest series of mutations of consecutive retinoma patients. The present data suggest that the type of inherited mutations underlying retinoma is undistinguishable from RB related ones, i.e., largely dominated by truncating mutants. This finding is in contrast with the RB1 genotypic spectrum of mutations associated with low-penetrance RB, i.e., nontruncating mutants. The molecular mechanism underlying low-penetrance and attenuated expressivity (retinomas) appeared to be distinct.

Low-penetrant RB allele in small-cell cancer shows geldanamycin instability and discordant expression with mutant ras

Certain kindreds with low-penetrant (lp) retinoblastoma carry mutant alleles which retain partial tumor suppressor activity and we previously showed that these alleles exhibit defective, temperature-sensitive binding in yeast. To investigate the molecular basis for incomplete penetrance, we studied three recurrent lp alleles and observed approximately 50% of wildtype activity measured by (i) phosphorylation at key regulatory sites, S780, S795, S807/S811, (ii) transcriptional co-activation, and (iii) 'flat-cell' differentiation in mammalian cells in vivo. In addition, we studied a small-cell carcinoma that is homozygous for the R661W allele providing the first analysis of the effect of a naturally occurring lp allele in a human tumor. While we detected abundant expression of the R661W protein, we noted marked instability of both endogenous and recombinant R661W following treatment in vivo with the Hsp90 inhibitor, geldanamycin and stabilization of R661W following heat shock. In addition, we observed a discordant phenotype in the tumor cells with induction of p16 and loss of cyclin D1 consistent with a null RB status combined with homozygous expression of mutant ras which had not been reported previously for RB (-) small-cell cancer. These findings show that a recurrent missense lp allele retains greater functional activity in vivo than predicted from earlier in vitro assays, proposing a role for stabilizing chaperone-like activity in vivo. In addition, these data suggest that reversible protein instability and the requirement for a cooperating mutation may provide a stochastic explanation for the molecular basis of incomplete penetrance in kindreds carrying these alleles.

Loss of RB1 induces non-proliferative retinoma: increasing genomic instability correlates with progression to retinoblastoma

Retinoblastoma clinical observations revealed the role of tumor suppressor genes in human cancer, Knudson's 'two-hit' model of cancer induction. We now demonstrate that loss of both RB1 tumor suppressor gene alleles initiates quiescent RB1(-/-) retinomas with low level genomic instability and high expression of the senescence-associated proteins p16(INK4a) and p130. Although retinomas can remain unchanged throughout life, highly proliferative, clonal and aneuploid retinoblastomas commonly emerge, exhibiting altered gene copy number and expression of oncogenes (MYCN, E2F3, DEK, KIF14 and MDM4) and tumor suppressor genes (CDH11, p75(NTR)) and reduced expression of p16(INK4a) and p130. We suggest that RB1 inactivation in developing retina induces genomic instability, but senescence can block transformation at the stage of retinoma. However, stable retinoma is rarely clinically observed because progressive genomic instability commonly leads to highly proliferative retinoblastoma.

Structure-function analysis of the retinoblastoma tumor suppressor protein - is the whole a sum of its parts?

Biochemical analysis of the retinoblastoma protein's function has received considerable attention since it was cloned just over 20 years ago. During this time pRB has emerged as a key regulator of the cell division cycle and its ability to block proliferation is disrupted in the vast majority of human cancers. Much has been learned about the regulation of E2F transcription factors by pRB in the cell cycle. However, many questions remain unresolved and researchers continue to explore this multifunctional protein. In particular, understanding how its biochemical functions contribute to its role as a tumor suppressor remains to be determined. Since pRB has been shown to function as an adaptor molecule that links different proteins together, or to particular promoters, analyzing pRB by disrupting individual protein interactions holds tremendous promise in unraveling the intricacies of its function. Recently, crystal structures have reported how pRB interacts with some of its molecular partners. This information has created the possibility of rationally separating pRB functions by studying mutants that disrupt individual binding sites. This review will focus on literature that investigates pRB by isolating functions based on binding sites within the pocket domain. This article will also discuss the prospects for using this approach to further explore the unknown functions of pRB.

Genotype-phenotype correlations in hereditary familial retinoblastoma

We studied 50 unrelated pedigrees with a family history of retinoblastoma (Rb) (165 carriers of a RB1 mutation) to delineate the spectrum of RB1 germline mutations in familial Rb and to identify genotype-phenotype correlations as well as putative modifiers. Patients were followed at Institut Curie and they were examined by an ophthalmologist, a pediatrician, and a geneticist. All cases of familial Rb were determined via genetic counseling. Clinical features included disease status, laterality, age at diagnosis, mutation type, follow-up, and disease-eye ratio (DER). To eliminate mosaic cases, first-generation carriers displaying low-penetrance (LP) Rb were excluded from the analysis. Complete penetrance was the rule for nonsense and frameshift mutations (25 families) and high penetrance was observed for large rearrangements (eight families). Promoter (two families) and missense (two families) mutations displayed heterogeneous phenotypes and LP. Variable penetrance was observed for splice abnormalities (13 families) and was explained by in/out of frame mutations or respect of functional domains. Surprisingly, two families with the LP g.45867G>T/IVS6+1G>T mutation presented data that conflicted with the data reported in previous publications, as unaffected carriers had paternally inherited mutant alleles. Moreover, RNA analyses suggested that the lack of penetrance in unaffected carriers could be explained by an increase in expression levels of the wild-type allele. This observation prompted us to define a new class "3" of LP alleles. We believe this is the first large-scale study of familial Rb with a high level of homogeneity in the clinical and genetic analysis of patients and their relatives, thereby allowing for reliable intrafamilial genotype-phenotype correlations. Our analysis suggests in some cases the influence of modifier factors probably involved in mRNA level regulation and/or pRB pathway regulation.

The retinoblastoma tumor-suppressor gene, the exception that proves the rule

The retinoblastoma tumor-suppressor gene (Rb1) is centrally important in cancer research. Mutational inactivation of Rb1 causes the pediatric cancer retinoblastoma, while deregulation of the pathway in which it functions is common in most types of human cancer. The Rb1-encoded protein (pRb) is well known as a general cell cycle regulator, and this activity is critical for pRb-mediated tumor suppression. The main focus of this review, however, is on more recent evidence demonstrating the existence of additional, cell type-specific pRb functions in cellular differentiation and survival. These additional functions are relevant to carcinogenesis suggesting that the net effect of Rb1 loss on the behavior of resulting tumors is highly dependent on biological context. The molecular mechanisms underlying pRb functions are based on the cellular proteins it interacts with and the functional consequences of those interactions. Better insight into pRb-mediated tumor suppression and clinical exploitation of pRb as a therapeutic target will require a global view of the complex, interdependent network of pocket protein complexes that function simultaneously within given tissues.

pRb2/p130: a new candidate for retinoblastoma tumor formation

Retinoblastoma is the most common primary intraocular tumor in childhood. Mutations in both the alleles of the RB1 gene represent the causative agent for the tumor to occur. It is becoming evident that, although these alterations represent key events in the genesis of retinoblastoma, they are not sufficient per se for the tumor to develop, and other additional genetic or epigenetic alterations must occur. A supportive role in the genesis of retinoblastoma has recently been proposed for the RB1-related gene RB2/p130. Additionally, several other genetic alterations involving different chromosomes have been described as relevant in the tumorigenic process. In this review we will analyse current knowledge about the molecular mechanisms involved in retinoblastoma, paying particular attention to the mechanisms of inactivation of the biological function of the retinoblastoma family of proteins.

A molecular study of first and second RB1 mutational hits in retinoblastoma patients

RB1 mutations accountable for biallelic inactivation are crucial events in the development of retinoblastoma because a first mutation (M1) predisposes to retinoblastoma while a second mutation (M2) is required for tumor development. Mutational analyses of this gene showed a wide spectrum of genetic alterations (single base substitutions, insertions, or deletions, as well as small and large deletions). The most frequent second hit in retinoblastoma patients is loss of heterozygosity (LOH) followed by promoter methylation. Molecular analyses of RB1 mutations were conducted in 36 patients (20 unilateral and 16 bilateral) using polymerase chain reaction-mediated single-strand conformation polymorphism (SSCP) analysis, sequencing, and LOH analysis. Sixty-four amplified fragments showing abnormal SSCP patterns were sequenced, and mutations were confirmed in five patients (13.89%). Four mutations were located at coding regions, and a fifth one was found at an exon-intron junction. Two mutations were C-->T transitions, two were small-length deletions, and one was a G-->A transition. A total of 47.05% patients showed LOH. In one patient, the parental origin of the mutated allele was detected: the allele retained in the tumor was the paternal one. This work helps to characterize the spectrum of mutations in the Brazilian population, and to confirm that formaldehyde-fixed paraffin tissue can provide valuable information on the RB1 status in retinoblastoma patients.

Late-Onset Retinoblastoma in a Well-Functioning Fellow Eye

PURPOSE

To describe the documented growth, clinical course, and histopathology of retinoblastomas in an untreated and otherwise normal right eye of a 27-year-old white male with a g.153211T>A (p.Tyr606X) mutation in the retinoblastoma 1 gene, whose left eye was enucleated at age 2 years for 2 retinoblastomas.

DESIGN AND PARTICIPANTS

Retrospective interventional case report.

INTERVENTIONS

Over the years, the right eye was irradiated twice and underwent trans-pars plana vitrectomy, transscleral cryocoagulation, argon laser photocoagulation of tumors and their feeder vessels, extracapsular cataract extraction with posterior chamber lens implantation, and neodymium:yttrium-aluminum-garnet laser treatment of after-cataract in the form of Elschnig's pearls. Finally, the patient received combination chemotherapy with etoposide, methotrexate, actinomycin D, cisplatin, and vincristine.

RESULTS

The eye finally had to be removed 12 years later due to tumor recurrences and seeding, pseudohypopyon, and elevated intraocular pressure. Histopathology showed microcellular retinoblastoma cells in the anterior chamber angle and trabecular meshwork without subconjunctival extension and in the nasal ciliary body, pars plana, internal limiting membrane, and optic nerve head anterior to the cribriform plate. The patient is without local or systemic recurrences at age 50, 11 years after the last eye was enucleated.

CONCLUSIONS

This report shows that retinoblastoma patients may have tumor growth in their fellow eye 25 years after the first eye and also that Elschnig's after-cataract pearls still can arise after irradiation of a lens with 45 Gy.

An E2F binding-deficient Rb1 protein partially rescues developmental defects associated with Rb1 nullizygosity

Rb1 is essential for normal embryonic development, as null mice die in midgestation with widespread unscheduled cell proliferation. Rb1 protein (pRb) mediates cell cycle control by binding E2F transcription factors and repressing expression from E2F-dependent promoters. An increasing amount of evidence suggests that pRb loss also compromises cellular differentiation. Since differentiation is often dependent on cell cycle exit, it is currently unclear whether the effects of pRb on differentiation are an indirect consequence of pRb/E2F-mediated cell cycle control or whether they reflect direct cell-type-specific pRb functions. We have mutated Rb1 in the mouse to express a protein (R654W) specifically deficient in binding E2F1, E2F2, and E2F3. R654W mutant embryos exhibit cell cycle defects the same as those of Rb1 null embryos, reinforcing the importance of the interactions of pRb with E2F1, E2F2, and E2F3 for cell cycle control. However, R654W embryos survive at least 2 days longer than Rb1 null embryos, and increased life span is associated with improved erythrocyte and fetal liver macrophage differentiation. In contrast, R654W pRb does not rescue differentiation defects associated with pRb-deficient retinae. These data indicate that Rb1 makes important cell-type-specific contributions to cellular differentiation that are genetically separable from its general ability to stably bind E2F1, E2F2, and E2F3 and regulate the cell cycle.

Diagnóstico molecular del retinoblastoma: epidemiología molecular y consejo genético

BACKGROUND AND OBJECTIVE

Retinoblastoma, a prototype of hereditary cancer, is the most common intraocular tumor in children and a potential cause of blindness from therapeutic eye ablation, second tumors in germ line mutation carriers, and even death when untreated. The molecular scanning of RB1 in search of germ line mutations in 213 retinoblastoma patients from Spain, Cuba, Colombia and Serbia, has led to the detection of 106 mutations whose knowledge is important for genetic counselling and characterization of phenotypic-genotypic relations.

PATIENTS AND METHOD

Mutational study (PCR-sequentiation and microsatellites analysis) in patients with retinoblastoma, from Spain, Cuba, Colombia and Serbia.

RESULTS

45% of mutations, including most of the frame shift (FS), missense (MS) and splicing (SP), were new, while all nonsense mutations (NS) corresponded to hypermutable sites in RB1. Germ line mutations were found in 22% of unilateral sporadic patients. The incidence of SP plus MS mutations in this group of patients was greater (p = 0.018) than in bilateral patients. The frequency of SP mutations was higher (p = 0.0003) in Spain and France than in Germany and United Kingdom, while the incidence of NS mutations was lower (p = 0.0006). SP mutations were associated with the low penetrance phenotype and were also overrepresented (p = 0.018) in patients with delayed retinoblastoma onset.

CONCLUSIONS

Mutational scanning of unilateral patients is important for genetic counselling and may help decipher the molecular mechanisms leading to low penetrance or expressivity. The functional characterization of mutations associated with low-penetrance or expressivity phenotypes and the molecular classification of tumors using multiple expression profiling is important for a better understanding of the retinoblastoma pathogenesis.

Mutational screening of the RB1 gene in Italian patients with retinoblastoma reveals 11 novel mutations

Retinoblastoma (RB, OMIM#180200) is the most common intraocular tumour in infancy and early childhood. Constituent mutations in the RB1 gene predispose individuals to RB development. We performed a mutational screening of the RB1 gene in Italian patients affected by RB referred to the Medical Genetics of the University of Siena. In 35 unrelated patients, we identified germline RB1 mutations in 6 out of 9 familial cases (66%) and in 7 out of 26 with no family history of RB (27%). Using the single-strand conformational polymorphism (SSCP) technique, 11 novel mutations were detected, including 3 nonsense, 5 frameshift and 4 splice-site mutations. Only two of these mutations (1 splice site and 1 missense) were previously reported. The mutation spectrum reflects the published literature, encompassing predominately nonsense or frameshift and splicing mutations. RB1 germline mutation was detected in 37% of our cases. Gross rearrangements outside the investigated region, altered DNA methylation, or mutations in non-coding regions, may be the cause of disease in the remainder of the patients. Some cases, e.g. a case of incomplete penetrance, or variable expressivity ranging from retinoma to multiple tumours, are discussed in detail. In addition, a case of pre-conception genetic counselling resolved by rescue of banked cordonal blood of the affected deceased child is described.

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

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

RB1 gene mutation up-date, a meta-analysis based on 932 reported mutations available in a searchable database

Background

Retinoblastoma, a prototype of hereditary cancer, is the most common intraocular tumour in children and potential cause of blindness from therapeutic eye ablation, second tumours in germ line carrier's survivors, and even death when left untreated. The molecular scanning of RB1 in search of germ line mutations lead to the publication of more than 900 mutations whose knowledge is important for genetic counselling and the characterization of phenotypic-genotypic relationships.

Results

A searchable database (RBGMdb) has been constructed with 932 published RB1 mutations. The spectrum of these mutations has been analyzed with the following results: 1) the retinoblastoma protein is frequently inactivated by deletions and nonsense mutations while missense mutations are the main inactivating event in most genetic diseases. 2) Near 40% of RB1 gene mutations are recurrent and gather in sixteen hot points, including twelve nonsense, two missense and three splicing mutations. The remainder mutations are scattered along RB1, being most frequent in exons 9, 10, 14, 17, 18, 20, and 23. 3) The analysis of RB1 mutations by country of origin of the patients identifies two groups in which the incidence of nonsense and splicing mutations show differences extremely significant, and suggest the involvement of predisposing ethnic backgrounds. 4) A significant association between late age at diagnosis and splicing mutations in bilateral retinoblastoma patients suggests the occurrence of a delayed-onset genotype. 5) Most of the reported mutations in low-penetrance families fall in three groups: a) Mutations in regulatory sequences at the promoter resulting in low expression of a normal Rb; b) Missense and in-frame deletions affecting non-essential sequence motifs which result in a partial inactivation of Rb functions; c) Splicing mutations leading to the reduction of normal mRNA splicing or to alternative splicing involving either true oncogenic or defective (weak) alleles.

Conclusion

The analysis of RB1 gene mutations logged in the RBGMdb has shown relevant phenotype-genotype relationships and provided working hypothesis to ascertain mechanisms linking certain mutations to ethnicity, delayed onset of the disease and low-penetrance. Gene profiling of tumors will help to clarify the genetic background linked to ethnicity and variable expressivity or delayed onset phenotypes.

Rb plays a role in survival of Abl-dependent human tumor cells as a downstream effector of Abl tyrosine kinase

The retinoblastoma (Rb) gene product is a tumor suppressor that is mutated or inactivated in many types of human cancers. Although Rb is known to be an upstream negative regulator of Abl protein tyrosine kinase, we propose here that Rb also functions as a downstream effector of Abl that plays a positive role in survival of Abl-dependent human tumor cells, including Bcr/Abl-positive chronic myelogenous leukemia (CML). We show that Rb is constitutively phosphorylated at tyrosine in Abl-dependent tumor cells, and that Abl phosphorylates Rb specifically at Y805 within the C-terminal domain of the molecule. We also show that ectopic expression of Rb induces apoptosis in Abl-dependent tumor cells by inhibiting the Abl tyrosine kinase activity, and that Rb-induced apoptosis is compromised by Abl-catalysed phosphorylation of Rb at Y805. Furthermore, the silencing of endogenous Rb by RNA interference induced apoptosis in Abl-dependent tumor cells. Thus, our findings suggest that Abl-catalysed tyrosine phosphorylation of Rb is necessary for survival of Abl-dependent human tumor cells, and raises the possibility that this phosphorylated Rb can be a molecular target for cancer therapy aimed at inducing apoptosis of Abl-dependent tumor cells, such as Bcr/Abl-positive CML.

Molecular determinants for the complex formation between the retinoblastoma protein and LXCXE sequences

The retinoblastoma tumor suppressor protein (pRb) is a key negative regulator of cell proliferation that is frequently disregulated in human cancer. Many viral oncoproteins (for example, HPV E7 and E1A) are known to bind to the pRb pocket domain via a LXCXE binding motif. There are also some 20 cellular proteins that contain a LXCXE motif and have been reported to associate with the pocket domain of pRb. Using NMR spectroscopy and isothermal calorimetry titration, we show that LXCXE peptides of viral oncoproteins bind strongly to the pocket domain of pRb. Additionally, we show that LXCXE-like peptides of HDAC1 bind to the same site on pRb with a weak (micromolar) and transient association. Systematic substitution of residues other than conserved Leu, Cys, and Glu show that the residues flanking the LXCXE are important for the binding, whereas positively charged amino acids in the XLXCXEXXX sequence significantly weaken the interaction.

Analysis of the RB1 gene in children with retinoblastoma having residential connections to West Cumbria, England

Six of eight cases of retinoblastoma previously identified as having a residential association with West Cumbria, England, in which the Sellafield nuclear installation is situated, were examined for the presence of a constitutional RB1 mutation. No mutations were detected, thus providing strong evidence against an environmental or occupational genotoxic effect causing germline mutations in the parents of these children.

Genetic interaction between Rb and K-ras in the control of differentiation and tumor suppression

Although the retinoblastoma protein (pRb) has been implicated in the processes of cellular differentiation, there is no compelling genetic or in vivo evidence that such activities contribute to pRb-mediated tumor suppression. Motivated by cell culture studies suggesting that Ras is a downstream effector of pRb in the control of differentiation, we have examined the tumor and developmental phenotypes of Rb and K-ras double-knockout mice. We find that heterozygosity for K-ras (i) rescued a unique subset of developmental defects that characterize Rb-deficient embryos by affecting differentiation but not proliferation and (ii) significantly enhanced the degree of differentiation of pituitary adenocarcinomas arising in Rb heterozygotes, leading to their prolonged survival. These observations suggest that Rb and K-ras function together in vivo, in the contexts of both embryonic and tumor development, and that the ability to affect differentiation is a major facet of the tumor suppressor function of pRb.

A second primary tumor in a patient with retinoma

AIM

To evaluate the frequency of the co-occurrence of a retinoma and another primary tumor.

METHOD

Presentation of a case report and review of the literature.

RESULTS

A cutaneous melanoma was observed in a 52-year-old man, who was known to have two retinomas in the right eye. Five other cases were found in a review of the literature, suggesting the possibility of an increased risk of developing a second primary tumor in patients with retinoma. The occurrence of cutaneous melanoma as a second primary tumor after retinoma and/or retinoblastoma is discussed.

CONCLUSION

There might be an increased risk for patients with retinoma to develop another primary tumor, necessitating regular follow-up of these patients.

Comprehensive screening for constitutional RB1 mutations by DHPLC and QMPSF

Constitutional mutations of the RB1 gene are associated with a predisposition to retinoblastoma. It is essential to identify these mutations to provide appropriate genetic counseling in retinoblastoma patients, but this represents an extremely challenging task, as the vast majority of mutations are unique and spread over the entire coding sequence. Since 2001, we have implemented RB1 testing on a routine basis as part of the clinical management of retinoblastoma. As most screening techniques do not meet the requirements for efficient RB1 testing, we have devised a semi-automated denaturing high-performance liquid chromatography (DHPLC) method for point mutation detection combined with a quantitative multiplex PCR of short fluorescent fragments (QMPSF) approach to screen for gene rearrangements. We report the results of this comprehensive screening of all exons and promoter of RB1 in 192 unrelated patients, mostly of French origin. Among 102 bilateral and/or familial cases and 90 unilateral sporadic probands, mutations were identified in 83 (81.5%) and 5 (5.5%) cases, respectively. A total of 43 mutations have not been previously reported. The mutational spectrum was found to be significantly different from previous published series, displaying a surprising amount of splice mutations and large deletions. This study demonstrates the reliability of DHPLC for RB1 analysis, but also illustrates the need for a deletion scanning approach. Finally, considering the benefits to retinoblastoma patients, RB1 testing should be widely implemented in routine healthcare because our study clearly illustrates its feasibility.

Two new mutations and three novel polymorphisms in the RB1 gene in Ecuadorian patients

RB1 is the gene responsible for retinoblastoma, the most common malignant intraocular tumor of infancy and early childhood. There are no reports about this gene in Ecuadorian populations, and only a few studies have been published in Latin America about this subject. There is a spectrum of more than 370 mutations described in the RB1 gene mutation database (http://www.d-lohmann.de/Rb/mutations.html), and alterations have been found in 25 of the 27 exons. During the exon-by-exon analysis of 31 tumor and blood samples from Ecuadorian patients, we found two new mutations and three novel polymorphisms. One of the polymorphisms is located in intron 26 where no alterations of the gene have been described previously. The polymorphisms were found in all of the patients' tumor samples, but not in normal population, suggesting there might be a relationship between these polymorphisms and the development of retinoblastoma in the Ecuadorian population.

Sensitive and efficient detection of RB1 gene mutations enhances care for families with retinoblastoma

Timely molecular diagnosis of RB1 mutations enables earlier treatment, lower risk, and better health outcomes for patients with retinoblastoma; empowers families to make informed family-planning decisions; and costs less than conventional surveillance. However, complexity has hindered clinical implementation of molecular diagnosis. The majority of RB1 mutations are unique and distributed throughout the RB1 gene, with no real hot spots. We devised a sensitive and efficient strategy to identify RB1 mutations that combines quantitative multiplex polymerase chain reaction (QM-PCR), double-exon sequencing, and promoter-targeted methylation-sensitive PCR. Optimization of test order by stochastic dynamic programming and the development of allele-specific PCR for four recurrent point mutations decreased the estimated turnaround time to <3 wk and decreased direct costs by one-third. The multistep method reported here detected 89% (199/224) of mutations in bilaterally affected probands and both mutant alleles in 84% (112/134) of tumors from unilaterally affected probands. For 23 of 27 exons and the promoter region, QM-PCR was a highly accurate measure of deletions and insertions (accuracy 95%). By revealing those family members who did not carry the mutation found in the related proband, molecular analysis enabled 97 at-risk children from 20 representative families to avoid 313 surveillance examinations under anesthetic and 852 clinic visits. The average savings in direct costs from clinical examinations avoided by children in these families substantially exceeded the cost of molecular testing. Moreover, health care savings continue to accrue, as children in succeeding generations avoid unnecessary repeated anaesthetics and examinations.

Structural basis for the recognition of the E2F transactivation domain by the retinoblastoma tumor suppressor

Repression of E2F transcription activity by the retinoblastoma (Rb) tumor suppressor through its interaction with the transactivation domain of the E2F transcription factor is one of the central features of G1/S arrest in the mammalian cell cycle. Deregulation of the Rb-E2F interaction results in hyperproliferation, lack of differentiation, and apoptosis, and can lead to cancer. The 2.2-A crystal structure of the Rb pocket complexed with an 18-residue transactivation-domain peptide of E2F-2 reveals that the boomerang-shaped peptide binds to the highly conserved interface between the A-box and the B-box of the Rb pocket in a bipartite manner. The N-terminal segment of the E2F-2 peptide in an extended beta-strand-like structure interacts with helices from the conserved groove at the A-B interface, whereas the C-terminal segment, which contains one 3(10) helix, binds to a groove mainly formed by A-box helices. The flexibility in the middle of the E2F-2 peptide is essential for the tight association of E2F to the Rb pocket. The binding of Rb to the E2F-2 peptide conceals several conserved residues that are crucial for transcription activation of E2F. We provide the structural basis for the Rb-mediated repression of E2F transcription activity without the requirement of histone-modifying enzymes.