Identification of a mutation in exon 27 of the RB1 gene associated with incomplete penetrance retinoblastoma

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.

Genetics and Molecular Diagnostics in Retinoblastoma--An Update

Retinoblastoma is the prototype genetic cancer: in one or both eyes of young children, most retinoblastomas are initiated by biallelic mutation of the retinoblastoma tumor suppressor gene, RB1, in a developing retinal cell. All those with bilateral retinoblastoma have heritable cancer, although 95% have not inherited the RB1 mutation. Non-heritable retinoblastoma is always unilateral, with 98% caused by loss of both RB1 alleles from the tumor, whereas 2% have normal RB1 in tumors initiated by amplification of the MYCN oncogene. Good understanding of retinoblastoma genetics supports optimal care for retinoblastoma children and their families. Retinoblastoma is the first cancer to officially acknowledge the seminal role of genetics in cancer, by incorporating "H" into the eighth edition of cancer staging (2017): those who carry the RB1 cancer-predisposing gene are H1; those proven to not carry the familial RB1 mutation are H0; and those at unknown risk are HX. We suggest H0* be used for those with residual <1% risk to carry a RB1 mutation due to undetectable mosaicism. Loss of RB1 from a susceptible developing retinal cell initiates the benign precursor, retinoma. Progressive genomic changes result in retinoblastoma, and cancer progression ensues with increasing genomic disarray. Looking forward, novel therapies are anticipated from studies of retinoblastoma and metastatic tumor cells and the second primary cancers that the carriers of RB1 mutations are at high risk to develop. Here, we summarize the concepts of retinoblastoma genetics for ophthalmologists in a question/answer format to assist in the care of patients and their families.

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.

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.

Recent advances in retinoblastoma genetic research

PURPOSE OF REVIEW

Retinoblastoma is a pediatric eye tumor that serves as a paradigm for understanding the genetic basis of cancer. This review will highlight recent advances in retinoblastoma genetic research and discuss how these new findings influence our knowledge of retinoblastoma tumorigenesis and management.

RECENT FINDINGS

Current data demonstrate that retinomas, benign retinal tumors found in some retinoblastoma patients, exhibit bi-allelic mutations in RB1, the retinoblastoma gene, and lack of expression of the retinoblastoma protein. Interestingly, retinomas demonstrate a low level of genomic instability that becomes progressively more severe in retinoblastoma tumors. Additionally, a subset of retinomas share genomic alterations with retinoblastoma. Collectively, these data suggest that retinomas represent true premalignant lesions and not regressed retinoblastoma tumors, as previously thought. Translational advances in retinoblastoma genetic research include development of an allele-specific assay that now enables the identification of mutational mosaicism, thereby increasing the rate of RB1 mutation detection in bilaterally affected patients to as high as 95%.

SUMMARY

These and related research efforts reveal novel data that enhance our understanding of the biology of retinoblastoma. These observations may facilitate new therapeutic approaches to further decrease the morbidity and mortality associated with retinoblastoma and other more common forms of cancer.