404PAdvantages of next-generation sequencing in revealing low-level somatic mosaicism in blood samples of retinoblastoma patients

Background: Retinoblastoma (RB) is an embryonic malignant tumor of retina caused by inactivation of both alleles of the RB1 tumor suppressor gene. Identifying low-level mosaic mutations in RB1 gene in blood samples is challenging. Mosaic mutations arise in early embryogenesis and require high-resolution techniques for detection. In approximately 10% of families, the initial RB1 mutation is mosaic. The ability to identify RB1 mosaicism is important for genetic counseling because mosaicism increases the risk for developing RB in the other eye, second cancers and transmitting the mutation to progeny. Methods: Using NGS we have evaluated the spectrum and frequency of RB1 mutations mosaicism in peripheral blood of 120 patients with sporadic RB (82 unilateral and 38 bilateral). Results: In 5,8% (7/120) of patients a low-level mosaic mutation was found. The spectrum of identified mosaic RB1 mutations, the degree of mosaicism and clinical characteristics are shown in the table. Additionally, we analyzed the Sanger sequencing data from 72 blood samples from patients with sporadic RB (48 unilateral and 24 bilateral), performed earlier in our laboratory, and have not identified mosaic cases.Table: 404PThe spectrum of identified mosaic RB1 mutationsSampleForm (uni/bi-l ateral)Age of onsetMosaic mutationDegree of mosaicism,%Mutation in relativesRB1Uni-6 moc.C1363T (14ex)C:80, T:20NegativeRB2Uni-1yr 8 moc.C1363T (14ex)C:74, T:26NegativeRB3Bi-3 weeksc.2326delC (23ex)wt:80, del:20NegativeRB4Uni-3 yrc.C958T (10ex)C:80, T:20NegativeRB5Uni-3 moc.C751T (8ex)C:84, T:16NegativeRB6Uni-1 yr 8 moc.1215 + 1G-A (12ex)G:87, A:13NegativeRB7Uni-8 moc.C1735T (18ex)C:79, T:21NegativeRB8No symptomsNoc.887delT (9ex)wt:85, del:15Positive bilateral daughter Open table in a new tab Conclusions: NGS is an efficient method for detecting low-level mosaic mutations in blood samples from RB patients. Clinically, RB1 mosaicism is variable, it can manifest as a bilateral or a unilateral form with early or late onset, or without disease, which make the diagnosis even more difficult. Legal entity responsible for the study: FASO Russia. Funding: The research was carried out within the State assignment of FASO Russia. Disclosure: All authors have declared no conflicts of interest.