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

Abstract

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.

RB1 Germline Variant Predisposing to a Rare Ovarian Germ Cell Tumor: A Case Report

“…This allowed us to hypothesize a “parent-of-origin” effect, a broadly studied mechanism in retinoblastoma. Hypomorphic variants affecting the paternal allele are more likely to predispose to RB than the ones on the maternal allele, while, maternally-inherited hypomorphic variants usually don't lead to RB development, but can predispose to other neoplasms in the adult age ( 14 , 18 , 20 , 21 ). Here the proband was not affected by retinoblastoma in her childhood, and, as expected, the variant was inherited from the mother.…”

“…RB1 is regulated by an imprinting mechanism which is responsible for a skewed gene expression in favor of the maternal allele, that is about 3 times more transcribed than the paternal one (18,19). A parent-of-origin effect, due to this imprinting mechanism, has been proposed as an explanation for low penetrant pedigrees, in whom hypomorphic RB1 variant inherited from the mother retain sufficient activity to suppress retinoblastoma development and carriers can show only benign lesions named retinoma or developing other types of cancer in adulthood (20)(21)(22).…”

Retinoblastoma (hereditary predisposition)

“…Predisposition to retinoblastoma is transmitted as an autosomal dominant trait trait with incomplete penetrance (Vogel F, 1979;Eloy P et al, 2016;Imperatore V et al, 2018);; it is caused by mutations in the RB1 gene; there is also a non-hereditary form of retinoblastoma (mostly in children with isolated unilateral retinoblastoma) that is caused by RB1mutations confined to somatic cells…”

“…These mutations, mostly represented by nonsense and frameshift changes resulting in premature termination codons (PTC), are generally associated with full penetrance (Valverde JR et al, 2005). RB1 amorphic mutations can be associated to reduced penetrance in association with somatic mosaicism (Imperatore V et al, 2018). Hypomorphic mutations partially inactivating protein function or reducing gene expression combined with a parent-of-origin effect can also be associated to incomplete penetrance or variable expressivity (Kanber D et al, 2009;Eloy P et al, 2016;Imperatore V et al, 2018).…”

“…RB1 amorphic mutations can be associated to reduced penetrance in association with somatic mosaicism (Imperatore V et al, 2018). Hypomorphic mutations partially inactivating protein function or reducing gene expression combined with a parent-of-origin effect can also be associated to incomplete penetrance or variable expressivity (Kanber D et al, 2009;Eloy P et al, 2016;Imperatore V et al, 2018). Another important class of RB1 oncogenic events is represented by large rearrangements (~15%;Taylor M et al, 2007).…”