Parental germline mosaicism in genome-wide phased de novo variants: recurrence risk assessment and implications for precision genetic counselling

Abstract

Background: De novo mutations (DNMs) significantly impact health, particularly through developmental disorders. DNMs occur in both paternal and maternal germlines via diverse mechanisms including parental early embryonic mosaicism, which increases recurrence risk for future pregnancies through germline mosaicism. Embryonic mosaicism is divided based on primordial germ cell specification (PGCS): pre-PGCS events may affect both germline and somatic tissues, while post-PGCS events are only found in the germline. The specific contribution of germline mosaicism to DNMs across the genome is not well defined. We aimed at categorizing DNMs and their recurrence risk by detecting a large set of DNMs followed by systematic deep sequencing of parental blood and sperm DNA. Methods: We performed trio-based short-read genome sequencing for initial DNM detection and long-read genome sequencing for phasing, followed by high-depth targeted sequencing of parental blood and paternal sperm to detect germline mosaicism. Results: We detected a total of 428 DNMs (on average 85.6 per trio, n = 5 trios), with an expected paternal bias of 80%. Targeted resequencing of parental blood and sperm (depth > 5000x) unveiled 20/334 parental germline mosaics (2–5 per trio) with variant allele fractions (VAFs) ranging from 0.24–14.7%, including 7 that were detected in paternal sperm exclusively (1–2 per trio). We estimate that individual genomes harbour about 2 paternal and 2 maternal pre-PGCS DNMs and 2 paternal post-PGCS DNMs (detectable in sperm only). Due to paternal bias, maternally phased variants appear 3.4x more likely to be mosaic in blood. By using average VAFs in sperm as a direct indicator, we estimate recurrence risk of genome -wide paternally phased de novo variants to be 0.3%, prior to any sperm sequencing assessment. This estimate is an average between a majority of variants with a null recurrence risk and a handful of variants with a high recurrence risk. Conclusions: Genetic counselling of DNM may not rely anymore on empirical estimates of recurrence risk. Sperm sequencing may be an effective method to reliably specify the recurrence risk of most individual DNMs. Long-read sequencing, allowing the phasing of DNMs, may also become critical in this process.