Whole-genome sequencing of spermatocytic tumors provides insights into the mutational processes operating in the male germline

Abstract

Adult male germline stem cells (spermatogonia) proliferate by mitosis and, after puberty, generate spermatocytes that undertake meiosis to produce haploid spermatozoa. Germ cells are under evolutionary constraint to curtail mutations and maintain genome integrity. Despite constant turnover, spermatogonia very rarely form tumors, so-called spermatocytic tumors (SpT). In line with the previous identification of FGFR3 and HRAS selfish mutations in a subset of cases, candidate gene screening of 29 SpTs identified an oncogenic NRAS mutation in two cases. To gain insights in the etiology of SpT and into properties of the male germline, we performed whole-genome sequencing of five tumors (4/5 with matched normal tissue). The acquired single nucleotide variant load was extremely low (~0.2 per Mb), with an average of 6 (2–9) non-synonymous variants per tumor, none of which is likely to be oncogenic. The observed mutational signature of SpTs is strikingly similar to that of germline de novo mutations, mostly involving C>T transitions with a significant enrichment in the ACG trinucleotide context. The tumors exhibited extensive aneuploidy (50–99 autosomes/tumor) involving whole-chromosomes, with recurrent gains of chr9 and chr20 and loss of chr7, suggesting that aneuploidy itself represents the initiating oncogenic event. We propose that SpT etiology recapitulates the unique properties of male germ cells; because of evolutionary constraints to maintain low point mutation rate, rare tumorigenic driver events are caused by a combination of gene imbalance mediated via whole-chromosome aneuploidy. Finally, we propose a general framework of male germ cell tumor pathology that accounts for their mutational landscape, timing and cellular origin.

Molecular Heterogeneity and Early Metastatic Clone Selection in Testicular Germ Cell Cancer Development

“…TGCC carried low numbers of SNV (~0.1 per Mb, Fig. 1B), somewhat lower than reported previously [12][13][14] and comparable to some of the pediatric cancers and spermatocytic tumors 28,30 . The total number of SNV with a predicted impact on the encoded protein (3 -16, Table 1) was similar to other reports [11][12][13] .…”

Selfish mutations dysregulating RAS-MAPK signaling are pervasive in aged human testes

“…This would represent a mechanism by which the testis "filters" the transmission of pathogenic mutations across generations, although proof of this concept would require the development of ultrasensitive assays to screen large numbers of sperm samples. It is noteworthy that despite the relative abundance of strongly oncogenic mutations in the adult male germline, testicular tumors originating from adult spermatogonia (spermatocytic tumors) are extremely rare, with an incidence of about one per million men and are mostly benign in nature (Ghazarian et al 2015;Giannoulatou et al 2017).…”

Cytokines in Male Fertility and Reproductive Pathologies: Immunoregulation and Beyond

“…However, the mechanisms underlying GCNIS establishment and progression into either subtype are largely obscure, particularly because this process may take years to decades. Genome-wide association studies (GWAS) have most frequently identified KITL and KRAS alleles as the highest genetic risk factor ( 81 ), providing clues to processes or events that may enable inappropriate survival of germ cells that do not differentiate. Mice and other common laboratory animals do not develop TGCTs similar to those found in humans; the timespan from PGC formation to onset of spermatogenesis in a mouse is less than one month, while in humans this transformation typically lasts several years.…”