Cellular correlates of selfish spermatogonial selection

Parental mosaicism detection and preimplantation genetic testing in families with multiple transmissions of de novo mutations

BACKGROUND

De novo mutations (DNMs) are linked with many severe early-onset disorders ranging from rare congenital malformation to intellectual disability. Conventionally, DNMs are considered to have an estimated recurrence rate of 1%. Recently, studies have revealed a higher prevalence of parental mosaicism, leading to a greater recurrence risk, resulting in a second child harbouring the same DNM as a previous child.

METHODS

In this study, we included 10 families with DNMs leading to adverse pregnancy outcomes. DNA was extracted from tissue samples, including parental peripheral blood, parental saliva and paternal sperm. High-throughput sequencing was used to screen for parental mosaicism with a depth of more than 5000× on average and a variant allele fraction (VAF) detection limit of 0.5%.

RESULTS

The presence of mosaicism was detected in sperms in two families, with VAFs of 2.8% and 2.5%, respectively. Both families have a history of multiple adverse pregnancies and DNMs shared by siblings. Preimplantation genetic testing (PGT) and prenatal diagnosis were performed in one family, thereby preventing the reoccurrence of DNMs.

CONCLUSION

This study is the first to report the successful implementation of PGT for monogenic/single gene defects in the parental mosaicism family. Our study suggests that mosaic detection of paternal sperm is warranted in families with recurrent DNMs leading to adverse pregnancy outcomes, and PGT can effectively block the transmission of the pathogenic mutation.

Germline selection of PTPN11 (HGNC:9644) variants make a major contribution to both Noonan syndrome's high birth rate and the transmission of sporadic cancer variants resulting in fetal abnormality

Some spontaneous germline gain-of-function mutations promote spermatogonial stem cell clonal expansion and disproportionate variant sperm production leading to unexpectedly high transmission rates for some human genetic conditions. To measure the frequency and spatial distribution of de novo mutations we divided three testes into 192 pieces each and used error-corrected deep-sequencing on each piece. We focused on PTPN11 (HGNC:9644) Exon 3 that contains 30 different PTPN11 Noonan syndrome (NS) mutation sites. We found 14 of these variants formed clusters among the testes; one testis had 11 different variant clusters. The mutation frequencies of these different clusters were not correlated with their case-recurrence rates nor were case recurrence rates of PTPN11 variants correlated with their tyrosine phosphatase levels thereby confusing PTPN11's role in germline clonal expansion. Six of the PTPN11 exon 3 de novo variants associated with somatic mutation-induced sporadic cancers (but not NS) also formed testis clusters. Further, three of these six variants were observed among fetuses that underwent prenatal ultrasound screening for NS-like features. Mathematical modeling showed that germline selection can explain both the mutation clusters and the high incidence of NS (1/1000-1/2500).

Ageing in men with normal spermatogenesis alters spermatogonial dynamics and nuclear morphology in Sertoli cells

BACKGROUND

Ageing in men is believed to be associated with fertility decline and elevated risk of congenital disorders for the offspring. The previous studies also reported reduced germ and Sertoli cell numbers in older men. However, it is not clear whether ageing in men with normal spermatogenesis affects the testis and germ cell population dynamics in a way sufficient for transmitting adverse age effects to the offspring.

OBJECTIVES

We examined men with normal spermatogenesis at different ages concerning effects on persisting testicular cell types, that is the germ line and Sertoli cells, as these cell populations are prone to be exposed to age effects.

MATERIAL AND METHODS

Ageing was assessed in testicular biopsies of 32 patients assigned to three age groups: (i) 28.8 ± 2.7 years; (ii) 48.1 ± 1 years; and (iii) 70.9 ± 6.2 years, n = 8 each, with normal spermatogenesis according to the Bergmann-Kliesch score, and in a group of meiotic arrest patients (29.9 ± 3.8 years, n = 8) to decipher potential links between different germ cell types. Besides morphometry of seminiferous tubules and Sertoli cell nuclei, we investigated spermatogenic output/efficiency, and dynamics of spermatogonial populations via immunohistochemistry for MAGE A4, PCNA, CREM and quantified A-pale/A-dark spermatogonia.

RESULTS

We found a constant spermatogenic output (CREM-positive round spermatids) in all age groups studied. In men beyond their mid-40s (group 2), we detected increased nuclear and nucleolar size in Sertoli cells, indirectly indicating an elevated protein turnover. From the 7th decade (group 3) of life onwards, testes showed increased proliferation of undifferentiated spermatogonia, decreased spermatogenic efficiency and elevated numbers of proliferating A-dark spermatogonia.

DISCUSSION AND CONCLUSION

Maintaining normal sperm output seems to be an intrinsic determinant of spermatogenesis. Ageing appears to affect this output and might provoke compensatory proliferation increase in A spermatogonia which, in turn, might hamper germ cell integrity.

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

Mosaic mutations present in the germline have important implications for reproductive risk and disease transmission. We previously demonstrated a phenomenon occurring in the male germline, whereby specific mutations arising spontaneously in stem cells (spermatogonia) lead to clonal expansion, resulting in elevated mutation levels in sperm over time. This process, termed "selfish spermatogonial selection," explains the high spontaneous birth prevalence and strong paternal age-effect of disorders such as achondroplasia and Apert, Noonan and Costello syndromes, with direct experimental evidence currently available for specific positions of six genes (FGFR2, FGFR3, RET, PTPN11, HRAS, and KRAS). We present a discovery screen to identify novel mutations and genes showing evidence of positive selection in the male germline, by performing massively parallel simplex PCR using RainDance technology to interrogate mutational hotspots in 67 genes (51.5 kb in total) in 276 biopsies of testes from five men (median age, 83 yr). Following ultradeep sequencing (about 16,000×), development of a low-frequency variant prioritization strategy, and targeted validation, we identified 61 distinct variants present at frequencies as low as 0.06%, including 54 variants not previously directly associated with selfish selection. The majority (80%) of variants identified have previously been implicated in developmental disorders and/or oncogenesis and include mutations in six newly associated genes (BRAF, CBL, MAP2K1, MAP2K2, RAF1, and SOS1), all of which encode components of the RAS-MAPK pathway and activate signaling. Our findings extend the link between mutations dysregulating the RAS-MAPK pathway and selfish selection, and show that the aging male germline is a repository for such deleterious mutations.

Paternal age and risk of testicular germ cell tumors: a cohort study of 1,000,000 men

Testicular germ cell tumors (TGCT) are the most frequent cancer among young men, with increasing incidence worldwide. Advanced paternal age has been linked to adverse health outcomes in offspring, but reports on the association of paternal age with TGCT are few and inconsistent. We aimed to examine the relationship of paternal age (PAB) at birth with the risk of TGCT and by histologic type: seminoma and non-seminoma. A population-based cohort of 1,056,058 males, examined at ages 16-19 between the years 1980-2011, was linked to the Israel National Cancer Registry to obtain incident TGCT through 2012. We applied multivariable Cox regression. During 16.5 million person-years of follow-up, 1247 incident cases (604 seminomas and 643 non-seminomas) were detected. Increasing PAB was linearly associated with lower risk of TGCT (HR_{per year}  = 0.983, 95% CI: 0.974-0.993, p = 0.001), after adjustment for year of birth, years of education, height, cryptorchidism history and origin, and also with additional adjustment for maternal age at birth (MAB) (HR_{per year}  = 0.980: 0.965-0.995, p = 0.008). The association was stronger for seminoma (HR_{per year}  = 0.968: 0.946-0.989, p = 0.004) and persisted in a subset adjusted for sibship size (HR_{per year}  = 0.950: 0.917-0.983, p = 0.003). In the fully adjusted model, young PAB (15-24 vs. ≥30) was a risk factor for seminoma (HR = 1.41: 1.07-1.85, p = 0.014). In models adjusted for PAB, MAB was not associated with risk of TGCT. In conclusion, our findings suggest that young paternal age is a risk factor of TGCT, especially seminoma. The findings warrant further investigation into the possible impact of young paternal age on their offsprings' testes.