Paternal MTHFR deficiency leads to hypomethylation of young retrotransposons and reproductive decline across two successive generations

Abstract

5, 10-Methylenetetrahydrofolate reductase (MTHFR) is a crucial enzyme in the folate metabolic pathway with a key role in generating methyl groups. As MTHFR deficiency impacts male fertility and sperm DNA methylation, there is the potential for epimutations to be passed to the next generation. Here, we assessed whether the impact of MTHFR deficiency on testis morphology and sperm DNA methylation is exacerbated across generations. While MTHFR deficiency in F1 fathers has only minor effects on sperm counts and testis weights and histology, F2 generation sons show further deterioration in reproductive parameters. Extensive loss of DNA methylation is observed in both F1 and F2 sperm, with >80% of sites shared between generations, suggestive of regions consistently susceptible to MTHFR deficiency. These regions are generally methylated during late embryonic germ cell development and are enriched in young retrotransposons. As retrotransposons are resistant to reprogramming of DNA methylation in embryonic germ cells, their hypomethylated state in the sperm of F1 males could contribute to the worsening reproductive phenotype observed in F2 MTHFR- deficient males, findings compatible with the intergenerational passage of epimutations.

Metabolic pathways regulating the development and non-genomic heritable traits of germ cells

“…Providing betaine-supplemented water to Mthfr homozygous mice from the embryonic stage to 3 months of age results in recovery of spermatogenesis and sperm number and a marked increase in fertility [ 49 ]. These results highlight the importance of methionine production in spermatogenesis and consistently suggest the involvement of DNA hypomethylation in spermatogenic failure [ 50 ]. Mthfr deficiency causes DNA hypomethylation in young retrotransposons, such as LINE-1 in the sperm, suggesting that hypomethylation-associated abnormal expression of LINE-1 could result in DNA damage and consequent spermatogenesis defects [ 50 ], as in Shmt2 KD oocytes described in the previous section [ 43 ].…”

“…These results highlight the importance of methionine production in spermatogenesis and consistently suggest the involvement of DNA hypomethylation in spermatogenic failure [ 50 ]. Mthfr deficiency causes DNA hypomethylation in young retrotransposons, such as LINE-1 in the sperm, suggesting that hypomethylation-associated abnormal expression of LINE-1 could result in DNA damage and consequent spermatogenesis defects [ 50 ], as in Shmt2 KD oocytes described in the previous section [ 43 ]. In addition, clinical evidence suggests that a functional homozygous polymorphism of MTHFR (677C→T) in humans causes reduced male fertility [ 51 ], suggesting an evolutionarily conserved role for MTHFR in spermatogenesis.…”

“…Collectively, these results indicate that proper folate cycle activity leading to an adequate methionine supply ( Fig. 4B ) in male germ cells is necessary to establish the normal epigenetic profiles required for normal development of the offspring as well as for spermatogenesis, as discussed in the previous section [ 49 , 50 , 51 ].…”

Andrology and humanities

“…In addition, environmental factors promote epigenetic transgenerational inheritance of disease such as sperm defects, prostate and kidney abnormalities and metabolic disfunctions. [2][3][4] Therefore, in addition to studying reproductive fitness and the impact of the environment, there is the need to implement policies to prevent exposure to hazardous chemicals that impair fertility, and regulatory requirements concerning the effect of pharmaceuticals on sperm quantity, quality, and function need to be strengthened. Education programs raising awareness that agricultural, economical, and life choices might impact the reproductive health of populations and individuals of both sexes are critical.…”

“…Further, there is growing evidence that testis function and semen quality correlate with a man's health and that the father's age affects offspring fitness. In addition, environmental factors promote epigenetic transgenerational inheritance of disease such as sperm defects, prostate and kidney abnormalities and metabolic disfunctions 2–4 . Therefore, in addition to studying reproductive fitness and the impact of the environment, there is the need to implement policies to prevent exposure to hazardous chemicals that impair fertility, and regulatory requirements concerning the effect of pharmaceuticals on sperm quantity, quality, and function need to be strengthened.…”