Meiotic arrest of the male germline

In western societies, ~7% of all males are infertile, but the underlying causes often remain elusive. Azoospermia is the most severe form and is defined by the complete lack of spermatids in the ejaculate. If, in addition, no sperm can be found in the testis this is referred to as non-obstructive azoospermia (NOA), which is often caused by meiotic arrest (MeiA). Here, the germ cells arrest at various stages of their differentiation and the production of mature spermatids is hindered.

Within one of our projects, we recently identified meiosis 1 arresting protein (M1AP) as a novel autosomal-recessive candidate gene causing meiotic arrest, NOA and leading to male infertility.

Figure 1: Wildtype M1AP genotype (A) with complete spermatogenesis in the testis (B/C) compared to a LoF variant in M1AP (D) leading to MeiA in the patient (E/F). The duplication (c.676dup) results in a frameshift and premature stop codon (p.Trp226fsTer4). Transfection of HEK293T cells reveals expression of a truncated protein (G).

In our MERGE cohort (Male Reproductive Genomics) comprising more than 900 exomes of infertile men, including 64 with meiotic arrest, we identified three men with a homozygous loss-of-function (LoF) variant in M1AP resulting in a severely truncated and therefore very likely non-functional protein. Our finding was validated by the identification of three additional men with NOA in cohorts from our collaboration partners of the International Male Infertility Genomics Consortium (IMIGC). Independently of our finding, M1AP was identified as a causal gene for male infertility in a consanguineous family from Turkey. A previously published M1AP knockout mouse was described as infertile and showed a histological phenotype comparable to the patients. In men and mice, M1AP is predominantly expressed in the testis. We therefore provide multiple lines of evidence that M1AP plays a crucial role during spermatogenesis and meiosis.

Very little is known about the precise expression pattern, the subcellular localisation or the cellular function of the M1AP protein. Therefore, we aim to investigate M1AP on several levels: by expressing M1AP in cellular model systems, analysing human testicular samples, and generating a new knockout mouse using molecular biological, protein biochemical, and microscopical techniques, we will examine M1AP under impaired and wildtype conditions. Finally, we aim to identify interaction partners of M1AP using biomolecular mass spectrometry. This will identify additional candidate genes for male infertility and further elucidate the cellular network of M1AP.

In perspective, our projects will shed more light onto specific meiotic processes and the origin of meiotic arrest leading to male infertility and further decrease the percentage of unsolved male infertility cases.