Role of testis‑specific serine kinase 1B in undiagnosed male infertility

Abstract

Male infertility is a global problem affecting a considerable part of the male population. Current guidelines and practices aimed at diagnosing the cause of this problem still have low diagnostic yield. As novel candidate genes for infertility emerge, their functional role needs to be investigated in patient populations. The present study aimed to investigate testis‑specific serine kinase 1B (TSSK1B), which was discovered in a previously diagnosed patient. Sanger sequencing of the coding regions and exon borders of TSSK1B was performed in a cohort of 100 male Bulgarian patients with unresolved infertility causes. Missense mutations were discovered in 10% of patients and were associated with clinical data on sperm dysmorphology. Two previously unreported mutations were discovered, p.3D>N and p.52F>L. All mutations were scored via in silico predictors and protein modelling using AlphaFold2. The present findings indicated an association between TSSK1B mutations and asthenoteratozoospermia, with further missense mutations in patients with azoospermia and teratozoospermia. Mutations in TSSK1B may be a cause of undiagnosed cases of male infertility and should be considered when molecular diagnostics are warranted.

Keywords: asthenoteratozoospermia; genetics; male infertility; protein modelling; sequencing; testis‑specific serine kinase 1B.

Figure 1.

Tree diagram of the evolutionary origin of Tssk1B. Dashed lines represent multiple branches not shown here. Solid lines represent direct descendants. White, pseudogene; gray, normally expressed genes. Strepsirrhini, Tarsiiformes and other mammalian species appearing before 100 million years ago possess Tssk1 and Tssk2 genes located on the same chromosome, separated by a sequence of ~3 kb. Semiiformes have a Tssk1B gene translocated on another chromosome, and in the case of Platyrrhini multiple copies of the gene. In Semiiformes Tssk1A becomes a pseudogene due to acquisition of Tssk1B. Tssk, testis-specific serine kinase.

Figure 2.

Illustrative representation of spermatid elongation and chromatoid body segregation. (A) Round spermatid carrying a chromatoid body containing primarily MIWI proteins, located around the nucleus. (B) Early stage of spermatid elongation, where CB loses MIWI proteins but acquires Tssk1, 2 and TSSK and segregates into a satellite and ring. (C) During spermatid elongation the ring shrinks and moves downstream of the spermatid tail along with the annulus. Above the ring, mitochondria interact with the axoneme to form a mitochondrial sheath. Later, the centriole degrades and cytoplasmic volume decreases. Tssk, testis-specific serine kinase; CB, chromatoid body; MIWI, murine P-element Induced wimpy testis family member proteins.

Figure 3.

Alignment of the canonical TSSK1B protein and models created with AlphaFold2. Arrows indicate folding alterations. (A) canonical TSSK1B protein, where A1 and A2 show the normal folding. (B) Protein model of the p.66M>V mutation, showing two small helix motifs not found in the canonical TSSK1B protein (B1 and B2). Tssk, testis-specific serine kinase.