Gene-knockout by iSTOP enables rapid reproductive disease modeling and phenotyping in germ cells of the founder generation

Cytosine base editing achieves C•G-to-T•A substitutions and can convert four codons (CAA/CAG/CGA/TGG) into STOP-codons (induction of STOP-codons, iSTOP) to knock out genes with reduced mosaicism. iSTOP enables direct phenotyping in founders' somatic cells, but it remains unknown whether this works in founders' germ cells so as to rapidly reveal novel genes for fertility. Here, we initially establish that iSTOP in mouse zygotes enables functional characterization of known genes in founders' germ cells: Cfap43-iSTOP male founders manifest expected sperm features resembling human "multiple morphological abnormalities of the flagella" syndrome (i.e., MMAF-like features), while oocytes of Zp3-iSTOP female founders have no zona pellucida. We further illustrate iSTOP's utility for dissecting the functions of unknown genes with Ccdc183, observing MMAF-like features and male infertility in Ccdc183-iSTOP founders, phenotypes concordant with those of Ccdc183-KO offspring. We ultimately establish that CCDC183 is essential for sperm morphogenesis through regulating the assembly of outer dynein arms and participating in the intra-flagellar transport. Our study demonstrates iSTOP as an efficient tool for direct reproductive disease modeling and phenotyping in germ cells of the founder generation, and rapidly reveals the essentiality of Ccdc183 in fertility, thus providing a time-saving approach for validating genetic defects (like nonsense mutations) for human infertility.

Progress in the genetic studies of spermatogenesis abnormalities

About 15% couples suffer from infertility, half of which are caused by male factors. Male infertility usually manifests as teratozoospermia, oligospermia and/or asthenospermia, of which the most severe form is azoospermia. In this review, we summarize the recent progress in the study of genetic factors involved in nonobstructive azoospermia and teratozoospermia, Recently, with the rapid development of high-throughput chips and sequencing technologies, many genetic factors of spermatogenesis have been discovered and analyzed. For the nonobstructive azoospermia, genome-wide association studies (GWAS) and high-throughput sequencing revealed many risk loci of nonobstructive azoospermia. For the teratozoospermia, the application of whole-exome sequencing (WES) revealed a series of disease-causing genes, greatly enriching our knowledge of teratozoospermia including multiple morphological abnormalities of the flagella (MMAF). The discovery of lots of disease genes helped the characterization of the pathological mechanisms of male infertility. Therefore, a comprehensive and in-depth understanding of genetic factors in spermatogenesis abnormalities will play important roles in the clinical diagnosis, treatment and genetic counseling of male infertility.