Sperm motility is vital to human reproduction. Malformations of sperm flagella can cause male infertility. Men with multiple morphological abnormalities of the flagella (MMAF) have abnormal spermatozoa with absent, short, coiled, bent, and/or irregular-caliber flagella, which impair sperm motility. The known human MMAF-associated genes, such as DNAH1, only account for fewer than 45% of affected individuals. Pathogenic mechanisms in the genetically unexplained MMAF remain to be elucidated. Here, we conducted genetic analyses by using whole-exome sequencing and genome-wide comparative genomic hybridization microarrays in a multi-center cohort of 30 Han Chinese men affected by MMAF. Among them, 12 subjects could not be genetically explained by any known MMAFassociated genes. Intriguingly, we identified compound-heterozygous mutations in CFAP43 in three subjects and a homozygous frameshift mutation in CFAP44 in one subject. All of these recessive mutations were parentally inherited from heterozygous carriers but were absent in 984 individuals from three Han Chinese control populations. CFAP43 and CFAP44, encoding two cilia-and flagella-associated proteins (CFAPs), are specifically or preferentially expressed in the testis. Using CRISPR/Cas9 technology, we generated two knockout models each deficient in mouse ortholog Cfap43 or Cfap44. Notably, both Cfap43-and Cfap44-deficient male mice presented with MMAF phenotypes, whereas the corresponding female mice were fertile. Our experimental observations on human subjects and animal models strongly suggest that biallelic mutations in either CFAP43 or CFAP44 can cause sperm flagellar abnormalities and impair sperm motility. Further investigations on other CFAP-encoding genes in more genetically unexplained MMAF-affected individuals could uncover novel mechanisms underlying sperm flagellar formation.

Male infertility is a major concern affecting human reproductive health. Asthenoteratospermia can cause male infertility through reduced motility and abnormal morphology of spermatozoa. Several genes, including DNAH1 and some CFAP family members, are involved in multiple morphological abnormalities of the sperm flagella (MMAF). However, these known genes only account for approximately 60% of human MMAF cases. Here, we conducted further genetic analyses by using whole-exome sequencing in a cohort of 65 Han Chinese men with MMAF. Intriguingly, bi-allelic mutations of TTC21A (tetratricopeptide repeat domain 21A) were identified in three (5%) unrelated, MMAF-affected men, including two with homozygous stop-gain mutations and one with compound heterozygous mutations of TTC21A. Notably, these men consistently presented with MMAF and additional abnormalities of sperm head-tail conjunction. Furthermore, a homozygous TTC21A splicing mutation was identified in two Tunisian cases from an independent MMAF cohort. TTC21A is preferentially expressed in the testis and encodes an intraflagellar transport (IFT)-associated protein that possesses several tetratricopeptide repeat domains that perform functions crucial for ciliary function. To further investigate the potential roles of TTC21A in spermatogenesis, we generated Ttc21a mutant mice by using CRISPR-Cas9 technology and revealed sperm structural defects of the flagella and the connecting piece. Our consistent observations across human populations and in the mouse model strongly support the notion that bi-allelic mutations in TTC21A can induce asthenoteratospermia with defects of the sperm flagella and head-tail conjunction.

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