Lack of CCDC146, a ubiquitous centriole and microtubule-associated protein, leads to non-syndromic male infertility in human and mouse

Reviewer #1 (Public Review):

In this study, Muronova et al., demonstrate the physiological importance of a centriole and microtubule-associated protein, CCDC146, in sperm flagellar formation and male reproduction. In a previous study, the authors identified two loss-of-function mutations in CCDC146 from the sterile males with multiple morphological abnormalities in flagellar (MMAF) phenotype. To further test physiological significance of the CCDC146, the authors generate its knockout mouse model. The knockout males share the MMAF phenotypes with severely impaired flagellar morphology due to the defective sperm generation in testes. Using CCDC146 knock-in mouse model and expansion microscopy techniques, the authors observed CCDC146 localizes at human and mouse sperm flagella, which is different from the somatic cells. The authors also observed impaired manchette and head-tail coupling apparatus in developing spermatid lacking CCDC146 and address CCDC146 loss-of-function induces molecular and structural defects at axoneme in developing male germ cells, which finally causes MMAF phenotype and male infertility.

This reviewer agrees that identifying and analyzing new pathogenic molecules and variants is hugely valuable to establish male infertility in genetic level. As the authors have done, this study also enlarges the genetic causality underlying MMAF and male infertility. In addition, this study applies new techniques, expansion microscopy, which is also an innovative approach. Although many approaches are used, unfortunately, this study misses the molecular mechanisms to explain pathogenicity to cause MMAF by the CCDC146. Only intracellular localization of the molecule is heavily examined. Although the authors show defective intracellular localization of the centriole and manchette, how CCDC146 loss-of-function and the developmental defects are linked is not examined. These limits provide the impression that this study could be simply another identification of the MMAF-causing gene, which were heavily performed by the authors. Also, in many parts, the results do not clearly support the authors claim. Therefore, this reviewer thinks the current manuscript requires additional results to clearly explain molecular mechanisms underlying the pathogenicity by CCDC146 loss-of-function.

Reviewer #2 (Public Review):

In whole exome sequencing of two patients suffering from MMAF syndrome, mutations of CCDC146 gene that result in premature stop codons were identified. The position of mutations could result in a truncated form of protein, thus whether these patients do indeed lack CCDC146 protein or if present, whether the truncated protein is functional, is unanswered by showing the CCDC146 protein localization only in the sperm from healthy donors. The main claim that CCDC146 protein is microtubule associated protein in the axoneme is well supported imaging expanded sperm flagellum to increase spatial resolution. However, the author's claim that the signal in the mid-piece is not specific is less supported by experimental evidence. The detection of CCDC146 in the sperm head is not further explored while TEM images show spermatogenesis defects in the manchette and acrosome formation. Increased detection of the CCDC146 protein in mouse sperm with sarkosyl supports its association with microtubules but does not exclude its potential role in the formation of sperm head. Overall, this study provides valuable information on CCDC146 function in male germ cells during spermatogenesis.

Reviewer #3 (Public Review):

Male infertility is an important health problem. Among pathologies with multiple morphological abnormalities of the flagellum (MMAF), only 50% of the patients have no identified genetic causes. It is thus primordial to find novel genes that cause the MMAF syndrome. In the current work, the authors follow up the identification of two patients with MMAF carrying a mutation in the CCDC146 gene. To understand how mutations in CCDC146 lead to male infertility, the authors generated two mouse models: a CCDC146-knockout mouse, and a knockin mouse in which the CCDC146 locus is tagged with an HA tag. Male CCDC146-knockout mice are infertile, which proves the causative role of this gene in the observed MMAF cases. Strikingly, animals develop no other obvious pathologies, thus underpinning the specific role of CCDC146 in male fertility.

The authors have carefully characterised the subcellular roles of CCDC146 by using a combination of expansion and electron microscopy. They demonstrate that all microtubule-based organelles, such as the sperm manchette, the centrioles, as well as the sperm axonemes are defective when CCDC146 is absent. Their data show that CCDC146 is a microtubule-associated protein, and indicate, but do not prove beyond any doubt, that it could be a microtubule-inner protein (MIP).
This is a solid work that defines CCDC146 as a novel cause of male infertility. The authors have performed comprehensive phenotypic analysis to define the defects in CCDC146 knockout mice. Surprisingly, the authors provide virtually no information on the penetrance of those defects - in most cases they simply show descriptive micrographs. The message of this manuscript would have been more convincing if the key phenotypes of the CCDC146 knockout mice were quantified, in particular those shown in Fig. 2E, 7A, 11B, 13.

The manuscript text is well written and easy to follow also for non-specialists. The introduction and discussion chapters contain important background information that allow putting the current work into the greater context of fertility research. The figures could have been designed more carefully, with additional information on the genotype and other details such as the antibodies used etc. directly added to the figure panels, which would improve their readability. The author might also consider pooling small figures with complementary content into one bigger figure in order to group related information together, and again facilitate the reading of the manuscript.

Overall, this manuscript provides convincing evidence for CCDC146 being essential for male fertility, and illustrates this with a large panel of phenotypic observations, which however mostly lack quantification in order to judge their penetrance. Together, the work provides important first insights into the role of a so-far unexplored proteins, CCDC146, in spermatogenesis, thereby broadening the spectrum of genes involved in male infertility.