A Homozygous Loss-of-Function Mutation in MSH5 Abolishes MutSγ Axial Loading and Causes Meiotic Arrest in NOA-Affected Individuals

Biallelic mutations in RNA-binding protein ADAD2 cause spermiogenic failure and non-obstructive azoospermia in humans

STUDY QUESTION

What are some pathogenic mutations for non-obstructive azoospermia (NOA) and their effects on spermatogenesis?

SUMMARY ANSWER

Biallelic missense and frameshift mutations in ADAD2 disrupt the differentiation of round spermatids to spermatozoa causing azoospermia in humans and mice.

WHAT IS KNOWN ALREADY

NOA is the most severe cause of male infertility characterized by an absence of sperm in the ejaculate due to impairment of spermatogenesis. In mice, the lack of the RNA-binding protein ADAD2 leads to a complete absence of sperm in epididymides due to failure of spemiogenesis, but the spermatogenic effects of ADAD2 mutations in human NOA-associated infertility require functional verification.

STUDY DESIGN SIZE DURATION

Six infertile male patients from three unrelated families were diagnosed with NOA at local hospitals in Pakistan based on infertility history, sex hormone levels, two semen analyses and scrotal ultrasound. Testicular biopsies were performed in two of the six patients. Adad2 mutant mice (Adad2^Mut/Mut) carrying mutations similar to those found in NOA patients were generated using the CRISPR/Cas9 genome editing tool. Reproductive phenotypes of Adad2^Mut/Mut mice were verified at 2 months of age. Round spermatids from the littermates of wild-type (WT) and Adad2^Mut/Mut mice were randomly selected and injected into stimulated WT oocytes. This round spermatid injection (ROSI) procedure was conducted with three biological replicates and >400 ROSI-derived zygotes were evaluated. The fertility of the ROSI-derived progeny was evaluated for three months in four Adad2^WT/Mut male mice and six Adad2^WT/Mut female mice. A total of 120 Adad2^Mut/Mut, Adad2^WT/Mut, and WT mice were used in this study. The entire study was conducted over 3 years.

PARTICIPANTS/MATERIALS SETTING METHODS

Whole-exome sequencing was performed to detect potentially pathogenic mutations in the six NOA-affected patients. The pathogenicity of the identified ADAD2 mutations was assessed and validated in human testicular tissues and in mouse models recapitulating the mutations in the NOA patients using quantitative PCR, western blotting, hematoxylin-eosin staining, Periodic acid-Schiff staining, and immunofluorescence. Round spermatids of WT and Adad2^Mut/Mut mice were collected by fluorescence-activated cell sorting and injected into stimulated WT oocytes. The development of ROSI-derived offspring was evaluated in the embryonic and postnatal stages.

MAIN RESULTS AND THE ROLE OF CHANCE

Three recessive mutations were identified in ADAD2 (MT1: c.G829T, p.G277C; MT2: c.G1192A, p.D398N; MT3: c.917_918del, p.Q306Rfs*43) in patients from three unrelated Pakistani families. MT1 and MT2 dramatically reduced the testicular expression of ADAD2, likely causing spermiogenesis failure in the NOA patients. Immunofluorescence analysis of the Adad2^Mut/Mut male mice with the corresponding MT3 mutation showed instability and premature degradation of the ADAD2 protein, resulting in the spermiogenesis deficiency phenotype. Through ROSI, the Adad2^Mut/Mut mice could produce pups with comparable embryonic development (46.7% in Adad2^Mut/Mut versus 50% in WT) and birth rates (21.45 ± 10.43% in Adad2^Mut/Mut versus 27.5 ± 3.536% in WT, P = 0.5044) to WT mice. The Adad2^WT/Mut progeny from ROSI (17 pups in total via three ROSI replicates) did not show overt developmental defects and had normal fertility.

LARGE SCALE DATA

N/A.

LIMITATIONS REASONS FOR CAUTION

This is a preliminary report suggesting that ROSI can be an effective treatment for infertile Adad2^Mut/Mut mice. Further assisted reproductive attempts need to be carefully examined in humans during clinical trials.

WIDER IMPLICATIONS OF THE FINDINGS

Our work provides functional evidence that mutations in the ADAD2 gene are deleterious and cause consistent spermiogenic defects in both humans and mice. In addition, preliminary results show that ROSI can help Adad2^Mut/Mut to produce biological progeny. These findings provide valuable clues for genetic counselling on the ADAD2 mutants-associated infertility in human males.

STUDY FUNDING/COMPETING INTERESTS

This work was supported by the National Natural Science Foundation of China (32000587, U21A20204, and 32061143006), and the National Key Research and Developmental Program of China (2019YFA0802600 and 2021YFC2700202). This work was also supported by Institute of Health and Medicine, Hefei Comprehensive National Science Center, Hefei, China. The authors declare no competing interests.

Genetic variants underlying spermatogenic arrests in men with non-obstructive azoospermia

Spermatogenic arrest is a severe form of non-obstructive azoospermia (NOA), which occurs in 10-15% of infertile men. Interruption in spermatogenic progression at premeiotic, meiotic, or postmeiotic stage can lead to arrest in men with NOA. Recent studies have intensively focused on defining genetic variants underlying these spermatogenic arrests by making genome/exome sequencing. A number of variants were discovered in the genes involving in mitosis, meiosis, germline differentiation and other basic cellular events. Herein, defined variants in NOA cases with spermatogenic arrests and created knockout mouse models for the related genes are comprehensively reviewed. Also, importance of gene panel-based screening for NOA cases was discussed. Screening common variants in these infertile men with spermatogenic arrests may contribute to elucidating the molecular background and designing novel treatment strategies.

Loss of SUN1 function in spermatocytes disrupts the attachment of telomeres to the nuclear envelope and contributes to non-obstructive azoospermia in humans

One of the most severe forms of infertility in humans, caused by gametogenic failure, is non-obstructive azoospermia (NOA). Approximately, 20-30% of men with NOA may have single-gene mutations or other genetic variables that cause this disease. While a range of single-gene mutations associated with infertility has been identified in prior whole-exome sequencing (WES) studies, current insight into the precise genetic etiology of impaired human gametogenesis remains limited. In this paper, we described a proband with NOA who experienced hereditary infertility. WES analyses identified a homozygous variant in the SUN1 (Sad1 and UNC84 domain containing 1) gene [c. 663C > A: p.Tyr221X] that segregated with infertility. SUN1 encodes a LINC complex component essential for telomeric attachment and chromosomal movement. Spermatocytes with the observed mutations were incapable of repairing double-strand DNA breaks or undergoing meiosis. This loss of SUN1 functionality contributes to significant reductions in KASH5 levels within impaired chromosomal telomere attachment to the inner nuclear membrane. Overall, our results identify a potential genetic driver of NOA pathogenesis and provide fresh insight into the role of the SUN1 protein as a regulator of prophase I progression in the context of human meiosis.