Mutation Severity of FBXO43 Determines Spermatogenic Outcome and Informs Precision ART Strategies

FBXO43 is a known inhibitor of the anaphase-promoting complex/cyclosome (APC/C), a key E3 ubiquitin ligase that controls meiotic cell cycle progression. However, how different FBXO43 mutations affect APC/C inhibition and lead to divergent clinical phenotypes remains unclear. This study is aimed at clarifying how different FBXO43 mutations produce divergent clinical phenotypes in male infertility and to investigate their preliminary molecular mechanisms, thereby providing evidence-based guidance for precision-assisted reproduction in affected individuals.

Two infertile patients carrying distinct compound heterozygous FBXO43 variants-missense mutations in a macrozoospermia case and truncating mutations in a nonobstructive azoospermia (NOA) case-were identified through whole-exome sequencing. Sperm morphology, chromatin status, and aneuploidy were assessed, and a testicular biopsy was performed for the NOA case. Functional consequences of each mutation were evaluated using in vitro HEK 293T cell models, including protein stability, ubiquitination, and APC/C subunit interactions. Clinical outcomes of assisted reproductive technology (ART) were also reviewed.

Missense variants (NM_001029860, p.Pro641Leu/p.Arg660Gln) in the macrozoospermia patient allowed completion of meiosis but led to severe sperm head enlargement, chromatin condensation defects, and markedly elevated aneuploidy, resulting in repeated ICSI failure. In contrast, the truncating variants (p.Trp532 ^∗/p.Ser577Leufs ^∗11) in the NOA patient abolished the C-terminal functional domain and caused meiotic arrest with complete absence of mature sperm. Mechanistically, all mutations reduced FBXO43 protein stability and disrupted its specific binding to the APC/C substrate recognition subunit APC3, whereas interactions with APC2/6/8 remained intact. Loss of APC3 binding likely impairs APC/C inhibition, disturbing meiotic chromosome segregation in a mutation-severity-dependent manner.

Different types of FBXO43 mutations generate distinct infertility phenotypes through a dose-dependent mechanism: Truncating mutations cause profound meiotic arrest and NOA, whereas hypomorphic missense mutations permit meiotic completion but result in severe teratozoospermia with high aneuploidy. These findings link FBXO43 to a continuous phenotypic spectrum and highlight its relevance for precision reproductive counseling. Based on the identified mutation-specific risks, preimplantation genetic testing for aneuploidy (PGT-A) is recommended for affected couples to improve embryo selection and optimize ART outcomes.