Impact of tdrd9 Gene Mutations in the Therapeutic Response to L-carnitine in Oligoasthenozoospermic Men

Genetic Basis of Male Infertility with Emphasis on Oligozoospermia and the Role of TDRD9 Infertility represents a major global health concern, affecting approximately 10-15% of couples worldwide. Male factors contribute to infertility in nearly 20-50% of these cases, either as an isolated cause or in combination with female factors. Despite advances in diagnostic techniques, the etiology of male infertility remains unidentified in a substantial proportion of patients and is therefore classified as idiopathic infertility. Among the various causes of male infertility, abnormalities in sperm production and function constitute the most frequent underlying pathology.

Azoospermia, defined as the complete absence of spermatozoa in the ejaculate, is one of the most severe manifestations of male infertility. It affects approximately 1% of the general male population and accounts for more than 15% of infertility cases among men. Azoospermia may be obstructive or non-obstructive, with the latter reflecting intrinsic defects in spermatogenesis and frequently having a genetic basis.

Oligozoospermia, characterized by reduced sperm concentration, is another major contributor to male infertility and often represents a milder but clinically significant spermatogenic failure. According to the World Health Organization (WHO) 2021 reference values, oligozoospermia is defined as a sperm concentration below 16 million spermatozoa per milliliter (95% confidence interval: 15-18 million/mL). Although environmental, lifestyle, endocrine, and infectious factors have been implicated, accumulating evidence indicates that oligozoospermia has a strong genetic component. Nevertheless, its molecular and genetic underpinnings remain only partially understood.

Genetic Abnormalities in Oligozoospermia and Azoospermia Genetic defects are increasingly recognized as major contributors to otherwise unexplained oligozoospermia and azoospermia. Classical genetic abnormalities include numerical and structural chromosomal anomalies, such as Klinefelter syndrome (47,XXY), balanced translocations, and inversions. Microdeletions within the azoospermia factor (AZF) regions of the Y chromosome (AZFa, AZFb, and AZFc) are among the most well-established genetic causes of impaired spermatogenesis. In addition, mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene are commonly associated with congenital bilateral absence of the vas deferens and obstructive azoospermia.

Beyond these well-characterized abnormalities, next-generation sequencing technologies have enabled the identification of hundreds of genes potentially involved in spermatogenesis. To date, more than 400 genes have been reported to be specifically or potentially associated with male fertility regulation, reflecting the remarkable genetic heterogeneity underlying dyszoospermia. Mutations in genes such as RPL10L, which plays a role in ribosomal function during spermatogenesis, and MAGEB, involved in germ cell development, have been implicated in oligozoospermia. However, despite these discoveries, pathogenic variants in only a limited number of genes have been conclusively linked to male infertility, highlighting the need for continued investigation into novel candidate genes.

TDRD9 and Its Role in Spermatogenesis Recent studies have highlighted the Tudor Domain Containing 9 (TDRD9) gene as a critical regulator of spermatogenesis. TDRD9 encodes a germline-specific RNA helicase that plays an essential role in the PIWI-interacting RNA (piRNA) pathway, a key defense mechanism that suppresses transposable elements during germ cell development. Proper transposon silencing is crucial for maintaining genomic stability in germ cells, and disruption of this process can lead to impaired spermatogenesis and infertility.

TDRD9 is highly expressed in the testis, with expression detected across multiple stages of germ cell development, including mitotic spermatogonia, meiotic spermatocytes, and haploid spermatids. Functional studies have demonstrated that TDRD9 participates in the silencing of long interspersed nuclear element-1 (LINE-1) retrotransposons, thereby protecting germ cell integrity. Defects in this pathway have been associated with spermatogenic arrest and non-obstructive azoospermia.

A recent 2024 study provided compelling clinical evidence for the involvement of TDRD9 in human male infertility. In a Chinese family with idiopathic oligozoospermia, compound heterozygous mutations in TDRD9-namely a splice-site variant (c.1115+3A>G) and a frameshift variant (c.958delC)-were identified. These mutations resulted in aberrant mRNA splicing and truncated protein products, leading to impaired TDRD9 function. This study strengthened the causal link between TDRD9 dysfunction and defective spermatogenesis and expanded the spectrum of genes implicated in oligozoospermia.