Integrating Precision Medicine into the Standard of Care for Male Infertility: What Will it Take?

Despite the high prevalence of male infertility, which affects ~8% of men worldwide, the genetic mechanisms underlying the condition are poorly understood: approximately half of cases are idiopathic, lacking a known molecular or genetic cause [1]. Therefore, we cannot implement precision medicine approaches to the clinical management of infertile men and their families as we do not understand how a diagnosis of infertility impacts the reproductive health of offspring who are conceived naturally or through assisted reproductive technologies (ARTs).

An extreme yet common form of male infertility is azoospermia, the complete absence of sperm in the ejaculate, which accounts for 15% of infertility cases [2]. Genetic testing for obstructive azoospermia (OA), clinically defined as a physical blockage in the male reproductive tract, involves the detection of mutations in CFTR. Functional mutations in CFTR frequently lead to congenital bilateral absence of the vas deferens, for which procedures including testicular sperm extraction (TESE) are indicated to facilitate assisted pregnancies. However, besides well-defined Y-chromosome microdeletions and karyotype analyses, monogenic gene-disease relationships that would be informative in the care for nonobstructive azoospermic (NOA) remain largely unknown.

Foundational studies in 2019 by our group [3] and in 2021 by Wrywoll and colleagues [4] exhaustively curated the existing literature to better elucidate monogenic causes of male infertility. This seminal work helped to prioritize genes implicated in different subtypes of male infertility on the basis of standardized criteria, including gene expression data, experimental design and quality, in vitro and in vivo animal modeling, and patient phenotype assessment. In total, 120 genes were linked to male infertility with at least moderate evidence, and an additional 138 genes had limited evidence. However, the diagnostic, prognostic, and general clinical utility for a vast majority of these genes, even those with definitive evidence as a monogenic cause for male infertility, has yet to be defined.

The drastic difference in outcomes related to OA and NOA treatment highlight the need to identify clinically informative genes. Unlike OA, patients with NOA present with low success rates for TESE [5] and require microTESE. Application of genomic medicine in urology and andrology clinics via comprehensive genetic evaluations can guide clinical care strategies and predict outcomes in cases such as these. To this end, a deeper understanding of informative genetic biomarkers in terms of microTESE success, intracytoplasmic sperm injection, and other forms of ART is necessary to advance our approach to treating men with NOA and other forms of male infertility.

Work published in this issue of European Urology by Wrywoll et al. [6] characterized functional variants in key genes involved in spermatogenesis and azoospermia that could guide clinical approaches to diagnosing and treating men with spermatogenic failure. To this end, Wrywoll et al performed a comprehensive analysis of whole-exome sequencing and matched clinical data from 647 men from the Centre of Reproductive Medicine and Andrology (Münster, Germany). Their robust experimental design assessed the functional and clinical relevance of each variant using American College of Genomic Medicine and ClinGen guidelines. Within the broader context of male infertility research, this critical work builds on the aforementioned results published in 2019 and 2021, as well as a large number of smaller case-control and family-based studies, to investigate variants in putatively disease-relevant genes, setting a standard for further genomic evaluations related to infertility.

The identification of likely pathogenic and pathogenic variants in 22 genes approximately doubled the diagnostic yield for idiopathic NOA (and cryptozoospermic) cases to >6.5% across their cohort. This increase demonstrates and further addresses the clinical importance of genetic testing beyond well-established evaluations for AZF microdeletions, CFTR mutations, and karyotyping. With this genetic information, physicians can better inform infertile men and their families on the success rates associated with specific procedures related to sperm retrieval and ART procedures. However, for this scenario to become a reality, extensive research is required to uncover the genetic mechanisms underlying spermatogenic failure, as a vast majority of infertility cases still lack a known cause. Importantly, future studies must include individuals from diverse populations, ancestries, and socioeconomic backgrounds to facilitate the equitable distribution of findings related to the integration of precision medicine and male infertility.

Establishing definitive relationships between specific variants and infertility phenotypes can complement the findings of Wrywoll et al. [6] (and future related work) to facilitate a complete integration of precision medicine in male infertility care. For example, robust functional assays such as large-scale CRISPR screens with MIC-Drop technologies can assess monogenic gene-disease relationships in vivo [7]. In addition, hundreds of variants with unknown significance (VUS) exist that complicate the clinical and pathogenic assessment of mutations. We believe that examining these mutations in the three-dimensional structure of a protein using groundbreaking protein structure prediction algorithms, especially AlphaFold and RoseTTAFold [8], can identify additional mutations potentially relevant to spermatogenic failure. For example, as in developmental and congenital disorders [9], VUSs that colocalize with definitively functional mutations in a folded protein can illuminate important mutational events that drive male infertility.

One obstacle to the treatment of infertile men is the heterogenic nature of the disease. With more than 2000 genes involved in spermatogenesis [10], future research must consider the polygenic nature of male infertility. In addition, integrated analyses that focus on different combinations of genomic, transcriptomic, epigenomic, proteomic, and metabolomic data sets can identify factors related to the pathogenesis and progression of infertility, especially in idiopathic cases. For example, long-read sequencing with PacBio HiFi technologies allows genomic investigations related to single-nucleotide, copy-number, DNA-repeat, and structural variants in addition to concurrent epigenetic explorations [11]. Resolving these molecular complexities can illuminate biomarkers for specific phenotypic features of male infertility and identify those who would benefit the most from certain procedures. Lastly, the success of this relatively large study and the remaining gap in our understanding of the genetic basis for male infertility highlight the necessity for continued collaborative efforts and consortia focused on the investigation of this important disease (http://www.imigc.org/, https://gemini.conradlab.org/).

Overall, the work by Wrywoll et al. [6] represents an essential step towards the identification of informative genetic biomarkers that could guide clinical management strategies for infertile men. As costs continue to decline, the real-world application of modern DNA sequencing technologies in urology and andrology clinics is more feasible. To fully integrate precision medicine in the practice of male infertility care, additional research efforts are required to holistically explore and ultimately define genetic and molecular causes of spermatogenic failure. However, for cases of repeated sperm retrieval or pregnancy failures, there exists an unmet need to resolve the root cause of infertility and “restore” an individual’s spermatogenic potential. To this end, continued research efforts that mirror the experimental design of Wrywoll et al [6] may uncover therapeutic targets that could finally illuminate strategies to treat spermatogenic failure itself in infertile men.