Epigenetic Alterations in Cryopreserved Human Spermatozoa: Suspected Potential Functional Defects

Background: Gene set enrichment analysis (GSEA) was conducted on raw data, and alternative splicing (AS) events were found after mRNA sequencing of human spermatozoa. In this study, we aimed to compare unknown micro-epigenetics alternations in fresh and cryopreserved spermatozoa to evaluate the effectivity of cryopreservation protocols. Methods: Spermatozoa were divided into three groups: fresh spermatozoa (group 1), cryoprotectant-free vitrified spermatozoa (group 2), and conventionally frozen spermatozoa (group 3). Nine RNA samples (three replicates in each group) were detected and were used for library preparation with an Illumina compatible kit and sequencing by the Illumina platform. Results: Three Gene Ontology (GO) terms were found to be enriched in vitrified spermatozoa compared with fresh spermatozoa: mitochondrial tRNA aminoacylation, ATP-dependent microtubule motor activity, and male meiotic nuclear division. In alternative splicing analysis, a number of unknown AS events were found, including functional gene exon skipping (SE), alternative 5′ splice sites (A5SS), alternative 3′ splice sites (A3SS), mutually exclusive exon (MXE), and retained intron (RI). Conclusions: Cryopreservation of spermatozoa from some patients can agitate epigenetic instability, including increased alternative splicing events and changes in crucial mitochondrial functional activities. For fertilization of oocytes, for such patients, it is recommended to use fresh spermatozoa whenever possible; cryopreservation of sperm is recommended to be used only in uncontested situations.