ArticlePolymorphism in DNMT1 may modify the susceptibility to oligospermia
Introduction
Infertility is a common and complex reproductive health problem that affects approximately 10–15% of couples seeking to conceive, and in half of these couples the cause is male factor infertility (De Kretser and Baker, 1999). Spermatogenesis impairment is the most common disorder leading to male infertility and can be caused by genetic deficiency and environmental factors (Massart et al., 2012, Skakkebaek et al., 1994, Toshimori et al., 2004). Genetic deficiency was considered as one of the major aetiologies for spermatogenic impairment (Ferlin et al., 2006, Ferlin et al., 2007).
Spermatogenesis is a complex process, including proliferation and differentiation of the spermatogonia, meiosis and differentiation from round spermatids to spermatozoa, in which highly regulated expression of numerous genes are involved. DNA methylation can repress the expression of genes, which plays an important role in gene regulation during spermatogenesis (Dada et al., 2012, Hisano et al., 2003, Li, 2002). DNA methylation of certain genes is essential for normal spermatogenesis (Marchal et al., 2004, Rajender et al., 2011). It has been demonstrated that abnormal DNA methylation might lead to spermatogenesis impairment (Hartmann et al., 2006, Takashima et al., 2009). For instance, hypomethylation of DNA could disrupt the differentiation of germ cells into spermatocyte and lead to sperm count decrease (Kobayashi et al., 2007, Raman and Narayan, 1995). In addition, DNA methylation is also important for germ-cell-specific organization of chromatin required for meiosis and spermatogenesis (Oakes et al., 2007a, Oakes et al., 2007b). Therefore, it is speculated that the genes playing roles in DNA methylation may be implicated in spermatogenesis impairment and male infertility.
DNA methylation is a biochemical process where methyl groups are added from S-adenosyl-methionine to the 5′-position of cytosine residues of CpG dinucleotides, which is catalysed by DNA methyltransferases (DNMT). Three main DNA methyltransferases, including DNMT1, DNMT3A and DNMT3B, are responsible for DNA methylation and play crucial roles in establishing specific DNA methylation during gametogenesis (Biermann and Steger, 2007, Rodriguez-Osorio et al., 2010). DNMT1 methylates hemimethylated DNA and maintains already-established methylation patterns (Bestor and Ingram, 1983, Gruenbaum et al., 1982), whereas DNMT3A and DNMT3B act to establish de-novo methylation patterns (Lei et al., 1996, Okano et al., 1999). DNMT1 is located at 19p13.2. It is expressed in all stages of human spermatogenesis including proliferation, meiosis, spermatids differentiation and in ejaculated spermatozoa (Marques et al., 2011), which indicates the importance of DNMT1 for spermatogenesis. Decreased expression of DNMT1 in mouse germline stem cells could result in apoptosis and spermatogenic defects (Takashima et al., 2009). In humans, abnormal DNMT1 expression was also observed in testis of patients with impaired spermatogenesis (Omisanjo et al., 2007). These data suggested that DNMT1 is crucial for spermatogenesis and may play a role in spermatogenesis impairment and male infertility.
Since there is dearth of data about the effect of DNMT1 on male infertility with spermatogenesis impairment, the current study selected three exonic single-nucleotide polymorphisms (SNP; rs16999593, rs2228612 and rs2228611) in DNMT1 according to the SNP database (dbSNP) at NCBI and carried out a case–control study on the association between the polymorphisms of these SNP and male infertility with spermatogenesis impairment in a Chinese cohort, including 342 patients with idiopathic azoospermia or oligospermia and 232 controls, to explore the possible role of DNMT1 in spermatogenesis impairment and male infertility.
Section snippets
Subjects
The patient group included 342 patients with idiopathic azoospermia or oligospermia (sperm count <15 × 106/ml) aged 25–42 years. Of these patients, 141 were azoospermic and 201 were oligospermic. For patients with oligospermia, the ejaculate volume, sperm concentration, sperm motility and sperm morphology were 1.75 ± 0.64 ml, 9.38 ± 4.89 million/ml, 65.15 ± 18.9% and 45.46 ± 15.3%, respectively. Patients with diseases known to affect spermatogenesis, including 42 with orchitis, eight with maldescensus of
Results
The polymorphic distributions of SNP rs16999593, rs2228612 and rs2228611 in DNMT1 were investigated using PCR-RFLP in 342 infertile patients with idiopathic azoospermia or oligospermia and 232 fertile controls. The allele and genotype distributions of the three SNP are listed in Table 2. The distributions of genotypes of the three SNP followed the Hardy–Weinberg equilibrium in patients and controls (data not shown). As shown in Table 2, there were no significant differences in the distributions
Discussion
A genetic factor is one of main causes for spermatogenesis impairment. Although some genetic deficiencies that can lead to spermatogenesis impairment have been identified in last decades, the majority of genetic causes for spermatogenesis impairment remain unknown. With the progress in research on human genome, numerous genes that play roles in spermatogenesis have been identified. These genes are considered good candidates for spermatogenesis impairment. It is likely that genetic variants or
Acknowledgements
This work was supported by the National Natural Science Foundation of China (Grant No. 30770809).
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A Zhou-Cun, professor of medical genetics, is the director of the teaching and research section at College of Agriculture and Biology of Dali University, China. He received his MSC at West China Medical University, Chengdu in 1999 and obtained his PhD in medical genetics at Sichuan University, Chengdu in 2006. His research interest is the genetic aetiology of male infertility.