Azoospermic men found to have repair gene deficiencies

May 1, 2011

Evidence of MSH5 mismatch repair gene deficiencies in nonobstructive azoospermic men suggests that these men should receive genetic counseling if they plan to have children via intracytoplasmic sperm injection.

Key Points

Denver-Evidence of MSH5 mismatch repair (MMR) gene deficiencies in nonobstructive azoospermic (NOA) men suggests that these men should receive genetic counseling if they plan to have children via intracytoplasmic sperm injection (ICSI), according to the authors of a recent study.

"DNA repair is essential for maintenance of genomic integrity," said first author Sarmistha Mukherjee, PhD, research associate in urology at Baylor College of Medicine, Houston. "When there is an impairment or inactivation of this pathway, there can be errors in DNA replication, as well as recombination."

NOA men possess a severe sperm production defect that can result in Sertoli cell only (SCO) sperm, hypospermatogenesis, maturation arrest, or a combination of these pathologies, she said. However, with the advent of ICSI, treatment for these infertile couples has changed drastically, she added. Testicular sperm extraction (TESE) ICSI uses sperm obtained from a testis biopsy-rather than ejaculate-to achieve fertilization, she explained.

Dr. Mukherjee said that some samples from NOA men showed little to no expression of MSH4/5.

"MSH5 is very important in meiosis, so we wanted to look at the two functional domains of the protein, particularly the MSH4 binding site and the nuclear localization site, which encompasses exon 15," said Dr. Mukherjee, working with Dolores J. Lamb, PhD, and colleagues.

Sequencing data revealed variants in the key functional domains in infertile men and none in the controls. The change of the amino acids in the variants may lead to a conformational change that could compromise the function of proteins it creates, Dr. Mukherjee said.

When the MMR proteins are defective, she stated, "The rate of mutations increases spontaneously. And we find that it may be associated with malignancies and infertility in mice. So our main concern was, what are the health consequences for children conceived by ICSI, and are their fathers more prone to cancer development later in life?"

MMR proteins remove toxic alkyl moieties

To answer these questions, the researchers plated fibroblast cells and induced them with different doses of alkylating agents, then performed a cytotoxicity assay (WST) on harvested cells. They found that MMR proteins from fertile subjects could recognize and initiate the removal of toxic alkyl moieties, thereby protecting cells from cytotoxic and mutagenic effects, said Dr. Mukherjee. A deficiency of mismatch repair proteins yielded a different result.

"When mismatch repair proteins are deficient, mismatches with alkylated adducts are not recognized. The cells continue to proliferate, leading to a mutator phenotype," Dr. Mukherjee said. Overall, she added, 60% of NOA patients were resistant to the alkylating agents.

Custom gene expression arrays moreover revealed significant (p<.001 vs. controls) downregulation in multiple DNA repair pathway genes, she said. Investigators also found similar downregulation of MSH5 gene expression versus normal controls.

"The mutation of the single-nucleotide polymorphism may lead to weakened protein interaction with MSH4, thus affecting genetic stability and DNA recombination in a way that could impair spermatogenesis," Dr. Mukherjee concluded. Accordingly, she added, MMR protein deficiencies could carry health consequences for NOA men and their offspring conceived through ICSI.

"As our knowledge of the genetic cause of infertility is increasing, we should encourage NOA patients to undergo genetic counseling before ICSI," Dr. Mukherjee said.

Going forward, added Dr. Mukherjee, "It's very important that we find what the functional consequences of these mutations are. That's what we are studying in ongoing research."