Determining the underlying genetic basis in male infertility

Genetic factors are responsible for up to 15% of male infertility cases. It is necessary to determine the underlying genetic basis of male factor infertility to develop appropriate screens for abnormal phenotypes, and to discover more effective solutions for the problems of infertile couples. The most common genetic causes of male infertility include sex chromosome aneuploidies, Y chromosome microdeletions, gene polymorphisms and congenital absence of the vas deferens.

Klinefelter syndrome
Klinefelter syndrome is one of the most frequent cytogenetic anomalies found in infertile men. The most frequent type of karyotype present in men with Klinefelter syndrome is 47, XXY. The syndrome can also be related to mosaicism 46XY/47 XXY; also higher number of X chromosomes such as 48, XXXY; 48, XXYY or even 49, XXXXY and structurally abnormalities in sex chromosomes can be found. Notably, men with Klinefelter syndrome present hypogonadism, azoospermia, small testes, erectile dysfunction, and higher gonadotropin levels compared to normal and fertile men.

Patients with XX male syndrome (46, XX) are less common than Klinefelter syndrome. Uneven crossing over between X and Y chromosomes may result in an additional X chromosome bearing the SRY gene through a translocation process. Patients with XX male syndrome are infertile and may develop male external genitalia, micropenis, hypospadias and cryptorchidism.

Klinefelter syndrome is easily detected through conventional cytogenetic analysis but XX male syndrome requires molecular cytogenetic with SRY probe to be performed. Also, several X chromosome linked genes, such as AKAP4 and TGIF2LX, affect the ability of a man to have children.

Sperm aneuploidy
Male infertility is commonly due to deficiencies in the semen, and semen quality is used as a surrogate measure of male fecundity. However, the role of sperm chromosome level in male infertility remains unclear.

Normal karyotyping of blood cells, sperm DNA fragmentation and routine semen analysis cannot exclude the presence of chromosomal abnormalities in spermatozoa. The evaluation of autosome and sex chromosome aneuploidy in sperm of men with history of infertility and/or recurrent pregnancy loss (RPL) and/or failed in vitro fertilization (IVF) should be done routinely. Fluorescence in situ hybridization (FISH) technique based screening is recommended for investigating sperm aneuploidy in 13, 18, 21, X, and Y chromosomes.

Recent studies have focused in RPL, which affects 1–2% of couples. The ESHRE Guidelines defines RPL as having two or more consecutive pregnancy losses before week 20 of gestation. Dr. Ranjith Ramasamy and co-authors found that men with RPL had a greater percentage of sperm aneuploidy within the sex chromosomes and chromosomes 13,18, 21 (1.04% vs. 0.38%; 0.18% vs. 0.03%; 0.26% vs. 0.08%). In total, 40% of men with normal sperm density and motility had abnormal sperm aneuploidy in all the chromosomes analysed. Men with abnormal sperm density and motility had a higher proportion of sperm sex chromosome aneuploidy than men with normal density/motility (62% vs. 45%). Men with normal strict morphology (>4%) had lower rates of sex chromosome and sperm aneuploidy than men with abnormal strict morphology (28% vs. 57%). There was no association between sperm DNA fragmentation and sperm aneuploidy.

The conclusions of the study are:

• Men in couples with RPL have increased sperm aneuploidy compared with controls.
• A total of 40% of men with RPL and normal sperm density/motility had abnormal sperm aneuploidy.
• Men with oligoasthenozoospermia and abnormal strict morphology had a greater percentage of sperm aneuploidy compared with men with normal semen parameters.

Furthermore, it is important to identify reasons for failure after IVF and intracytoplasmic sperm injection (ICSI). One of the greatest challenges with ICSI is the identification of “normal sperm” for micromanipulation. Selection of euploid spermatozoa could possibly improve the chances of these couples of successfully carrying a pregnancy to term. However, as yet no such technique is available. With current technologies, we can only identify sperm with grossly abnormal morphology rather than detecting underlying genetic abnormalities such as aneuploidy.

Preimplantation genetic screening (PGS) could also be useful in managing sperm aneuploidy by screening for genetically normal embryos that improve chances of successful implantation and pregnancy. Therefore, men presenting with recurrent pregnancy loss or recurrent unexplained failure with assisted reproductive techniques (ART) should consider sperm aneuploidy testing to determine an underlying etiology to enable better and informed reproductive choices.

FISH can detect the rate of aneuploidy in different samples including ejaculated, epididymal, and testicular sperm for diagnostic purposes in male infertility. Clinically, results from this screening tool can be used in genetic counselling of couples suffering from male factor infertility to make informed decisions concerning their ART cycles.

Y chromosome microdeletion
Mammalian sex chromosomes evolved from autosomes at least 180 million years ago. The first step in differentiation of the Y chromosome involved the acquisition of the testis determining gene followed by large-scale inversions and sequential suppression of recombination between the X and Y chromosomes in a stepwise fashion.

Human Y is an acrocentric chromosome composed of two pseudoautosomal regions (PARs), a short arm (Yp) and the long arm (Yq) that are separated by a centromere. The Y chromosome is an obvious area of interest in the study of male factor infertility because it contains many of the genes that are critical for spermatogenesis and the development of male gonads.

The AZF gene is one of the most investigated Y chromosome genes related to infertility. The prevalence of microdeletions in azoospermic men was found to range from 10%–15%, in oligozoospermic men the prevalence of microdeletions was 5%–10%. Infertile patients with AZF deletions showing at least minimal levels of spermatogenesis could have children through sperm aspiration followed by intracytoplasmic sperm injection. Unfortunately, the AZF deletion is inherited by the male offspring (AZFc region microdeletion). AZFc deletions cause approximately 12% of nonobstructive azoospermia and 6% of severe oligozoospermia. It is critical that azoospermic and severely oligozoospermic men be tested for microdeletions both for accurate diagnosis and genetic counseling before performing ART.

Translocations
Translocations can cause the loss of genetic material at the break points of genes, which can corrupt the genetic message. Autosomal translocations were found to be four to 10 times more likely in infertile males in comparison with normal males. Robertsonian translocations, which occur when two acrocentric chromosomes fuse, are the most frequent structural chromosomal abnormalities in humans, and they affect fertility in one out of 1,000 men. Although the prevalence of Robertsonian translocations is only 0.8% in infertile males (oligozoospermic and azoospermic men with rates of 1.6% and 0.09% respectively), this figure is nine times higher than in the general population. The translocations can result in a variety of sperm production phenotypes from normal spermatogenesis to an inability to produce spermatogonia. Due to the risk of passing on the translocation to offspring, fluorescent in situ hybridization, with additional probes added for common translocations, is recommended to determine the chromosomal composition of the sperm.

CFTR gene
The CFTR gene, located on chromosome 7, is mutated in 60%–90% of patients with congenital bilateral absence of the vas deferens (CBAVD). Sperm aspiration and ICSI are useful methods of treatment for men with the CFTR mutation as long as the female does not also carry the CFTR mutation. Partners who both carry the mutation should be advised to have PGD to avoid passing the abnormality to their offspring.

Careful clinical observations coupled with detailed genetic information will provide important insights into these unanswered basic questions and give a different perspective to the field of androgenetics.

The reference list can be made available to interested readers upon request by sending an email to: communications@uroweb.org.