7 Genetics and Male Infertility Alaa Hamada, Sandro C Esteves, Ashok Agarwal Currently, genetic diseases contribute to 15–30% of CHAPTER CONTENTS causes of male infertility.1 Nevertheless, male factor infer- tility is responsible for more than 50% of cases of infer- ♦ Genetic Concepts tility2 and more than 50% of these cases are of unknown ♦ Evidence of Genetic Contribution for Male Infertility origin.3–5 Genetics may partly or entirely conduce to the ♦ Sperm Genome: Definitions and Concepts problem of infertility in such men. Exploring the exact ♦ Genetic Disorders Causing Male Infertility hereditary background of male infertility may lead to ♦ Importance of Genetic Testing and Counseling development of the therapeutic intervention which can ♦ Preimplantation Genetic Diagnosis modify the genetic error and prevent transmission of ♦ Genetic Tests such error to the offspring. ♦ Work-Up Plan for Specific Genetic Diagnosis and With the advent of assisted reproductive techniques Testing and particularly intracytoplasmic sperm injection (ICSI) ♦ Novel Technologies as a therapeutic option in severe male factor infertility, even sperm with genetic insufficiencies can successfully bypass the genetic barriers that select the healthiest sperm and fertilize the egg. Eventually, embryogenesis and successful pregnancy outcome will depend on the INTRODUCTION integrity of the genetic information. Genetic abnormali- ties may explain the 50–60% failure rate of ART in the enes master all aspects of sperm physiology best centers. Notwithstanding, the final outcome will Gand control not only the hypothalamic pituitary not end at this part since the male progeny may harbor gonadal (HPG) axis necessary for providing the suit- the same genetic defect that rendered their fathers infer- able hormonal milieu for spermatogenesis but also the tile and that will ultimately lead the sons to be infertile molecular events of sperm production. Additionally, as well. genes bring about the formation of ductal system essen- tial for sperm transport and orchestrate the sperm func- tions during fertilization events. However, the role of GENETIC CONCEPTS genetics in male infertility is not completely understood and a large number of studies are currently ongoing to Human genome is composed of 23 pairs of nuclear elucidate its complex relationship. chromosomes, 22 pairs are autosomes and one pair of Section 2 Male Factor Infertility sex chromosome (XX in female and XY in male). Body’s cells are of two types: somatic cells and germ cells (gametes). While somatic cells (nonreproductive cells) are diploid (46 chromosome) and forms the basic units in organ tissues, germ cells (eggs and sperm) are haploid (23 chromosomes and only present in the ovaries and testes respectively. In addition, human genome includes mitochondrial chromosomes where each mitochon- drion contains multiple copies of a small chromosome. Mitochondrial chromosomes are of maternal origin because they are entirely inherited from the cytoplasm of the fertilized ovum. Chromosomes are composed of chromatin which is a complex of unbroken long, double stranded and tightly wound DNA which carries the genes, and proteins which help in packing chromosomes in the nucleus. DNA wrapped around histones forms a nucleosome, which is the basic subunit of chromatin. Single DNA strand is composed of simple units called nucleotides which are essentially formed of nitrogenous bases (adenine, guanine, cytosine and thymine; abbreviated A, G, C and T), sugar (deoxyribose) and phosphate. The two DNA strands are interconnected through specific hydrogen bonds between specific nitrogenous bases forming base pairs. The human genome is formed of approximately 3.1 billion base pairs. Chromosomes are composed of variable lengths of these polynucleotide Figure 1 Basic organization of chromosome, chromatin and chains ranging from 50 million base pairs in chromo- DNA somes 21 to 250 million base pairs in chromosome 1. Figure 1 shows the basic organization of chromosome, chromatin and DNA. Genes are stretches of DNA sequence that encode individual cannot be changed by environmental factors. specific functions such as synthesis of protein through Phenotype is the discernible characteristic or trait of an mRNA transcription or synthesis of functional RNA. individual such as specific external feature, biochemical Human genome contains about 40 million genes. or physiological properties or behavior. Phenotype is Structurally, genes usually consist of coding regions the result of the expression of the genotype. However, called exons interrupted by at least one or several environmental factors may play a role in alteration of noncoding segments called introns. Then by process genotype-phenotype correlation. of splicing, introns are removed from the specifically In general, there are four types of variations in DNA synthesized RNA and exons are joined together to form nucleotide sequence. Firstly, single nucleotide polymor- mRNA coding for synthesis of specific protein (Figure 2). phism (SNP), pronounced as “snip”, is a single nucle- Gene length is variable and ranges from few kilobases otide (A, T, C or G) alteration along the DNA sequence, (1 kilobase equal to 1000 base pairs) to millions of base occurring at a rate of 1 in every 1,250 bases along the pairs. Interestingly, introns rather than exons constitute 3 billion base pairs human genome. It has been esti- greater part of gene length. The exact site or position of mated that up to 20 million SNPs are present in human a particular gene on a chromosome is called locus while genome; however, they are not evenly distributed. For a variant form of a gene is called allele. A gene is usually instance, SNP may alter the DNA nucleotide sequence represented by two alleles. If both alleles are similar the from (AAGGTAA) to ATGGTAA. Such variation is individual is homozygous for this gene. On the other reported in at least 1% of the human population and hand, presence of two different alleles of a gene renders occurs in both coding region of the gene (exon) and an individual heterozygous for such gene. noncoding region (introns). The majority of these SNPs Genotype is the actual genetic code that controls are harmless and very few are coding for new amino 114 physical and performance traits. The genotype of an acids or act as stop codon. Recently, many disease states Chapter 7 Genetics and Male Infertility Expansion of such repeats has been reportedly associ- ated with certain diseases such as neurodegenerative disorders and fragile X syndrome. Lastly, the fourth type of structural genetic sequence change is copy number variation (CNV). It is the most common type and accounts for 8–12% of human genome. CNV is defined when long segment of DNA spanning at least 1000 bases or more that has been added, deleted or inserted, deviating from the normal diploidy doctrine. CNV usually affects one gene or complete set of genes and an increase in CNV of a gene results in an increase in the frequency of its expression and the amount of the produced protein. Although CNV is usually harm- less, recent association with diseases such as cancer and increased susceptibility to systemic lupus erythematous have been demonstrated. Genetic diseases are classified into four major types, i.e. chromosomal disorders, single gene mutation related disorders, multifactorial disorders and mitochondrial genetic disorders. Chromosomal disorders include abnormalities in chromosomal number (aneuploidy) or structure. Single gene disorders may follow Mendelian inheritance laws or may be sex-linked. Multifactorial disorders involve alterations in the expression of multiple genes with a postulated role of the environ- Figure 2 Gene organization and splicing process ment to affect such expression. Mitochondrial genetic disorders are of maternal origin and contribute to certain debilitating human diseases. such as infertility or certain susceptibility traits have been linked to certain SNPs; however, further studies are needed to thoroughly elucidate their significance. EVIDENCE OF GENETIC CONTRIBUTION Secondly, mutations that are the other variation of FOR MALE INFERTILITY DNA nucleotide sequence alterations. Some authors consider mutations as a type of SNPs, however, their Infertility is denoted by failure of a couple to conceive frequency is less than 1% in the general population and after 12 months of unprotected regular intercourse. In most of them implicate harmful events. Mutations are the United States, 10–15% of couples face difficulties defined as alterations in DNA sequence contributing to to conceive and more than 50% of cases are attributed diseases or adverse effects on the host or offspring. Two to male factor either as the sole reason or contributory types of mutations are known: germinal and somatic problem.2 However, 36–58% of male factor infertility is mutations. Germinal mutations often occur during of unknown origin and research is embarked in looking gametogenesis (sperm or egg formation) and usually at this dilemma from various views.3–5 Genetics certainly are transmitted to the offspring or render the patient has a significant role in these unidentified causes and infertile. The transmitted mutations may cause various an increasing bulk of evidence are being added to shed effects in the progeny ranging from minor physiologic light on novel genes or proteins, essential for sperm modifications to serious disease states, infertility and production and/or function. even death. Somatic mutations usually affect