The Plasticity of Sex: The Molecular Biology and Clinical Features of Genomic Sex, Gender Identity and Sexual Behavior

By Marianne Legato J

The Plasticity of Sex: The Molecular Biology and Clinical Features of Genomic Sex, Gender Identity and Sexual Behavior provides a comprehensive view on the development of human sexuality. As there has been a crescendo of interest over the past several decades about the nature and diversity of human sexuality, this reference brings the evidence-based research into one place. The emergence of issues surrounding gender identity, genital ambivalence and the transition from one sex to another is striking, with the public and treating physicians alike clamoring for an evidence-based, comprehensive treatment of human sexuality and all its variations.

This is a must-have reference for biomedical researchers in endocrinology, neuroscience, development biology, medical students, residents, and practicing physicians from all medical areas.

Winner of the 2021 PROSE Award in Biomedicine from the Association of American Publishers!

• 2021 PROSE Awards - Winner: Category: Biomedicine: Association of American Publishers
• Discusses the role of biology in gender identity from research in genetics, endocrinology and neuroscience
• Addresses important health disparities and how to address them when treating the transgender patient
• Reviews evidence-based information on the biological basis and impact of environmental and hormonal factors at different life stages
• Outlines schema for treating variations in the sexuality and sexual function of the individual patient

• Language: English
• Publisher: Academic Press
• Release date: May 19, 2020
• ISBN: 9780128159699

Book preview

.

Chapter 1

What determines biological sex?

Marianne J. Legato¹, ²,    ¹Department of Medicine, Columbia University, New York, NY, United States,    ²Department of Medicine, Johns Hopkins University, Baltimore, MD, United States

Abstract

The X and Y chromosomes establish biological sex, have a unique pattern of interaction, and are established in the haploid male and female gametes through the specialized process of cell replication called meiosis. Meiosis generates zygotes with randomly distributed chromosomes, guaranteeing the potential for extensive variation in the embryonic phenotype. In the process of replication, epimarks that regulate gene activity are erased and reestablished in a pattern reflecting the sex of the zygote. The features of the two chromosomes vary significantly; the X has over a thousand genes—5% of the total genes in the human genome. The Y’s ability to establish male sex is jealously guarded by protection from recombination with the X along a significant portion of the chromosome. The nascent gonad is neutral and composed of both germ and somatic cells that are bipotential; they have the unique property of being capable of differentiating into either a female or male lineage. The two different pathways are regulated by the complex interaction of and balance between genes that function in the first phases of gonadal development.

Keywords

Biological sex; mitotic process; meiosis; male and female gametes; gametocyte; mtDNA

Most adult mammals exhibit clear sexual dimorphism that manifests externally and internally.¹

The X and Y chromosomes are arguably the most fascinating members of the human genome: they determine biological sex. The XX combination produces a female and the XY a male. These chromosomes have a unique pattern of interaction compared with autosomes and in a specialized process of duplication called meiosis, produce gametes which are the source of a virtually infinite series of variations in gene content.

Figure 1.1 The X and Y chromosomes. NIH Image Libraries.

Figures 1.2.1 and 1.2.2 This graphic of the sex chromosomes illustrates the size difference in the two. The X chromosome contains 153 million base pairs, the Y only 50 million. The male-specific region of the Y makes up 95% of the chromosome and contains the majority of genes needed for sperm production. NIH Image Libraries.

1.1 Cell multiplication and the mechanisms of heredity

1.1.1 Mitosis

The mitotic process is the method in which cells produce two identical copies of themselves. It is responsible for the organism’s normal growth and the maintenance of tissue. Mitosis begins with a duplication of each chromosome into two sister chromatids in anticipation of the distribution of these chromatids into daughter cells. Each DNA molecular pair in a chromosome is unzipped and copied. The duplicated DNA is distributed to each of an identical pair, called sister chromatids. These will separate and each will ultimately be transmitted to a daughter cell, identical in chromosome number and composition to the parent cell. Mitosis is essential for normal growth and tissue repair/regeneration.

Figure 1.3.1 The phases of cell division in mitosis.

A failure of chromatids or autologous chromosomes to separate in cellular fission is called nondisjunction: one daughter cell gets NO copy of the original chromosome and the other gets TWO. The three most common examples in children that survive include Down’s syndrome, in which the zygote has an extra copy of chromosome 21: two from the mother and one from the father. This phenomenon of trisomy is reproduced in all the somatic cells. The rate of nondisjunction increases with age, which is why the chances for Down’s syndrome increase between 35 and 45, going from 1 in 350 at age 35 to 1 in 10 at 45. Other examples of trisomy are Trisomy 13 (Patau syndrome) in which there are three copies of chromosome 13 causing intellectual impairment and many physical abnormalities.² Edward’s syndrome is an extra chromosome 18 which occurs in about 1 in 2500 pregnancies and 1 in 6000 live births.³

Figure 1.3.2 The phases of mitosis, in which the parent cell becomes two daughter cells with genetic information that is identical and reproduces the exact genetic information of the parent cell.

1.1.2 Meiosis

In contrast to the mitotic process the sequence of events that produces the sexual gamete (either a sperm or an ovum) begins in the process of meiosis.⁴ Meiosis is a system unique to the production of male and female gametes. Meiosis generates four gametes from a single primordial germ cell each of which contains half the number of the full complement of chromosomes (haploid) so that when fertilization occurs, the resulting zygote has the full complement of 46 chromosomes—half from each of its two parents. This reduction of the number of chromosomes is achieved in two sequential processes known as meiosis 1 and meiosis 2.

Figures 1.4.1 and 1.4.2 In meiosis I, prophase is divided into five subphases: leptotene, in which chromosomes begin to condense; zygotene, in which homologous chromosomes synapse and form four chromatids; pachytene, in which portions of homologous chromosomes cross over to form chiasmata; diplotene, in which homologous chromosomes start to separate but remain attached; and diakinesis, in which homologous chromosomes separate.

The enormous variety of genetic composition in each of the gametes creates the advantage of sexual reproduction and ensures a potentially almost infinite diversity of traits in the phenotype. This is achieved in two stages: the first is the result in the first cell division (meiosis 1) of an extensive exchange of DNA between tightly connected autologous chromosomes (one from each parent) prompted by a programmed induction of DNA double-strand breaks, in the phenomenon called crossover. The distribution of the sites of these breaks is not random but is controlled by PR domain-containing 9 (PRDM9).⁵ This system avoids the targeting of basic regulatory elements in the DNA strand. There are also newly identified partners in the process of coordinating and regulating the crossover and noncrossover pathways; the significance of noncrossover pathways has not yet been completely elucidated.

The second meiotic cell division (meiosis 2) further promotes genetic variation when the newly constituted chromosomes are randomly arranged into two groups. These are separated in turn and delivered to two new cells, called haploid gametocytes. In males, this results in two secondary spermatocytes, but in females, a large secondary oocyte and a much smaller polar body are produced. [Importantly, in spite of its significantly smaller compliment of cytoplasm, the polar body contains all the genetic material (both chromosomal and mitochondrial) of the parent gametocyte. Each polar body continues to meiotically divide.]

In females, meiosis I is initiated synchronously in all the cells of the fetal ovaries early in development, producing secondary oocytes, but is arrested after birth until a second meiotic division (meiosis 2) resumes in selected primary oocytes after puberty. In males the first and second meiotic divisions go on in an unbroken sequence throughout reproductive life; each primordial germ cell produces four haploid spermatids. In females the second meiotic division occurs after fertilization; the fertilized egg contains two haploid pronuclei (one from the father and the other from the mother) and three haploid polar bodies.

Figures 1.4.3 and 1.4.4 In meiosis II the events are analogous to a mitotic division, although the number of chromosomes involved has been halved in meiosis I.

The polar body is of special interest because like its larger sister cell, the polar body contains—and is a potential source of—the entire compliment of both chromosomal and mitochondrial DNA; thus it has several potential uses. Ma et al. describe the generation of functional human oocytes, for example, following the injection of polar body genomes from metaphase II oocytes into enucleated donor cytoplasm.⁶ This is an important contribution to the treatment of infertility; current assisted reproductive techniques are limited by the number and quality of oocytes, which decline as women age; similarly, the time to conception and the likelihood of miscarriage increase as women grow older. Leridon, writing in 2004, commented that all the then available techniques for assisted reproduction could not compensate for the natural decline of fertility after age 35.⁷ Similarly, polar bodies have the capacity for mitochondrial-replacement therapy to prevent transmission to a subsequent generation of mitochondrial DNA (mtDNA) disease.⁸ When mitochondrial mutation is over 60%, progeny may develop severe systemic disease, such as myopathies, neurodegenerative diseases, diabetes, cancer, and infertility. Wang comments, currently, inherited mitochondrial diseases are incurable and the treatments available are predominantly supportive. Although mtDNA contains only 37 genes, over 700 mutations in mtDNA have been identified.⁹ Wolf et al. discuss the possibility of mitochondrial-replacement therapies in oocytes or zygotes to prevent second-generation transmission of mtDNA defects.¹⁰ Finally, Verlinsky et al. have demonstrated the feasibility of performing preconception genetic analysis by the removal and analysis of the first polar body, which bypasses the need to biopsy preembryonic or extraembryonic cells and allows embryo transfer without the need for cryopreservation required for blastomere biopsy at the eight cell stage.¹¹

Meiosis in the male involves an additional phenomenon, meiotic sex chromosome inactivation. This silencing the X component of the male germ cell as it enters meiosis protects against aneuploidy in subsequent generations.¹² (Even this process is not clear-cut; some genes on the silenced X may escape inactivation.)

Primary gametocytes of both sexes have chromosomes composed of two sister chromatids. However, the timing of entry into meiosis is not the same for the two sexes. As we have said, female oocytes enter meiosis 1 in a completely synchronized fashion toward the last part of embryogenesis. The meiotic process is halted then until puberty, when oocytes enter meiosis 2 to produce oocytes capable of fertilization; in contrast, germ cells in the testes are arrested in G1–G2 phase of replication until birth; as spermatogonia mature they initiate meiosis and generate sperm continuously throughout the life span of the male.

Figure 1.5 Summarizing the difference between mitosis and meiosis.

1.2 Epigenetics: how genes are regulated

In the process of cellular replication the faithful reproduction of core DNA structure in daughter cells is not the only element involved. In an important review, Tchurikov summarizes our rapidly evolving current concept of the functional genome, pointing out that genetic expression is tightly regulated by a system of small chemical elements that attach to DNA without altering its core structure, as well as to proteins, RNA molecules, and perhaps other macromolecules in the chromatin network.¹³ These elements that regulate the expression of genes, called epimarks, are also faithfully transmitted to the daughter cell.

Tchurikov writes

The progeny obtains from the parents not pure DNA but DNA as a part of a structured genome, and moreover, as a constituent of a fertilized cell that exists under strictly controlled conditions. Some programs responsible for reading the information encoded in DNA are in epigenomic structures, or the epigenome…These structure are retained during mitosis, play an important role in development and are inherited in the set of cell generations. During differentiation, new structures appear and are retained and this is how tissues and organs are developed from stem cells.

Specific modifications of gene expression, among others, involve methylation of specific sites in DNA (e.g., in the phenomenon of parent-specific imprinting of some genes), histone modification, elements involved in the production of mRNA, and mobile elements (movable sequences in the genome that include promoters, insulators, and other regulatory elements).

Epigenetic control of gene expression is essential to maintain cell identity; if all genes were permitted to produce indiscriminately, no cell group could maintain its identity. Reik’s thoughtful insights into what epimarks must be stable, and others flexible in development summarize the dynamic sequence of erasure and reestablishment of epigenetic programming as development proceeds:

During the early stages of development, genes that are required later in development are transiently held in a repressed state by histone modifications, which are highly flexible and easily reversed when expression of these genes is needed. During differentiation, genes that are crucial for pluripotency are silenced by histone modifications as well as by DNA methylation. Some of these genes are also silent in mature germ cells, meaning that epigenetic marks probably need to be reversed rapidly after fertilization to allow reexpression of pleopotency-associated genes in the next generation. By contrast, long-term silencing of transposons and imprinted genes needs to be stably maintained from the gamete into the early embryo and the adult organism. ¹⁴

While many epigenetic marks inherited by germ cells are erased in early embryogenesis, some epimarked genes can be passed along to future generations when the modifying mark is resistant to erasure [as is the case with DNA modified by the protein stella (also known as DPPA3)].

Epigenetic changes are sex-specific: the heterochromatic X is reactivated in oogonia 2 months before birth, presumably to reap the benefits of recombination before the imprinting of the X chromosome occurs in the fertilized cell. In contrast the X and Y chromosomes are both silenced in the male since recombination of these two might result in harmful mutations as a function of crossover in meiosis 1.