Cytogenetics

Chromosome Structure, Cytogenetic Analysis; Classes of Chromosome Aberrations; Autosomal Abnormalities

Structure and Behavior of Sex Chromosomes; Sex Chromosome Abnormalities

M.R. Seashore, M.D.

Textbooks: Gelehrter, Collins, and Ginsburg, Chapter 8

Strachan and Read, Chapter 2; and Chapter 15 pp 416-418

Summary

Chromosome structure and function

Idiograms of chromosomes

Attempts to study human chromosomes have been made for nearly 100 years as seen in the last lecture. However, most of the knowledge of human chromosomes and chromosomal disorders has come since 1956 when the correct number of 46 chromosomes was first determined and especially since 1959 when the first disorder, trisomy 21 in Down Syndrome, was discovered.  Until 1970 the only available chromosome staining methods stained for the most part stained chromosome arms uniformly making specific identification of most chromosomes impossible; chromosomes could be classified by total length and arm ratios. Therefore only numerical abnormalities and some structural aberrations affecting relatively large portions of chromosomes could be detected. When chromosome banding techniques became available in the early 1970's previously obscure structural abnormalities were found, adding to the list of deletion or duplication syndromes. Further improvements in banding, known as "high resolution banding", in the late 1970's made possible the detection of even smaller structural abnormalities and furthered the understanding of the cause of additional conditions, both those with constitutional (congenital) abnormalities and those with those with acquired abnormalities, particularly those found in cancers. This lecture will deal only with the constitutional abnormalities while those on cancer will consider the significance of acquired chromosome abnormalities.   Most recently the availability of DNA molecular methods, frequently combined with the traditional cytogenetic methods, has aided the delineation or detection of structural changes that are at or beyond the limits of microscopic resolution.

For chromosome study various sources of dividing cells are available, including peripheral blood, bone marrow, tissue biopsies, amniotic fluid, and chorionic villi. A variety of staining methods are available; most useful are those that give chromosomes a banded appearance. Other methods highlight or differentially stain certain regions of the chromosomes and are therefore useful in distinguishing between heteromorphisms (normal variants), which are benign, and duplications or deletions of euchromatin, which are deleterious.

How the long strand DNA of a chromosome is packaged into a chromosome has interested investigators for many years; some partial answers have been obtained. Packaging can be viewed as a stepwise or ordered process.  The steps of mitosis and meiosis have been well known for many years; these processes should be reviewed. Unfortunately, little is known regarding the forces behind mitosis and meiosis or the factors which lead to errors of chromosome segregation.

Chromosomal abnormalities

Chromosomal abnormalities can be classified as numerical or structural. Numerical abnormalities are those in which there is an extra set(s) of the basic or haploid number (triploidy with 3 sets and tetraploidy or 4 sets), those with a missing chromosome (monosomy) and those with an extra chromosome(s) (trisomy, double trisomy). Structural abnormalities include: deletions, rings, duplications, inversions, isochromosomes and translocations.

In the numerical abnormalities and deletion, ring, duplications and isochromosomes there is a variable amount of chromosomal material lost or gained. These imbalances lead to phenotypic abnormalities of varying severity. Loss of chromosomal material is more deleterious than gain of the same portion of a chromosome. Generally, abnormalities of autosomal chromosomes have more serious consequences than similar abnormalities of the sex chromosomes.

Chromosome abnormalities have their highest frequency during fetal life and are a major cause of fetal loss. By live birth the frequency has decreased and, because several the major autosomal trisomies lead to early death, the frequency in older children and adults is even lower.

Numerical and structural abnormalities of chromosomes both lead to phenotypic effects. The nature of the relationship between the chromosomal abnormality and the phenotype is not clear in most cases. Unlike the single gene traits, these phenotypes are the effect of more than one gene. Deletion of genetic material is more deleterious than an excess of a similar amount.  There is a range of phenotypic effects within each of the disorders or syndromes due to the same chromosomal abnormality. Further variability can result from mosaicism. In particular, the presence of a normal cell line in the same individual can ameliorate the effects of a chromosomal abnormality, sometimes to the extent that the phenotype appears normal.

The phenotypic features which are found in the various conditions due to different chromosome abnormalities are not specific for any one abnormality. However, when a certain group or constellation of features occurs commonly in persons with the same cytogenetic abnormality a clinical syndrome can be established.

Although balanced rearrangements (translocations and inversions) usually do not affect development, carriers of such abnormalities are at increased risk of having offspring with unbalanced rearranged chromosomes which cause congenital malformations.  Many deletions, both microscopically visible and submicroscopic, have been identified which produce recognizable clinical syndromes. The phenotype in individuals with these deletions seems to be the result of loss or interruption of several contiguous genes along the involved chromosome. Improved resolution of microscopic techniques and addition of molecular techniques to the examination of chromosomes will probably elucidate more such deletions. To further cloud things, at least one condition due to a chromosomal deletion has two different phenotypes, depending on the parental origin of the chromosome bearing the deletion.

Special features of sex chromosomes

All cells have a pair of sex chromosomes, XX in the case of females and XY in the case of males. The X chromosome was the first to have specific disease genes mapped to it. Now there are at least fifty diseases and 200 genetic markers which have been assigned to the X-chromosome. Beyond the embryonic stage all somatic cells are hemizygous with respect to the X, since males have only one X and in females one X is inactivated early in embryonic life.

The sex chromosomes play the major but poorly understood role of determining the direction of sexual differentiation in the developing fetus. One of the few genes which have been definitively assigned to the Y-chromosome is the gene for testis-determining factor, believed to play a major role in sex-determination. While the formal proof for this hypothesis remains to be developed, and the mechanism is as yet unknown, there is good evidence for the existence of this gene from studies of humans with sex reversal: females with XY karyotype and males with XX karyotype. There is a region of homology between the X and the Y called the pseudoautosomal region in which there is recombination between X and Y.

The X-chromosome is the only one to undergo inactivation in somatic cells. This inactivation is a form of dosage compensation which acts in females to create a balance in gene expression between the X-chromosome and autosomal genes. This is necessary because of the fact the males have only one X-chromosome. An important consequence of this phenomenon is somatic cell mosaicism for genes coded for by the X-chromosome. In each cell the selection of which X is to be inactive, the maternally derived or the paternally derived, is random, and all the progeny of that cell have the same X inactivated.

Abnormalities in number and structure of the sex chromosomes occurs among spontaneous abortuses and among liveborn. In general sex chromosome abnormalities in contrast to abnormalities of autosomes, are associated with less severe phenotypic abnormalities. Indeed, in some cases an extra X or Y chromosome may have no obvious effect on the development of an individual. In both males and females an extra X lowers to some extent the intelligence of the individual, increasing the frequency of mental retardation (MR) in these groups of persons. There is a correlation between the number of extra Xs and severity of MR and physical abnormalities so that those with three additional Xs are much more defective than those with one extra X. An extra X causes sterility in the male, but not the female.

In contrast with additional sex chromosomes the presence of only a single sex chromosome has profound effect. A single Y (45,Y) or lack of any X is highly lethal, never having been found in recognized pregnancies. Although not always lethal, a single X (45,X) usually results in spontaneous abortion of the fetus; a small number of such conceptuses survive to livebirth and have a constellation of anomalies known as Turner syndrome. They are almost always infertile as adults.

A fragile site at the distal end of the long arm of the X is seen in males who have mental retardation and a characteristic facies, along with large testes after puberty. This condition, known as fragile X syndrome, is the most common cause of familial mental retardation. This and other mutations on the X chromosome account for the excess of mental retardation in males as compared to females. Although it is inherited as a Mendelian, X-linked condition, it is discussed here because it has a characteristic chromosome marker, the so-called fragile site on the X-chromosome. There are a number of unusual aspects to this syndrome relating to phenotypic expression and inheritance that make it an apparently unique genetic disorder.