S. Dahoun-Hadorn and C. Williamson
Division of Medical Genetics, Department of Genetics and Microbiology,
University Medical Centre, 1211 Geneva 4, Switzerland
The pathology of the human chromosomes (the " substance " of heredity) is the subject of cytogenetics, in which the chromosomes are examined and a karyotype established, at the resolution level of the optical microscope.
Cytogenetic analysis is performed by pairing our forty-six human chromosomes in a well-defined order based on characteristic bands inherent to each chromosome. This analysis is undertaken on a cell undergoing mitosis (metaphase).
A karyotype may be established on any cell line capable of division (mitosis), but readily accessible lineages are generally used i.e.: lymphocytes extracted from whole blood, or, in case of postnatal diagnosis, fibroblasts grown from skin biopsies. For prenatal purposes, trophoblast-derived cell lines (choriocentesis), amniocytes (amniocentesis) or lymphocytes from fetal blood (cordocentesis) are studied. Cell lines grown from curettage material (abortion) may also be used. Sperm and oocyte karyotypes may also be established on the basis of cross-fertilization techniques recently developed.
The cells obtained are processed in the following manner:
- Culture: the duration is determined by the type of tissue and can be omitted in the case of cells with a high spontaneous mitotic capacity such as trophoblastic cells of placental origin.
- Colchicine treatment: abruptly stops the mitosis in metaphase.
- Hypotonic shock: swells the cells and allows the cytoplasm to be eliminated.
- Production of bands by various denaturing techniques which give rise to patterns specific to each chromosome.
- Examination of several dividing nuclei. The chromosomes are paired (23 pairs) and numbered in order of size, the first pair being the longest.
A human male has 44 autosomes and 2 gonosomes (XY).
A human female has 44 autosomes and 2 gonosomes (XX).
In normal chromosome structure the centromere or central core is flanked by two " arms ": the short arm (p) and the long arm (q). The basic structure is the same for all chromosomes, with the exception that the acrocentric chromosomes have little or no short arm. The 23 pairs differ in the length of the arms, and each shows unique banding pattern.
Numerical aberrations, which can affect either the autosomes or the gonosomes (sex chromosomes), involve the loss or gain of a part of or an entire chromosome. This gives rise in the latter case to a trisomy (a chromosome of which three copies are present) or in the former to a monosomy (a single copy).
Structural aberrations affect a portion of one or several chromosomes; these can be of several types, and imply in general that a break has taken place and given rise to one of the following rearrangements:
- Translocations, the most frequent type of rearrangement between two chromosomes or two arms of the same chromosome, involve an exchange of genetic material; depending on whether the quantity of chromatin is modified, they are either balanced or unbalanced.
- Insertions of chromosomal material in an unusual location.
- Inversions: a portion of a chromosome is upside-down.
- Ring chromosomes: annealing of the two extremities of a chromosome after loss of their terminal segments.
- Isochromosomes: one arm is present in two copies, separated from each other by a centromere.
Clinical characteristics of unbalanced chromosomal aberrations
Missing or excessive autosomal material (e.g. trisomies) gives rise to two or more of the following, and causes clinically-recognizable syndromes:
- Mental retardation.
- Dysmorphic features.
- Malformation of internal organs.
- Retarded growth (of prenatal or postnatal origin).
Although the number and importance of symptoms is variable even within a group of patients with the same anomaly, a certain pattern generally exists, producing a characteristic phenotype which usually permits clinical diagnosis. Mental retardation is the common denominator. Monosomies of entire autosomes are incompatible with life, but partial monosomies due to small or even large deletions can affect all chromosomes, generally producing severe clinical syndromes.
Clinical characteristics of balanced autosomal rearrangements
In this case, the normal quantity of chromosomal material is preserved, but in disorganized fashion either due to an exchange between two chromosomes (translocation), with no net loss or gain of genetic material, or due to a centromere fusion. Whatever the structural defect, there is rarely a major clinical abnormality, but reproductive problems may arise; in women spontaneous abortion is most often seen, whereas in men spermatogenesis may be impaired, resulting in primary sterility.
Clinical characteristics of gonosomal abnormalities
The clinical implications are different from autosomic abnormalities: growth retardation is less frequent (with the exception of X monosomy or Turner’s syndrome), and in some sex chromosomal disorders, growth may even be accelerated. Dysmorphism is more subtle, and internal malformations and mental retardation are infrequent. Borderline intelligence is however not unusual in XXY and XXX syndromes, and 45,X women have a higher incidence of internal (heart and renal) malformations than women with other disorders.
X chromosome monosomy is the only example of a monosomy compatible with life but even conceptuses with this defect are for the most part " eliminated " as spontaneous abortions.
Most important are fertility problems secondary to gonadal dysgenesis, which is an almost constant feature in large X chromosome structural defects. In X monosomy, however, the presence of two cell lines (a mosaicism) may lead to fertility, especially if the normal cell line predominates. Reproduction may also remain possible if the affected gonosomal segment is sufficiently small.
When is a karyotype indicated?
- In pregnancies of older women, the incidence of aneuploidy (the presence of an abnormal number of chromosomes) rises steeply after the age of 35 years (about 1/200 births) and reaches 1/50 by 40 years of age. See Table 1 for the incidence of Down Syndrome.
- In the case of familial chromosomal aberrations, whether they concern autosomal translocations or maternal gonosomal mosaicisms (45,X/ 46,XX/ 47,XXX).
- In familial hereditary disorders of unknown etiology, to exclude a visible chromosomal abnormality, in parallel with molecular studies.
Prenatal diagnosis is sometimes offered to reduce the anxiety of parents of a child with a congenital defect, whether of chromosomal origin or not; this is not, however, based on medical need.
When at least two of the following abnormalities are present:
- Internal malformations.
- Facial dysmorphism.
- Delayed psychomotor development.
- Small stature, or retarded growth.
- Delayed puberty.
- Recurrent spontaneous abortions: in this case, 1/17 couples will carry a familial aberration in a balanced form, most frequently a translocation. When these are transmitted in an unbalanced form, partial monosomy or trisomy may ensue, often leading to spontaneous abortion.
- Sterility in males with azoospermia or oligospermia (sperm count inferior to 10 million/ ml), with or without testicular atrophy. This may be due to gonosomal or autosomal abnormalities, the latter occasionally impairing fertility even in a balanced form (the degree of impairment varying from mild reproductive difficulties to total infertility).
- Sterility in females: primary or secondary amenorrhea; premature ovarian failure; gonadal dysgenesis. Usually the X chromosome is present in an abnormal number, in all cells or as a mosaic. See Table 2 for examples of chromosomal abnormalities found in 258 subfertile patients.
It is well to bear in mind that a single chromosome carries several thousand genes, and that monogenic disorders, or even those involving several genes cannot be determined by cytogenetic techniques alone, the purpose or which is to detect major aberrations, involving chromosome number or structure.
Nonetheless, karyotyping remains the essential tool for analysis of some forms of reproductive difficulties, in prenatal screening, and in the diagnosis of a number of relatively frequent clinical syndromes.
- Gelehter, T.D., and Collins, F.S. (1990): In: Principles of medical genetics. Williams & Wilkins, Baltimore.
- Navarrete, C., and Salamanca F. (1986): Ann. Genet. (Paris), 29:98-103.
- Thompson, M.W., McInnes, R.R., and Willard, H.F. (1991): In: Genetics in medicine. W.B. Saunders Co., Londres.