Chromosomes are structures made up of proteins and DNA and are found in the cell nucleus. They can be divided into autosomal chromosomes (responsible for inheriting unisex characteristics) and sex chromosomes (responsible for sex determination and inheriting sex-linked characteristics). The normal number of chromosomes in a human being is 46 (22 pairs of autosomal chromosomes and one pair of sex chromosomes) – any abnormality in their number and structure leads to genetic defects. The set of chromosomes in a healthy cell is called a karyotype.
Chromosome aberrations can be divided into two main types – structural aberrations and numerical aberrations. Structural aberrations are changes in the structure of a chromosome and include deletion (loss of a chromosome and the genes it contains), duplication (doubling of a chromosome fragment and the genes it contains), inversion (turning a chromosome fragment over 180 degrees), translocation (moving a chromosome fragment to another location on the same or another chromosome), An isochromosome (an abnormal chromosome that lacks one arm and doubles the other) or a chromosome ring (formed when a chromosome breaks at both arms and the distal arm is lost and the remaining chromosome pieces combine to form a ring).
The second type of chromosome aberrations are numerical aberrations – the changes caused by these are usually the loss of one chromosome (monosomy) or the appearance of an extra chromosome (trisomy). Numerical mutations are usually caused by abnormalities (e.g. chromosomes not separating) in the process of division of germ cells.
Among the diseases associated with the presence of chromosomal aberrations in children, a distinction is made between syndromes caused by trisomy (the most common are trisomies of 21, 18 or 13 chromosomes), chromosomal deletion and microdeletion syndromes, and sex chromosome aberration syndromes.
Genetic defects in children – trisomies
The most common trisomy is trisomy 21, also known as Down’s syndrome. It occurs with a frequency of 1:700 births, but the likelihood of having an affected child increases with the age of the mother (the risk of Down’s syndrome in a child of a pregnant woman aged 35 is 1:360 and increases to 1:100 at the age of 40 and 1:30 when the woman is 45). More than 60% of pregnancies with a trisomy of chromosome 21 in either the embryo or the fetus result in spontaneous abortion.
There are several forms of this trisomy – a full trisomy (all cells in the body have an extra chromosome 21), a mosaic trisomy (only some cells contain the extra chromosome) or a translocation trisomy (the extra material on chromosome 21 can translocate to another chromosome) – but the development of a child with Down’s syndrome does not depend on the type of trisomy.
- External features which can identify a child with trisomy include slanted eyelid crevices, a sunken ridge of the nose, drooping corners of the mouth, a short neck, short broad hands or bright spots regularly placed on the iris.
- The height of children with trisomy 21 is lower than that of their peers, and they tend to gain weight.
A characteristic feature of affected children is also a delay in the acquisition of verbal and motor skills. In addition, people with Down’s syndrome are more likely to suffer from acute myeloid leukaemia, and almost 90% of sufferers aged 35-40 have symptoms of Alzheimer’s disease. The average IQ of a sufferer is 35-49 (moderate mental retardation). Men are usually infertile and in women fertility is significantly reduced. Currently, people with Down syndrome live approximately 50-60 years.
Another of the trisomies is trisomy 13, called Patau syndrome – the incidence of this syndrome is 1:8000-12000 live births (the risk of the disease increases with the age of the mother). As with Down’s syndrome, a distinction is made between its full, moisomic and translocation forms. In the newborn with trisomy 13 there are numerous congenital defects – microcephaly, loss of skin on the head, flat nasal ridge, low set ears, oversized fingers or fusions of fingers in the limbs. In children of several years of age, muscular hypotonia, lack of speech and independent walking, and deafness are observed. However, most children with Patau syndrome die in the neonatal period, only 5% survive to 1 year of age.
Another condition associated with trisomy 18 is Edwards’ syndrome, which occurs at a rate of 1 in 8,000 births – as with Down’s and Patau’s syndrome, the risk of developing this condition increases significantly with maternal age. Newborns with trisomy 18 are characterised by low birth weight, a protruding occipital bone, a small mandible and mouth, narrow eyelids, low-set and malformed auricles, clenched hands, overlapping fingers and deformed feet. In addition, affected children have numerous defects of the heart, gastrointestinal tract, urinary tract and osteoarticular system. The syndrome is also characterised by seizures, difficult breathing and severe disturbance of psychomotor development. The majority of pregnancies with trisomy 18 are spontaneous abortions, and the children born usually die during the first weeks or months of life (only 5-10% of patients live to the age of one).