Genetic syndromes or chromosomal abnormalities are disorders in which pathological genes or chromosomes impact developmental aspects of a child. They may affect physical characteristics, linguistic, mental and social development, or provoke changes in behaviour. A child may be born with obvious physical deformity, organ dysfunction or neurological problems, or may appear completely normal at birth and develop problems gradually.
What are genetic syndromes due to?
Each human cell contains 46 chromosomes that form 23 homologous pairs. A child inherits half of the chromosomes from the mother and the other half from the father. Therefore, one chromosome in each homologous pair comes from the father and the one from the mother.
One of the pairs defines gender, XY for boys and XX for girls. The rest are common to all cells and are called autosomal. Each chromosome contains genes, which are responsible for the synthesis of proteins, which in turn are related to development and body functions.
Genetic syndromes or chromosomal abnormalities are created when one or more gene or chromosome is missing or mutated, or if there are extra ones. As a result, protein synthesis is inhibited, compromised or accelerated. Some of these abnormal chromosomes come from the parents, while others are produced by the body itself.
Genetic Syndromes: How are they classified?
Although there are many types of genetic syndromes / chromosomal abnormalities, they can be classified into two main categories: numerical chromosomal abnormalities and structural chromosomal abnormalities.
Numerical Abnormalities
When a person is missing one chromosome in a pair, the condition is called monosomy. When there are more than two chromosomes in a pair, the condition is called trisomy.
Down Syndrome is a typical example of a numerical chromosomal abnormality. The most common type is trisomy 21, where there are three copies of chromosome 21 instead of two. Down Syndrome is typically associated with cognitive impairment, learning difficulties, poor muscle tone (hypotonia), loose joints, short broad head (brachycephalia), etc.
Turner Syndrome is an example of a monosomal numerical abnormality (one instead of two chromosomes). The syndrome occurs in girls who have only one X chromosome.
Structural Abnormalities
The structure of the chromosome may vary due to:
- Deletion: Part of the chromosome is missing or deleted.
- Duplication: Part of the chromosome is copied.
- Translocation: Part of the chromosome is replaced by another chromosome or the entire chromosome is linked to another chromosome.
- Inversion: The chromosome breaks apart, turns upside down and reattaches.
- Rings: Part of the chromosome breaks and a ring is formed.
Genetic Syndromes: Why do they appear?
There are two factors associated with the occurrence of chromosomal abnormalities:
- Family history
- The age of the mother.
Genetic Syndromes: Turner Syndrome
Turner Syndrome is a rare, multi-systemic disease (1:2500) that occurs in girls and is due to the lack or incomplete formation of one of the two X chromosomes. There are two types of syndrome, classic and mosaic. Depending on the type of syndrome, the girls exhibit the corresponding symptoms, mainly:
- Short stature
- Delayed puberty
- Webbed neck (extra skin)
- Kidney or heart disease
- Swollen extremities
- Impaired hearing
- Thyroid disorders
- Type 1 diabetes mellitus.
Diagnosis
Turner Syndrome can be diagnosed during the first few months of the child’s life (swelling at the back of the neck, heart problems), or during puberty. The doctor will order a blood test (karyotype) to check for the lack of all or part of the X chromosome. Depending on the case, a pelvic ultrasound may also be performed.
It can also be diagnosed during pregnancy through amniocentesis or chorionic villus sampling (CVS).
Treatment
Once the syndrome is diagnosed, growth hormone is administered to improve the stature of the child. Estradiol and progesterone may be administered at about 12 years of age. Cardiac disorders are sometimes corrected surgically. Regular monitoring by an endocrinologist is very important.
Genetic Syndromes: Prader-Willi Syndrome (PWS)
Prader-Willi Syndrome is a rare neurodevelopmental genetic disorder caused by a defect in chromosome 15. Children with Prader-Willi exhibit:
- Poor muscle tone (hypotonia)
- Poor sucking reflex in infancy
- Overeating and weight gain after the age of two, leading to obesity
- Low stature
- Cognitive impairment
- Delayed puberty
- Underdeveloped genitals
- Behavioural problems
- Narrow forehead and almond shaped eyes.
Diagnosis
Diagnosis is based on clinical symptoms and is confirmed by blood tests (karyotype).
Treatment
Treatment focuses on the symptoms and includes the administration of growth hormone to develop muscle mass, increase stature and normalise facial features. By administering pituitary hormones, the secondary characteristics of the sex are improved.
Genetic Syndromes: Klinefelter’s Syndrome
Klinefelter’s Syndrome is the genetic disorder in which boys have two X and one Y chromosome. Children exhibit:
- Reduced muscle tone
- Slow motor development and delay in speaking
- Short torso and long arms
- Weak bones
- Taller than average stature
- Breast tissue development
- Reduced hair growth.
Diagnosis
The diagnosis can be made in prenatal tests. More often however, the diagnosis is made through the observance of symptoms and tests measuring testosterone and chromosomes.
Treatment
Treatment focuses on the symptoms. Testosterone replacement therapy is recommended starting from puberty, in order to achieve development of muscles, bones and secondary sex characteristics.