Chromosome Abnormalities Testing and Analysis
Chromosomes carry our genetic information. They can be thought of as a car, while the genes are the passengers. We call the make-up of a person’s chromosomes a karyotype. By performing a Karyotyping test we are able to detect major abnormalities in a person’s make up.
Chromosome disorders are of conditions, caused by constitutional numerical or structural abnormalities of chromosomes. Every somatic cell of the human body consists of 46
chromosomes, organized in 23 pairs (22 pairs of autosomes and one pair of sex
chromosomes - XX in females and XY in males).
Numerical Chromosome Abnormalities
Numerical abnormalities occur when a person has extra copies of chromosomes, or when the person is missing chromosomes. Some chromosomal abnormalities cause embryonic or fetal death. Sometimes, the pregnancy continues to term, producing an infant with a combination of physical problems, health problems or learning difficulties. The severity of the conditions depends on which regions of the chromosome(s) are involved.
The most common chromosomal abnormality in newborns is Down syndrome, with a karyotype of 47,XX,+21 or 47,XY,+21 (Fig. 1a). The incidence of Down syndrome is around 1 in 1,000. Typical Down Syndrome features include delayed physical and mental
development, low muscle tone, small stature, upward slant of the eyes (Fig. 1b). Other numerical abnormalities that could survive to term include Edwards syndrome (Trisomy 18, about 3 in 10,000) and Patau syndrome (Trisomy 13, about 2 in 10,000). Trisomy 13, 18, 21 occur more frequently in fetuses of women of Advanced Maternal Age (35 or older).
Figure 1a. Down syndrome Karyotype
Figure 1b. Down syndrome child
(ref: Center of Disease Control, USA)
Numerical chromosome disorders associated with sex chromosomes (X & Y) usually discovered at puberty or during infertility assessment. Common sex chromosome disorders include Turner syndrome (Monosomy X, incidence about 1 in 4,000), Triple X
syndrome (XXX, incidence about 1 in every 1,000), Klinefelter syndrome (XXY, incidence about 1 in 1,000), and XYY syndrome
(incidence about 1 in 1,000).
Structural Chromosome Abnormalities
Structural abnormalities occur because of breakages in chromosomes. They can occur spontaneously, or they can be inherited from a parent. Structural abnormalities include deletions, duplications, translocations, inversions, isochromosomes,
supernumerary marker and ring chromosomes. Some structural abnormalities such as balanced translocations are more common than others.
Rare Chromosome Disorders
Rare chromosome disorders include chromosomal deletion, duplicated, inversions, and unbalanced translocations. Individually, rare chromosome abnormalities are indeed rare, but collectively, they are common. At least 1 in 200 babies is born with a rare chromosome disorder.
The Different Abnormalities Present Themselves in a Variety
Numerical chromosome abnormalities are when the body produces too few or too many chromosomes. The most common abnormality is Down Syndrome. A notable subset of
numerical abnormalities is those to do with the sex chromosomes. These include disorders such as Turner Syndrome which can lead to infertility in later life. Early testing can provide this information at birth, allowing the families to plan for this along with any other
implications that the chromosomal disorders may lead to in the future.
Treatments for Chromosome Abnormalities
As chromosome abnormalities affect a person’s genetic make-up, there is no direct cure for them. However, certain chromosomal disorders do have treatments designed to make life easier for those affected. These can include hormone replacement therapies and occupational or physical therapy.
How can chromosome abnormalities be detected?
Common Chromosome abnormalities can be diagnosed through Cytogenetic analysis/karyotyping. The technique is based on
microscopic examination of chromosomes in lymphocytes of the peripheral blood. Conventional Cytogenetics analyzes the whole genome in a single assay, which allows simultaneous characterization of numerical and structural chromosome abnormalities involving size of 15 Mb or larger.
Array Comparative Genomic Hybridization (aCGH) and SNP Genotyping
Recent advance in array CGH allows for the detection of copy number variations (CNVs) down to ~400kb or sometimes even smaller. This allows for more precise localization of chromosomal breakpoints, which are important for patients with interstitial deletions or Heterozygosity (LOH) and Uniparental Disomy (UPD) via “SNP genotypings”.
Who Should Take a Chromosome Analysis Screening?
Mothers above the age of 35 are at greater risk of their child developing a numerical chromosomal abnormality. Structural
chromosome abnormality can also be inherited through the parent, so if you have a family member or members with a
chromosomal disorder or if you yourself have one, this can lead to greater risk of the child developing one.
Here at HKCMGC, we offer comprehensive chromosome screening services to ascertain your risk of having or passing on any
abnormalities. Through our Karyotype testing services, we can help you gain peace of mind. Contact our Hong Kong team to get started.