DNA testing of the most common forms of trisomy

What is a trisomy?

Every human cell contains 46 chromosomes, that is, 23 chromosome pairs. Half (23) of the chromosomes are inherited from the mother and half (another 23) from the father. Trisomy is a chromosomal pathology – an error during the cell division process when an extra, third chromosome is added to a pair of chromosomes. The cells of a foetus with this pathology contain more than 46 chromosomes. Unfortunately, such significant changes in a genome cause serious development disorders and lead to congenital pathologies.

Most common forms of trisomy:

  • Chromosome 21 (Down syndrome)
  • Chromosome 18 (Edward syndrome)
  • Chromosome 13 (Patau syndrome)
  • Sex chromosome aneuploidies (Turner (X monosomy) and Klinefelter (XXY) syndromes).

Risk factors for genetic pathologies:

  • Pregnancy in women over 35
  • A biological father older than 45 years
  • Hereditary organ pathologies in one or both parents
  • Changes in karyotype or hereditary chromosomal changes in one of the parents
  • Exposure to ionising radiation affecting one of the parents
  • A blood relationship between the parents
  • History of several miscarriages, pre-term birth or stillbirth
  • Acute viral infections, the use of some medicinal products during the first trimester of pregnancy
  • A family history of congenital foetal development anomalies

Description of the test

The trisomy DNA test requires a sample of amniotic fluid that is obtained by amniocentesis.

Amniocentesis – a method for obtaining amniotic fluid in which a fine needle is inserted though the mother's abdominal wall (through the dermal and epidermal layer and then through the wall of the uterus) to sample a small amount (10–15 mL) of amniotic fluid, accompanied by an ultrasound for control purposes. The puncture is made in the upper part of the uterus on one side of the navel – in an area away from the placenta and foetus. This procedure is performed without anaesthetic. After the needle is removed, a sterile bandage is applied on the puncture site. Amniocentesis is usually performed between the 14th and 19th gestational week. The need for amniocentesis and genetic analysis of foetal DNA is determined by the supervising physician of the pregnant woman (a gynaecologist, obstetrician, or a medical specialist in genetics). The physician assesses the risk of genetic pathologies, the course of the pregnancy and the results of previous analyses and examinations. Since amniocentesis is associated with a slight risk of miscarriage, it should not be performed without sufficient justification. Amniocentesis in Latvia is performed in Riga Maternity Hospital, the Medical Genetics Clinic of the Children’s Clinical University Hospital and Pauls Stradins Clinical University Hospital.

The GenEra laboratory provides diagnostics of the most common types of chromosomal changes (trisomies) by using the quantitative polimerase chain reaction (PCR) method with a reagent kit ChromoQuantTM (manufactured by CybergeneAB, Sweden). The obtained amniocytes are processed to isolate chromosomal DNA. The test accurately determines the number of 13, 18, and 21 chromosomes to confirm or exclude trisomy.

Test results are usually available after 24–48 hours. It is of utmost importance in cases where diagnostics are needed as soon as possible, for instance, in later stages of pregnancy, during the 18th–20th week of gestation.

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