PRENATAL GENETIC DIAGNOSIS OF DOWN SYNDROME BY USING QUANTITATIVE REAL TIME POLYMERASE CHAIN REACTION IN UNCULTURED AMNIOCYTES

Abstract

Prenatal genetic diagnosis is one of the exciting advances in medicine. It is an important option for many families with an increased risk for having a child with a birth defect or genetic disease and rapidly is becoming an important tool of preventive medicine.<1

Prenatal genetic diagnosis has been introduced early in 1960s when amniocentesis was performed to women with advanced maternal age as they were at an increased risk for having an aneuploid fetus mainly Down syndrome.< 2!

Chromosomal aneuploidy is one of the major causes of fetal abnormalities that cause fetal and neonatal morbidity and mortality. The most frequent of these chromosomal abnormalities is Down syndrome (trisomy 21), which has the birth prevalence of 1 in 800.<3> In Egypt, it is estimated to be 1 in 1000 livebirth. <4> Consequently it is considered as one of the most common genetic birth defect and the most frequently observed chromosomal anomaly among live births.

Chromosome analysis of fetal cells obtained by amniocentesis or chorioninc villous sampling was the ideal method for the diagnoses of Down syndrome. However, the high cost, difficulties in cell culture and the long time required for the results to be obtained are profoundly distressing to the parents. (S)

During the last decades, recent efforts have been directed at developing non-invasive prenatal screening techniques. There have been great improvement in the field of prenatal diagnosis and screening that could not only improve sensitivity of prenatal screening, but also be employed in the first trimester to offer earlier diagnostic and interventional opportunities. Currently, ultrasound scanning as well as maternal serum biochemical markers are an integral part of routine prenatal screening. < 6

Nuchal translucency has been proved to be an effective ultrasound based screening test that, when combined with serum markers (free beta human chorionic gonadotropin and pregnancy-associated plasma protein) in the first trimester, broadens the diagnostic possibilities and improves the diagnostic capabilities of current prenatal trisomy 21 screening methods.(7)

Besides routine karyotyping, fluorescence in situ hybridization (FISH) testing for trisomy 21 is the most common test performed in a prenatal setting. In contrast to the 7 to 14 day turnaround time for full karyotyping, FISH can provide accurate result within 24 to 48 hours. The main limitations of FISH is high cost and its unsuitability for automation.< 8>

Alternative methods have been developed which offer rapid, cheap and accurate prenatal diagnosis based on the analysis of fetal DNA that can be done early during pregnancy and provide results within a short period of time which relieves women from anxiety and provides an early decision on their pregnancy management when there is an abnormal result.

Polymerase chain reaction (PCR) is a very useful method for the detection of small amount of DNA but it is not an ideal method for quantitative analysis. The real-time