GENE DOSAGE IN CAPILLARY ELECTROPHORESIS - PRENATAL-DIAGNOSIS OF DOWNS-SYNDROME

Modern proposals for pre-natal genetic analysis of Down's syndrome con sist in isolating DNA from amniotic cells and amplifying a highly poly morphic small tandem repeat region of the chromosome 21-specific D21S1 1 marker. The polymerase-chain-reaction-amplified fragments are typica lly 5'-end labelled with a green or blue fluorescent reporter and data acquisition occurs on-lane in DNA sequencing gel-slabs and equipment. The following patterns are expected: for normal individuals, 1 peak o r two peaks in a 1:1 ratio. In the case of trisomy 21, the following p atterns are found: either three peaks in a 1:1:1 ratio or a two-peak p rofile with a 2:1 gene ratio. We have developed a capillary electropho retic system, offering precise diagnostic value by exploiting the intr insic DNA absorbance at 254 nm. The separation occurs in capillaries c oated with an extremely stable and hydrophilic layer of poly(N-acroylo yl amino ethoxy ethanol) and filled with a background electrolyte cons isting of 89 mM Tris-borate, 2 mM EDTA, 2.5 mu M ethidium bromide and 8% short-chain, low-viscosity, replaceable, liquid, linear, sieving po lyacrylamide. The technique offers high reproducibility and precise on -line, automated peak acquisition and quantitation.