Gene dosage in capillary electrophoresis: Pre-natal diagnosis of Down's syndrome

Modern proposals for pre-natal genetic analysis of Down's syndrome consist in isolating DNA from amniotic cells and amplifying a highly polymorphic small tandem repeat region of the chromosome 21-specific D21S11 marker. The polymerase-chain-reaction-amplified fragments are typically 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 or two peaks in a 1:1 ratio. In the case of trisomy 21, the following patterns are found: either three peaks in a 1:1:1 ratio or a two-peak profile with a 2:1 gene ratio. We have developed a capillary electrophoretic system, offering precise diagnostic value by exploiting the intrinsic DNA absorbance at 254 nm. The separation occurs in capillaries coated with an extremely stable and hydrophilic layer of poly(N-acroyloyl amino ethoxy ethanol) and filled with a background electrolyte consisting of 89 mM Tris-borate, 2 mM EDTA, 2.5 mu M ethidium bromide and 8% short-chain, low-viscosity, replaceable, liquid, linear, sieving polyacrylamide. The technique offers high reproducibility and precise on-line, automated peak acquisition and quantitation.