Down syndrome: Gene dosage at the transcriptional level in skin fibroblasts

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Abstract
Chromosome imbalance (aneusomy) is the leading known cause of spontaneous abortion and mental retardation in human beings. The primary abnormality supposedly results from quantitative changes of transcription products from the unbalanced genetic material. To document this point, chromosome 21-specific transcription in skin fibroblasts from subjects with monosomy 21, disomy 21 (normal) and trisomy 21 (Down syndrome) was compared. Polyadenylylated RNA [poly(A)-RNA], which is enriched in messenger and messenger-precursor RNA sequences, was isolated from the above fibroblast lines. Radioactive DNA (cDNA) complementary to these RNA was synthesized with reverse transcriptase (RNA-dependent DNA polymerase). These cDNA were hybridized with (i) DNA from a cell line with a mouse genome plus human chromosome 21 and (ii) mouse DNA. Subtraction of the amount of hybridization in experiment ii from that in experiment i yielded a measure of human chromosome 21-specific RNA sequences. Gene dosage may exist at the transcriptional level: for monosomy 21-derived cDNA, 0.6% (of the total cDNA) hybridized specifically to human chromosome 21; for disomy 21-derived cDNA, 2% hybridized; and for trisomy 21-derived cDNA, 3% hybridized. For DNA sequences on chromosome 21 in human skin fibroblasts, transcription depends on DNA dosage. Characterization of the chromosome 21-specific RNA sequences quantitated in these experiments could help to elucidate the mechanisms by which abnormal karyotypes result in abnormal phenotypes.