Isolation and mapping of human chromosome 21 cDNA: progress in constructing a chromosome 21 expression map.

We have isolated 175 cDNA clones from a fetal brain library by direct cDNA selection using genomic DNA isolated from pools of human chromosome 21 (HC21) cosmids. DNA sequences have revealed that 16 of these cDNA clones contain overlapping sequences. Of the other 159 cDNA sequences, 10 match previously identified HC21 genes, and 9 match previously determined cDNA sequences, including the Wilms tumor related transcript (QM), the human testican cDNA, the mammalian calponin cDNA, and 6 anonymous expressed sequence tags. All isolated cDNAs were hybridized to their corresponding cosmids, which suggests that they originated from HC21. We have localized 92 cDNA clones to previously reported HC21q YACs. The remaining unmapped cDNAs contain either sequences not included in the isolated HC21q YACs or sequences that hybridize to yeast DNA. The cDNAs not included in the YACs should be useful in isolating new YACs to bridge the gaps. PCR primers were derived from 4 novel cDNA sequences that had been mapped to the YACs in the suspected Down syndrome region and used in RT-PCR analysis. All 4 primer sequences amplified RNA fragments with the expected sizes, suggesting that these sequences could be used for expression analysis. The construction of a chromosome 21 cDNA map not only is important in the refinement of physical maps, but also will identify a set of genes in the disease regions for detailed characterization.