Application of molecular and somatic cell genetics to the study of chromosome 21

An extra copy of human chromosome 21 has been known for over twenty years to be the chromosomal abnormality in Down's syndrome; however, the biochemical and molecular basis governing expression of the phenotype is still poorly understood. Using the methods of somatic cell and molecular genetics, we have been studying genes and DNA sequences on chromosome 21 by constructing hamster/human hybrids containing a whole or partial chromosome 21 and assigning their locations on the chromosome. In particular, a family of repetitive sequences, some having only a few thousand copies in the human genome, have been used as cloned DNA markers to define deletions in these somatic cell hybrids. We have shown that this approach can significantly improve the resolution of fine chromosomal structures over the conventional cytogenetic analysis. The rationale behind this approach is the observation that a repetitive sequence probe often forms multiple bands after hybridizing to a Southern blot of digested hybrid DNA, and the band pattern appears to be unique for each human chromosome. Therefore, each band (sequence) can be assigned to a particular region of human chromosome 21 by comparing the band patterns from hybrids containing different portions of the chromosome. Results presented here showed that a 0.58-kb repetitive sequence probe can be used to identify deletions, translocations, and other more complicated rearrangements of chromosome 21 seen in patients with abnormalities of this chromosome. The advantage of using such a repetitive sequence probe over a unique sequence is that it can serve both as a repetitive sequence defining multiple sites (multiple bands on a Southern blot) in the genome and at the same time serve as a unique sequence defining a particular site (individual band). For the detection of deletions and other rearrangements, especially in small chromosomes such as 21, it is the former property that makes it very efficient in the initial assignment of a chromosome location.