CLONING AND CHARACTERIZATION OF A 135-KB TO 500-KB REGION OF HOMOLOGYON THE LONG ARM OF HUMAN-CHROMOSOME-21

The polymorphic marker D21S190 was initially isolated from a chromosom e 21 phage library and mapped to two loci: one in 21q11.1 (proximal lo cus) and the other one in 21q22.1 (distal locus). To characterize the region of homology revealed with D21S190, we have screened two differe nt chromosome 21 YAC libraries and one chromosome 21 cosmid library. F luorescence in situ hybridization on normal human chromosomes of YACs, cosmids, and phages positive with D21S190 confirmed the existence of two homologous regions on the long arm of chromosome 21. Among the pos itive YACs, four (HY67, 2D7y21, 2D11y21, and 1B1y21) were selected and oriented relative to each other, forming a 2-Mb contig in the distal locus, including D21S54. Hybridization of YAC extremities to a panel o f somatic cell hybrids containing various portions of chromosome 21 sh owed that the proximal locus is located between the breakpoints of 2Fu (r)1 and ACEM and the distal locus between the breakpoints of ACEM and 6918. The proximal and the distal breakpoints of JC6 are both include d in the region of homology. We have constructed a restriction map of HY67, 2D7y21, and 2D11y21 spanning 1 Mb and including several markers: D21S294, D21S296, and the new STSs corresponding to YAC extremities. The region of homology encompasses 135-500 kb and has the same orienta tion in the distal and in the proximal locus, which are at least 12 Mb apart. It is lacking a NotI site but does contain clusters of GC-rich restriction sites, which are candidate regions for as yet unidentifie d genes. (C) 1994 Academic Press, Inc.