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Dipartimento di Genetica, Biologia e Biochimica, Università di Torino, Torino, Italy;
Departments of Medicine and Microbiology and Immunology, and Cancer Center, University of Rochester School of Medicine and Dentistry, Rochester, NY 14642; and
Dipartimento di Scienze Cliniche e Biologiche, Università di Torino, Torino, Italy
Human Ig heavy chain constant regions are encoded by a cluster of
genes, the IGHC locus, on 14q32.3. Several forms of
IGHC deletions and duplications spanning one to five
genes have been described in different populations, with frequencies of
1.53.5% and 4.544%, respectively. Despite the common occurrence
of these gene rearrangements, little is known about the breakpoint
sites; evidence obtained from deletions in the IGHC
locus and in other regions of the human genome suggests that they
preferentially occur in highly homologous regions and might be favored
by a variety of recombinogenic signals. We present here a detailed
study of three homozygotes for the most common type of
IGHC multiple gene deletion, spanning the
A1-GP-G2-G4-E genes. Using a combination of Southern
blotting, long-range PCR, and automated sequencing, the
unequal crossover events of all of the six studied haplotypes have been
mapped to a region of
2 kb with almost complete homology between
EP1-A1 and E-A2, flanked by two
minisatellites. These results are consistent with the hypothesis that
segments of complete homology may be required for efficient homologous
recombination in humans. The possible role of minisatellites as
recombination signals is inferred, in agreement with current
knowledge.
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