Zmediated Recombination Events Are Responsible For Gene Duplication

The results of a genome-wide effort to identify sequence elements at the junctions between LCRs and unique sequences were reported (69). A total of9464junctions and 2366 duplication alignments were examined throughout the genome and a significant enrichment of Alu elements in the vicinity of the junctions was found (27% of all LCRs had an Alu at least one end). This suggested that the duplication process forming the LCRs coincided with a primate-specific burst of Alu retroposition activity. Because the Alu sequences were identical in sequence, it was possible that they could undergo Alu-Alu based unequal recombination events forming mosaic Alu products. In the case of 22q11.2, the breakpoints in some of the Alu elements involved in the rearrangements occurred by unequal crossover mechanisms, whereas others occurred by different mechanisms, resulting in breakpoints at the end of Alu elements as opposed to the middle of Alu for unequal crossover events (68).

The IGSF3 gene maps to chromosome 1p13.1. Part of this gene became copied onto 22q11.2, to LCR22-2 and LCR22-4. It is adjacent to the GGT locus in these LCR22s. The rearrangement responsible for this "trans" rearrangement involving non-homologous chromosomes appeared to have been created in meiosis by a replication-dependent mechanism similar to the model proposed by Richardson et al. (70). In this model, recombination occurs by Alu-mediated misalignment of two different chromosomes. A gene conversion then takes place, thereby avoiding crossovers, which would lead to aberrant translocations. These results suggest that mammalian genomes have a mechanism scanning the entire genome to find stretches of homology (70,71).

Fig. 6. Low-stringency FISH mapping. Probes from LCR22-2 (GenBank AC008132) and LCR22-4 (GenBank AC009288) were used for low-stringency FISH mapping. Hybridization signals were detected on chromosomes 1p13, 2p11, 5p13, 13p11, and 20p12. The strongest signals were detected on chromosome 22q11 owing to the presence of multiple copies of sequences contained within the LCR22s (68).

Fig. 6. Low-stringency FISH mapping. Probes from LCR22-2 (GenBank AC008132) and LCR22-4 (GenBank AC009288) were used for low-stringency FISH mapping. Hybridization signals were detected on chromosomes 1p13, 2p11, 5p13, 13p11, and 20p12. The strongest signals were detected on chromosome 22q11 owing to the presence of multiple copies of sequences contained within the LCR22s (68).

Was this article helpful?

0 0

Post a comment