Characterization and regional mapping of new anonymous chromosome 20-specific DNA markers isolated from a flow-sorted DNA library

Molecular heterogeneity at the breakpoints of smaller 20q deletions

Deletions of the long arm of chromosome 20 [del(20q)] are recurring abnormalities in patients with myeloid disorders. Although variable in size, these deletions are usually interstitial. With the object of defining a commonly deleted region for smaller 20q deletions, we used quantitative Southern blot analysis complemented by restriction fragment length polymorphism (RFLP) analysis to determine the copy number at 15 loci spanning 20q. The proximal breakpoints of three such deletions were found to separate HCK and the growth hormone releasing factor (GHRF) locus near the centromeric boundary of band 20q11.2. The distal breakpoints were localized to the vicinity of the D20S22 locus in band q13.1. A candidate tumor suppressor gene, RBL2, and the SRC oncogene were both located within the commonly deleted region. Six loci in terminal region q13.2-q13.3 were conserved on these del(20q) chromosomes, thereby confirming that the deletions were interstitial. Molecular heterogeneity at one and possibly both deletion breakpoints rules out the pathological involvement of loci at these sites. Instead, loss of a tumor suppressor locus from within the commonly deleted region may contribute to deregulated hemopoiesis.

PAX1, a member of the paired box-containing class of developmental control genes, is mapped to human chromosome 20p11.2 by in Situ hybridization (ISH and FISH)

Pax-1, a member of a murine multigene family, belongs to the paired box-containing class of developmental control genes first identified in Drosophila. The Pax-1 gene encodes a sequence-specific DNA-binding protein with transcriptional activating properties and has been found to be mutated in the autosomal recessive mutation undulated (un) on mouse chromosome 2 with vertebral anomalies along the entire rostrocaudal axis. By radioactive in situ hybridization (ISH) using a fragment from the murine Pax-1 paired box that is almost identical to the respective sequences from the cognate human gene HuP48 and fluorescence in situ hybridization (FISH) using a complete mouse Pax-1 cDNA, we have assigned the human homologue of murine Pax-1, the PAX1 locus, to chromosome 20p. The map position of PAX1 after FISH (FL-pter value of 0.34 +/- 0.04) corresponds to band p11.2. These results confirm the exceptional homology between human chromosome 20 and the distal segment of mouse chromosome 2, extending from bands F to G, and add PAX1 to the group of genes on 20p like PTPA, PRNP, SCG1, BMP2A, which are located in proximity on both chromosomes.