8847

DLEU2

1B4|BCMSUN|DLB2|LEU2|LINC00022|MIR15AHG|NCRNA00022|RFP2OS|TRIM13OS;deleted in lymphocytic leukemia 2 (non-protein coding);DLEU2;deleted in lymphocytic leukemia 2 (non-protein coding)

deleted in lymphocytic leukemia, 2|leukemia associated gene 2|long intergenic non-protein coding RNA 22|mir-15a-16-1 cluster host gene (non-protein coding)|ret finger protein 2 opposite strand

13q14.3

ncRNA

Chromosome 13q14 deletions constitute the most common genetic abnormality in chronic lymphocytic leukemia (CLL). To identify the putative tumor suppressor gene targeted by 13q14 genomic loss, we completely sequenced and characterized a segment of 790 kb at 13q14 spanning the minimal region of loss in CLL. Transcribed sequences in the region were identified through database homology searches and exon-prediction analysis. Two-hundred kb at the centromeric end of the sequence contain five CpG islands, three previously identified genes LEU5/RFP2, LEU2, and LEU1, seven of seven EST clusters composed of >10 ESTs, and a large number of predicted exons. Homology searches against the mouse EST database have allowed us to identify a highly conserved alternative first exon of the LEU2 gene, giving rise to a novel transcript, ALT1 (GenBank accession no. AF380424), which originates within a G+C region in the vicinity of the D13S272 marker. Two novel 3 exons of LEU2 were also identified and are present in both LEU2 and ALT1 transcripts. However, we have not identified any mutations in leukemia cases, or alterations in expression of mRNAs in the region, that might directly implicate these mRNAs in the pathology of CLL. The centromeric end of the sequence, where all reported genes are located, contains twice the expected amount of ALU repeats, whereas the telomeric end is LINE1 rich and contains four LINE1 elements longer than 4 kb, including two full-length LINE1 sequences. This feature of the sequence may favor the occurrence of chromosomal rearrangements and may confer instability to the region, resulting in deletions that may inactivate an as yet unidentified tumor suppressor.

Previous studies have indicated the presence of a putative tumor suppressor gene on chromosome 13q14, commonly deleted in patients with B-cell chronic lymphocytic leukemia (B-CLL). We have previously defined a minimally deleted region of 130 kb centromeric to the marker D13S272, and constructed a PAC and cosmid contig encompassing this area. In the present study we have made a detailed restriction and transcriptional map of the region of interest. Using these tools we have screened a panel of 206 primary CLL clones and three cell lines. In five CLL cases we found limited deletions defining the region of interest to an area of no more than 10 kb. Two adjacent genes, termed Leu1 and Leu2 (leukemia-associated gene 1 and 2), were mapped to the minimally deleted region, with several patients showing deletion borders within these genes. The Leu1 and Leu2 genes show little homology to previously published genes at the nucleotide and expected translated amino acid sequence level. mutational analysis of the Leu1 and 2 genes in 170 CLL samples revealed no small intragenic mutations or point mutations. However, in all cases of 13q14 loss examined, the first exon of both genes, which are only 300 bp apart, were deleted. We conclude that the Leu1 and Leu2 genes are strong candidates as tumor suppressor gene(s) involved in B-CLL leukemogenesis.