Nucleotide sequence, transcription map, and mutation analysis of the 13q14chromosomal region deleted in B-cell chronic lymphocytic leukemia

Deletions of the 13q14 chromosome region are associated with B-cell chronic lymphocytic leukemia (B-CLL) and several other types of cancer, suggesting the presence of a tumor suppressor gene. In previous studies the minimal r egion of deletion (MDR) was mapped to a less than 300-kilobase (kb) interva l bordered by the markers 173a12-82 and 138G4/1.3R. For the identification of the putative tumor suppressor gene, the entire MDR (approximately 347 kb ) has been sequenced, and transcribed regions have been identified by exon trapping, EST-based full-length complementary DNA cloning, database homolog y searches, and computer-assisted gene prediction analyses. The MDR contain s 2 pseudogenes and 3 transcribed genes: CAR, encoding a putative RING-fing er containing protein; 1B4/Leu2, generating noncoding transcripts; and EST7 0/Leu1, probably representing another noncoding gene (longest open reading frame of 78 codons). These genes have been sequenced in 20 B-CLL cases with 13q14 hemizygous deletion, and no mutations were found. Moreover, no somat ic variants were found in the entire MDR analyzed for nucleotide substituti ons by a combination of direct sequencing and fluorescence-assisted mismatc h analysis in 5 B-CLL cases displaying 13q14-monoallelic deletion. The nond eleted allele of the CAR and EST70/Leu1 genes was expressed in B-CLL specim ens, including those with monoallelic loss, whereas no expression of 1B4/Le u2 was detectable in B-CLL, regardless of the 13q14 status. These results i ndicate that allelic loss and mutation of a gene within the MDR is an unlik ely pathogenetic mechanism for B-CLL. However, haplo-insufficiency of one o f the identified genes may contribute to tumorigenesis.