LARG at chromosome 11q23 has functional characteristics of a tumor suppressor in human breast and colorectal cancer

Deletion of 11q23-q24 is frequent in a diverse variety of malignancies, including breast and colorectal carcinoma, implicating the presence of a tumor suppressor gene at that chromosomal region. We examined a 6-Mb region on 11q23 by high-resolution deletion mapping, using both loss of heterozygosity analysis and customized microarray comparative genomic hybridization. LARG (leukemia-associated Rho guanine-nucleotide exchange factor) (also called ARHGEF12), identified from the analysed region, is frequently underexpressed in breast and colorectal carcinomas with a reduced expression observed in all breast cancer cell lines (n=11), in 12 of 38 (32%) primary breast cancers, 5 of 10 (50%) colorectal cell lines and in 20 of 37 (54%) primary colorectal cancers. Underexpression of the LARG transcript was significantly associated with genomic loss (P=0.00334). Hypermethylation of the LARG promoter was not detected in either breast or colorectal cancer, and treatment of four breast and four colorectal cancer cell lines with 5-aza-2'-deoxycytidine and/or trichostatin A did not result in a reactivation of LARG. Enforced expression of LARG in breast and colorectal cancer cells by stable transfection resulted in reduced cell proliferation and colony formation, as well as in a markedly slower cell migration rate in colorectal cancer cells, providing functional evidence for LARG as a candidate tumor suppressor gene.

Overexpression of RB1 transcript is significantly correlated with 13q14 allelic imbalance in colorectal carcinomas

RB1 gene expression has been reported to be upregulated in colorectal carcinomas (CRC) at both the mRNA and protein levels when compared to normal colonic mucosa. However, allelic loss at the genomic level has been detected in CRC with widely differing frequencies ranging from 11.5% to 50%. To determine whether there is indeed a correlation between RB1 allelic imbalance (AI) and expression, a consecutive series of 55 CRC from Singapore patients were analysed by microsatellite analysis, real-time RT-PCR and immunohistochemistry. Microsatellite analysis using 3 RB1 intragenic microsatellite markers and 2 markers flanking RB1 detected AI in 32.7% (18/55) of the cases, in at least 1 locus. The highest AI frequency (22.9%) was observed at the microsatellite marker D13S137 (Cu13), which maps 5 cM distal to RB1. AI was present in both early and late Dukes stages. Real-time RT-PCR revealed that all 40 cases analysed expressed RB1 mRNA, with mRNA overexpression in 37.5% (15/40) and pRB protein expression in 88.2% (30/34) of cases. Notably, a statistically significant correlation was found between AI of RB1 and mRNA overexpression of RB1 (p < 0.001, Fishers exact test). These findings provide evidence that despite AI, RB1 expression is not abrogated. Thus, our data suggests that RB1 may play a role in colorectal tumorigenesis through functional regulation of the transcript and protein rather than through its tumour suppressor role by gene inactivation.

Overlapping deletion regions at 11q23 in myelodysplastic syndrome and chronic lymphocytic leukemia, characterized by a novel BAC probe set

Translocations or deletions involving the 11q23 region have been observed in acute lymphoblastic leukemia (ALL), acute myelocytic leukemia (AML), myelodysplastic syndrome (MDS), and chronic lymphocytic leukemia (CLL). BAC probes encompassing the D11S29 and D11S924 markers and flanking the MLL gene were used in dual color fluorescence in situ hybridization. Fifteen patients with hematologic malignancies and cytogenetic abnormalities of 11q23 were analyzed. The BAC and MLL probes demonstrated split signals in five of 7 ALL or AML cases with translocations of 11q23. Of the remaining 2 cases, one had normal signals for both probe sets and the other had a submicroscopic deletion of the MLL 3' region. In one case of AML with del(11)(q23), deletion of the MLL 3' region and the region telomeric to the MLL gene was seen. Three CLL cases with deletion of part or the entire 11q23 region showed deletion of one copy of MLL, but retention of the region telomeric to MLL. However, in four MDS cases with deletions involving the 11q23 region, deletions of both the MLL gene and the flanking regions of the MLL gene were observed. Hence, the deletions on 11q23 are different but overlapping for CLL and MDS, implicating different genes involved for these diseases.