A novel isoform of the 8p22 tumor suppressor gene DLC1 suppresses tumor growth and is frequently silenced in multiple common tumors

• Published in: Oncogene Vol. 30; no. 16; pp. 1923 - 1935
• Main Authors: Low, J S W, Tao, Q, Ng, K M, Goh, H K, Shu, X-S, Woo, W L, Ambinder, R F, Srivastava, G, Shamay, M, Chan, A T C, Popescu, N C, Hsieh, W-S
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 21-04-2011; Nature Publishing Group
• Subjects: 631/208/2489/2487/2486; 631/67/581; Apoptosis; Base Sequence; Biological and medical sciences; Breast; Carcinogenesis; Cell Biology; Cell physiology; Cell transformation and carcinogenesis. Action of oncogenes and antioncogenes; Cellular biology; Cervix; Chromosome 8; Chromosomes, Human, Pair 8; CpG islands; Demethylation; DNA Methylation; DNA Primers; DNMT1 protein; Ectopic expression; Epigenetics; Epithelial cells; Esophagus; Fundamental and applied biological sciences. Psychology; Gene expression; Gene Silencing; Genes, Tumor Suppressor; Genetic aspects; GTPase-Activating Proteins - genetics; Health aspects; Human Genetics; Humans; Internal Medicine; Isoforms; Liver cancer; Lung carcinoma; Medicine; Medicine & Public Health; Methyltransferases; Molecular and cellular biology; Molecular Sequence Data; Multiple primary tumors; N-Terminus; Neoplasms - genetics; Neoplasms - pathology; Oncology; original-article; p53 Protein; Physiological aspects; Transcription; Tumor cell lines; Tumor cells; Tumor suppressor genes; Tumor Suppressor Proteins - genetics; Tumors; Singapore; carcinoma; DLC1; methylation; tumor suppressor gene; RhoGAP; p53; Molecular form; Carcinoma; TP53 Gene; Malignant tumor; Methylation; Carcinogenesis; Cancer; Tumor suppressor gene
• ISSN: 0950-9232; 1476-5594
• DOI: 10.1038/onc.2010.576

The critical 8p22 tumor suppressor deleted in liver cancer 1 (
DLC1
) is frequently inactivated by aberrant CpG methylation and/or genetic deletion and implicated in tumorigeneses of multiple tumor types. Here, we report the identification and characterization of its new isoform,
DLC1
isoform 4 (
DLC1
-i4). This novel isoform encodes an 1125-aa (amino acid) protein with distinct N-terminus as compared with other known
DLC1
isoforms. Similar to other isoforms,
DLC1-
i4 is expressed ubiquitously in normal tissues and immortalized normal epithelial cells, suggesting a role as a major
DLC1
transcript. However, differential expression of the four DLC1 isoforms is found in tumor cell lines: Isoform 1 (longest) and 3 (short thus probably nonfunctional) share a promoter and are silenced in almost all cancer and immortalized cell lines, whereas isoform 2 and 4 utilize different promoters and are frequently downregulated.
DLC1
-i4 is significantly downregulated in multiple carcinoma cell lines, including 2/4 nasopharyngeal, 8/16 (50%) esophageal, 4/16 (25%) gastric, 6/9 (67%) breast, 3/4 colorectal, 4/4 cervical and 2/8(25%) lung carcinoma cell lines. The functional
DLC1
-i4 promoter is within a CpG island and is activated by wild-type p53. CpG methylation of the
DLC1
-i4 promoter is associated with its silencing in tumor cells and was detected in 38–100% of multiple primary tumors. Treatment with 5-aza-2′-deoxycytidine or genetic double knockout of
DNMT1
and
DNMT3B
led to demethylation of the promoter and reactivation of its expression, indicating a predominantly epigenetic mechanism of silencing. Ectopic expression of
DLC1-
i4 in silenced tumor cells strongly inhibited their growth and colony formation. Thus, we identified a new isoform of
DLC1
with tumor suppressive function. The differential expression of various
DLC1
isoforms suggests interplay in modulating the complex activities of DLC1 during carcinogenesis.