HGNC Approved Gene Symbol: LMO4
Cytogenetic location: 1p22.3 Genomic coordinates (GRCh38) : 1:87,328,880-87,348,923 (from NCBI)
The LIM domain, an approximately 55-residue cysteine-rich zinc-binding motif, is present in a variety of proteins, including LIM homeobox proteins that contain 2 LIM domains and 1 homeodomain. The nuclear LIM-only proteins (e.g., LMO2; 180385) lack a DNA-binding homeodomain. To identify novel LIM domain transcription factors, Kenny et al. (1998) screened 2 mouse embryonic expression libraries using the LIM interaction domain of the widely expressed nuclear LIM interactor (NLI, or LDB1; 603451), to which the LIM domains of nuclear proteins bind with high affinity. They reported the isolation and characterization of LMO4, a novel LIM-only gene that is highly expressed in the T-lymphocyte lineage, cranial neural crest cells, somite, dorsal limb bud mesenchyme, motor neurons, and Schwann cell progenitors. LMO1 (186921), LMO2, and LMO4 were found to have distinct expression patterns in adult tissue. Mesenchymal and thymic blast cell expression patterns of LMO4 and LMO2 were consistent with the suggestion that LMO genes inhibit differentiation.
Using autologous patient serum to screen a human breast tumor cDNA expression library, followed by screening a testis cDNA library, Racevskis et al. (1999) cloned LMO4. The 5-prime end of the transcript is highly GC rich (74%), with some stretches exceeding 90%, and the 3-prime UTR contains an RNA destabilization motif. Except for a proline-rich N terminus, the deduced 165-amino acid protein is composed entirely of 2 tandem LIM domains. The mouse and human LMO4 proteins are identical. Northern blot analysis detected LMO4 expression in most human tissues analyzed, with highest levels in brain, skeletal muscle, testis, and ovary. No expression was detected in liver, kidney, and pancreas. The expression pattern was somewhat different than that found in mouse.
Using Northern blot analysis, Wittlin et al. (2003) detected LMO4 transcripts of about 2.1 and 2.3 kb in normal mammary epithelia and breast cancer cell lines. A transcript of about 1.7 kb was also detected in the cancer cell lines, but not in normal epithelial cells. The variation in transcript size was due to different promoter usage and splicing.
Visvader et al. (2001) explored a role for LMO4, initially described as a human breast tumor autoantigen, in developing mammary epithelium and breast oncogenesis. The gene was expressed predominantly in the lobuloalveoli of the mammary gland during pregnancy. Consistent with a role in proliferation, forced expression of this gene inhibited differentiation of mammary epithelial cells. Overexpression of LMO4 mRNA was observed in 5 of 10 human breast cancer cell lines. Moreover, in situ hybridization analysis of 177 primary invasive breast carcinomas revealed overexpression of LMO4 in 56% of specimens. Immunohistochemistry confirmed overexpression in a high percentage (62%) of tumors. These studies implied a role for LMO4 in maintaining proliferation of mammary epithelium and suggested that deregulation of this gene may contribute to breast tumorigenesis.
Using human and mouse expression plasmids in several protein interaction assays, Sum et al. (2002) identified CTIP (604124) and BRCA1 (113705) as LMO4-binding proteins. The LMO4-BRCA1 interaction required the C-terminal BRCT domains of BRCA1. LDB1 also associated with a complex containing LMO4, CTIP, and BRCA1 in transfected human embryonic kidney cells. In functional assays, LMO4 repressed BRCA1-mediated transcriptional activation in both yeast and mammalian cells.
Wittlin et al. (2003) found that the 2 LMO4 promoters exhibited strong activity in breast cancer cell lines that correlated with RNA levels. Promoter 2 appeared to be selectively activated in certain breast cancer cell lines but not in immortalized human mammary epithelial cells, implying aberrant activation of the LMO4 gene in the cancer cell lines.
By mammalian 2-hybrid analysis, Manetopoulos et al. (2003) showed that the helix-loop-helix protein HEN1 (162360) interacted with both LMO2 and LMO4. LMO4, but not LMO2, could augment HEN1-mediated repression of E47 (147141) transcriptional activity. LMO4 could also prevent HEN1-mediated neurite extension in rat hippocampal precursor cells. Manetopoulos et al. (2003) concluded that LMO4 can modulate the transcriptional activity of HEN1.
Sum et al. (2005) found that downregulation of LMO4 expression by RNA interference reduced proliferation of human breast cancer cells and increased differentiation of mouse mammary epithelial cells. Furthermore, transfection of small interfering RNA into breast cancer cells reduced the capacity of the cells to migrate and invade an extracellular matrix. Conversely, overexpression of LMO4 in noninvasive, immortalized human breast epithelial cells promoted cell motility and invasion. Sum et al. (2005) also found that high nuclear levels of LMO4 correlated with poor patient outcome in a cohort of 159 primary breast cancers. They concluded that deregulation of LMO4 in breast epithelium contributes directly to breast neoplasia by altering the rate of cellular proliferation and promoting cell invasion.
By genomic structure analysis, Tse et al. (1999) determined that the mouse Lmo4 gene contains 6 exons, the last 4 of which encode the 165-amino acid protein.
Wittlin et al. (2003) determined that the LMO4 gene contains 2 5-prime noncoding exons, exon 1a and 1b, and exon 2 encodes the first LIM domain. Promoter 1, upstream of exon 1a, and promoter 2, upstream of exon 1b, lack canonical TATA or CCAAT boxes, but both contain GC-rich regions. Promoter 1 also has recognition sites for SP1 (189906) and Kruppel-like factors (see KLF1; 600599), and promoter 2 has SP1-like recognition elements, GC boxes, and AP2 (107580) sites.
Tse et al. (1999) mapped the mouse Lmo4 gene to chromosome 3 by interspecific backcross analysis. Using FISH, they mapped the human LMO4 gene to 1p22.3, a region that is deleted in a number of human cancers.
Kenny, D. A., Jurata, L. W., Saga, Y., Gill, G. N. Identification and characterization of LMO4, an LMO gene with a novel pattern of expression during embryogenesis. Proc. Nat. Acad. Sci. 95: 11257-11262, 1998. [PubMed: 9736723] [Full Text: https://doi.org/10.1073/pnas.95.19.11257]
Manetopoulos, C., Hansson, A., Karlsson, J., Jonsson, J.-I., Axelson, H. The Lim-only protein LMO4 modulates the transcriptional activity of HEN1. Biochem. Biophys. Res. Commun. 307: 891-899, 2003. [PubMed: 12878195] [Full Text: https://doi.org/10.1016/s0006-291x(03)01298-1]
Racevskis, J., Dill, A., Sparano, J. A., Ruan, H. Molecular cloning of LMO4, a new human LIM domain gene. Biochim. Biophys. Acta 1445: 148-153, 1999. [PubMed: 10209267] [Full Text: https://doi.org/10.1016/s0167-4781(99)00037-8]
Sum, E. Y. M., Peng, B., Yu, X., Chen, J., Byrne, J., Lindeman, G. J., Visvader, J. E. The LIM domain protein LMO4 interacts with the cofactor CtIP and the tumor suppressor BRCA1 and inhibits BRCA1 activity. J. Biol. Chem. 277: 7849-7856, 2002. [PubMed: 11751867] [Full Text: https://doi.org/10.1074/jbc.M110603200]
Sum, E. Y. M., Segara, D., Duscio, B., Bath, M. L., Field, A. S., Sutherland, R. L., Lindeman, G. J., Visvader, J. E. Overexpression of LMO4 induces mammary hyperplasia, promotes cell invasion, and is a predictor of poor outcome in breast cancer. Proc. Nat. Acad. Sci. 102: 7659-7664, 2005. [PubMed: 15897450] [Full Text: https://doi.org/10.1073/pnas.0502990102]
Tse, E., Grutz, G., Garner, A. A., Ramsey, Y., Carter, N. P., Copeland, N., Gilbert, D. J., Jenkins, N. A., Agulnick, A., Forster, A., Rabbitts, T. H. Characterization of the Lmo4 gene encoding a LIM-only protein: genomic organization and comparative chromosomal mapping. Mammalian Genome 10: 1089-1094, 1999. [PubMed: 10556429] [Full Text: https://doi.org/10.1007/s003359901167]
Visvader, J. E., Venter, D., Hahm, K., Santamaria, M., Sum, E. Y. M., O'Reilly, L., White, D., Williams, R., Armes, J., Lindeman, G. J. The LIM domain gene LMO4 inhibits differentiation of mammary epithelial cells in vitro and is overexpressed in breast cancer. Proc. Nat. Acad. Sci. 98: 14452-14457, 2001. [PubMed: 11734645] [Full Text: https://doi.org/10.1073/pnas.251547698]
Wittlin, S., Sum, E. Y. M., Jonas, N. K., Lindeman, G. J., Visvader, J. E. Two promoters within the human LMO4 gene contribute to its overexpression in breast cancer cells. Genomics 82: 280-287, 2003. [PubMed: 12906853] [Full Text: https://doi.org/10.1016/s0888-7543(03)00147-2]