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Single-minded homolog 2 is a protein that in humans is encoded by the SIM2 gene.[1][2] It plays a major role in the development of the central nervous system midline as well as the construction of the face and head.[3]


SIM1 and SIM2 genes are Drosophila single-minded (sim) gene homologs. The Drosophila sim gene encodes a transcription factor that is a master regulator of neurogenesis of midline cells in the central nervous system. SIM2 maps within the so-called Down syndrome chromosomal region, specifically on the q arm of chromosome 21, band 22.2.[3] Based on the mapping position, its potential function as transcriptional repressor and similarity to Drosophila sim, it is proposed that SIM2 may contribute to some specific Down syndrome phenotypes[2]


SIM2 has been shown to interact with Aryl hydrocarbon receptor nuclear translocator.[4][5][6][7]

When the SIM2 gene is tranfected into PC12 cells, it effects the normal cycle of cell maturation. SIM2 inhibits the expression of cyclin E, which in turn inhibits the cell's ability to pass through the G1/S checkpoint and suppresses the cell's proliferation ability. it also up-regulates the presence of p27, a growth inhibitor protein. The presence of p27 inhibits the activation of cell cycle regulatory kinases.[8]

Disease state

There are three states of the gene: +/+, +/-, and -/-. When the gene is expressed as SIM2 -/-, it is considered disrupted and many physical malformations are seen, particularly in the craniofacial area. Individuals with SIM2 -/- have either a full or partial secondary palate cleft and malformations in the tongue and pterygoid processes of the sphenoid bone. These malformations cause aerophagia, or the swallowing of air, and postnatal death. Severe aerophagia leads to accumulation of air in the gastrointestinal tract, causing the belly to be distended.[3] It is thought that the over-expression of the SIM2 gene brings about some of the phenotypic deformities that are characteristic of Down syndrome. The presence of SIM2 mRNA in many parts of the brain known to show deformities in individuals with Down syndrome, as well as in the palate, oral and tongue epithelia, mandibular and hyoid bones.[3]

SIM2 Short (SIM2s)

There are two known isoforms of SIM2 which play different roles in various tissues. The isoform SIM2 Short (SIM2s) has been shown to be specifically expressed in mammary gland tissue.[9] SIM2s is a splice variant which lacks exon 11 of SIM2.[10] It has been researched that SIM2s acts in mammary gland development and has tumor suppressive characteristics specifically in breast cancer.[9][11][12] In a mouse specimen, when SIM2s was not expressed in mammary epithelial cells there were development defects leading to cancer-like characteristics in the cells.[12] The defects were increased cell proliferation, cellular invasion of local stroma, loss of cellular polarity, and loss of E-cadherin cellular adhesion molecules.[12] These observations suggest that SIM2s is essential for proper mammary gland development.[12] Experiments reintroducing SIM2s in human breast cancer cells allowed for the tumor suppressive characteristics to be observed. Comparing normal human breast cells to human breast cancer cells with immunohistochemical staining showed that SIM2s was expressed more in the normal than the cancerous.[9] Reintroducing SIM2s expression in breast cancer cells showed a decrease in growth, proliferation, and invasiveness.[9] SIM2s represses the actions of the matrix metalloprotease-3 gene (MMP3) which include cell migration, cancer progression, and epithelial to mesenchymal transitions (EMT).[9] SIM2s also represses the SLUG transcription factor which in turn suppresses EMT.[12] EMT suppression allows for E-cadherin to remain and for the cell to not undergo pathological EMT associated with tumor formation.[12] These actions show the tumor suppressive effects of SIM2s in mammary epithelium.

Knockout model

Scientists can purposefully "knockout" or cause the gene to be disrupted. To do this, they perform homologous recombination and eliminate the predicted start codon and the following 47 amino acids. Then the EcoRI restriction site is introduced into the chromosome.[3]


  1. Muenke M, Bone LJ, Mitchell HF, Hart I, Walton K, Hall-Johnson K, Ippel EF, Dietz-Band J, Kvaløy K, Fan CM (Nov 1995). "Physical mapping of the holoprosencephaly critical region in 21q22.3, exclusion of SIM2 as a candidate gene for holoprosencephaly, and mapping of SIM2 to a region of chromosome 21 important for Down syndrome". American Journal of Human Genetics. 57 (5): 1074–9. PMC 1801356. PMID 7485157.
  2. 2.0 2.1 "Entrez Gene: SIM2 single-minded homolog 2 (Drosophila)".
  3. 3.0 3.1 3.2 3.3 3.4 Shamblott, MJ; Bugg, EM; Lawler, AM; Gearhart, JD (2002). "Craniofacial abnormalities resulting from targeted disruption of the murine Sim2 gene". Developmental Dynamics. 224: 373–380. doi:10.1002/dvdy.10116. PMID 12203729.
  4. Probst MR, Fan CM, Tessier-Lavigne M, Hankinson O (Feb 1997). "Two murine homologs of the Drosophila single-minded protein that interact with the mouse aryl hydrocarbon receptor nuclear translocator protein". The Journal of Biological Chemistry. 272 (7): 4451–7. doi:10.1074/jbc.272.7.4451. PMID 9020169.
  5. Ooe N, Saito K, Mikami N, Nakatuka I, Kaneko H (Jan 2004). "Identification of a novel basic helix-loop-helix-PAS factor, NXF, reveals a Sim2 competitive, positive regulatory role in dendritic-cytoskeleton modulator drebrin gene expression". Molecular and Cellular Biology. 24 (2): 608–16. doi:10.1128/MCB.24.2.608-616.2004. PMC 343817. PMID 14701734.
  6. Woods SL, Whitelaw ML (Mar 2002). "Differential activities of murine single minded 1 (SIM1) and SIM2 on a hypoxic response element. Cross-talk between basic helix-loop-helix/per-Arnt-Sim homology transcription factors". The Journal of Biological Chemistry. 277 (12): 10236–43. doi:10.1074/jbc.M110752200. PMID 11782478.
  7. Moffett P, Reece M, Pelletier J (Sep 1997). "The murine Sim-2 gene product inhibits transcription by active repression and functional interference". Molecular and Cellular Biology. 17 (9): 4933–47. doi:10.1128/mcb.17.9.4933. PMC 232345. PMID 9271372.
  8. Meng, X; Shi, J; Peng, B; Zou, X; Zhang, C. "Effect of mouse Sim2 gene on the cell cycle of PC12 cell". Cell Biology International. 2006 (30): 349–353. doi:10.1016/j.cellbi.2005.11.012.
  9. 9.0 9.1 9.2 9.3 9.4 Kwak, Hyeong-Il; Gustafson, Tanya; Metz, Richard P.; Laffin, Brian; Schedin, Pepper; Porter, Weston W. (July 2006). "Inhibition of breast cancer growth and invasion by single-minded 2s". Carcinogenesis. 28: 259–266. doi:10.1093/carcin/bgl122.
  10. Metz, Richard P.; Kwak, Hyeong-Il; Gustafson, Tanya; Laffin, Brian; Porter, Weston W. (February 2006). "Differential Transcriptional Regulation by Mouse Single-minded 2s". The Journal of Biological Chemistry. 281: 10839–10848. doi:10.1074/jbc.m508858200.
  11. Wellberg, Elizabeth; Metz, Richard P.; Parker, Caitlin; Porter, Weston W. (March 2010). "The bHLH/PAS transcription factor singleminded 2s promotes mammary gland lactogenic differentiation". Development. 137: 945–952. doi:10.1242/dev.041657. PMC 2834457. PMID 20150276.
  12. 12.0 12.1 12.2 12.3 12.4 12.5 Laffin, Brian; Wellburg, Elizabeth; Kwak, Hyeong-Il; Burghardt, Robert C.; Metz, Richard P.; Gustafson, Tanya; Schedin, Pepper; Porter, Weston W. (March 2008). "Loss of Singleminded-2s in the Mouse Mammary Gland Induces an Epithelial-Mesenchymal Transition Associated with Up-Regulation of Slug and Matrix Metalloprotease 2". Molecular and Cellular Biology. 28: 1936–1946. doi:10.1128/mcb.01701-07. PMC 2268409.

Further reading