With more than 380 scientific citations, Oncomine has become an industry standard for cancer researchers. This series showcases recent scientific publications that use Oncomine in various and novel ways to further cancer research.

06 MAY 2010
EHMT1 and EHMT2 methylate TP53 at lysine 373, suggesting a post-translational mechanism for TP53 inactivation¹.

TP53 is a sequence-specific transcription factor with the important job of preventing proliferation of stressed or damaged cells. Given this function, it is not surprising that expression of TP53 in normal cells is relatively low compared to diseased tissue. Indeed, many analyses in Oncomine show much higher expression of TP53 in cancer populations when compared to normal counterparts.

Figure 1: Over-expression of TP53 in cancer cells as compared to normal in French Brain².

High expression of TP53 in cancer cells should be a good thing, since TP53 is expected to prevent proliferation of damaged cells. However, it is clear in proliferating tumor cells that TP53 tumor suppressor activity — whatever the expression level — is not high, since these highly abnormal cells continue to proliferate. Indeed, it is well established that about half of all human tumors harbor TP53 mutations in the DNA binding domain, providing one mechanism to explain how high expression levels of TP53 can fail to halt unwanted proliferation.

What about remaining tumors — those with high expression of wild type TP53 — that still fail to generate a robust tumor-suppressor response?

A new study shows that at least part of the answer may be post-translational modification of the TP53 protein at lysine 373, via one of two methylases, EHMT1 (Glp) or EHMT2 (G9a). Study authors took a number of approaches to demonstrate the relevance of EHMT1 and EHMT2 for TP53 activity:

• Tested full length TP53 protein for methylation by EHMT1 and by EHMT2, demonstrated that both enzymes are independently capable of methylating TP53 at lysine 373, and that no other sites are methylated by these enzymes.
• Showed that siRNA reduction of EHMT1 or EHMT2 causes decreased methylation of TP53 and increased apoptosis.
• Using Oncomine, showed that EHMT1 and EHMT2 are over-expressed in tumor samples relative to normal.

Altogether these results show that EHMT1 and EHMT2 methylate TP53 at lysine 373, knocking down these genes reverses TP53 methylation and restores TP53 activity, and EHMT1 and EHMT2 over-expression promotes tumor growth. Study authors conclude that EHMT1 and EHMT2 are putative oncogenes, and that EHMT2 may be a useful target for cancer treatment.  

A summary of EHMT2 results across analysis and cancer types in Oncomine is shown below (Figure 2). 

Figure 2: Summary of EHMT2 expression results across Oncomine. Note threshold settings at the top can be adjusted to expose or filter data based on statistical thresholds.

Interestingly, when the cancer vs. normal summary from EHMT1 is superimposed next to that of EHMT2, it appears that while some cancers over-express both genes (e.g. colorectal) most cancers over-express one of the two genes, but not the other (leukemia). Heat map views of data from individual studies show the actual range of EHMT1 and EHMT2 across samples, and clearly shows the similarity of EHMT1 and EHMT2 in colorectal cancer, but quite distinct patterns in leukemia (Figure 3).

Figure 3: Cancer types over-expressing EHMT2 in cancer vs. normal analyses (2nd summary map column) taken from Figure 2, superimposed against the same type of summary for EHMT1 (1st summary map column)to show that most cancers over-express one of the EHMT genes but not both. One exception is colorectal cancer, which appears to over-express both. An example of this is shown in the Kaiser Colon³ heat map (top right), which shows over-expression of both genes in the cancer samples as compared to normal. In contrast, the Andersson Leukemia⁴ heat map (bottom right) shows that in this cancer type only EHMT2 is over-expressed in this cancer vs. normal analysis.

Here we note that in addition to the EHMT results in cancer vs. normal analyses noted by study authors, several analyses on drug treatments (cells treated with a drug vs. untreated) or drug sensitivities (cells sensitive to a drug vs. cells resistant to a drug) also show significant results for the EHMT methylases.

For example, data from Wang CellLine⁵ shows a clear decrease in EHMT1 after dactinomycin treatment (Figure 4).

Figure 4. Under-expression of EHMT1 and EHMT2 in cells treated with dactinomycin in the prostate cancer cell line A549 in Wang CellLine 2⁵.

Finally, data from Palomero CellLine⁶ shows that cell lines that are sensitive to Compound E tend to over-express EHMT1 and EHMT2 as compared to their resistant counterparts.

Figure 5. Over-expression of EHMT1 and EHMT2 in cells sensitive to the gamma-secretase inhibitor Compound E in a panel of leukemia cell lines ⁶.

Altogether these results show how existing data aggregated and organized in Oncomine can be used to both validate and extend research findings — and even to generate hypotheses for further inquiry.

¹G9a and Glp methylate lysine 373 in the tumor suppressor p53.
Huang J, Dorsey J, Chuikov S, Zhang X, Jenuwein T, Reinberg D, Berger SL.
J Biol Chem. 2010 Mar 26;285(13):9636-41.

²Gene expression profiles associated with treatment response in oligodendrogliomas.
French PJ, Swagemakers SM, Nagel JH, Kouwenhoven MC, Brouwer E, van der Spek P, Luider TM, Kros JM, van den Bent MJ, Sillevis Smitt PA.
Cancer Res. 2005 Dec 15;65(24):11335-44.

³Transcriptional recapitulation and subversion of embryonic colon development by mouse colon tumor models and human colon cancer.
Kaiser S, Park YK, Franklin JL, Halberg RB, Yu M, Jessen WJ, Freudenberg J, Chen X, Haigis K, Jegga AG, Kong S, Sakthivel B, Xu H, Reichling T, Azhar M, Boivin GP, Roberts RB, Bissahoyo AC, Gonzales F, Bloom GC, Eschrich S, Carter SL, Aronow JE, Kleimeyer J, Kleimeyer M, Ramaswamy V, Settle SH, Boone B, Levy S, Graff JM, Doetschman T, Groden J, Dove WF, Threadgill DW, Yeatman TJ, Coffey RJ Jr, Aronow BJ.
Genome Biol. 2007;8(7):R131.

⁴Microarray-based classification of a consecutive series of 121 childhood acute leukemias: prediction of leukemic and genetic subtype as well as of minimal residual disease status.
Andersson A, Ritz C, Lindgren D, Eden P, Lassen C, Heldrup J, Olofsson T, Rade J, Fontes M, Porwit-Macdonald A, Behrendtz M, Hoglund M, Johansson B, Fioretos T.
Leukemia. 2007 Jun;21(6):1198-203.

⁵Synthesis and biologic properties of hydrophilic sapphyrins, a new class of tumor-selective inhibitors of gene expression.
Wang Z, Lecane PS, Thiemann P, Fan Q, Cortez C, Ma X, Tonev D, Miles D, Naumovski L, Miller RA, Magda D, Cho DG, Sessler JL, Pike BL, Yeligar SM, Karaman MW, Hacia JG.
Mol Cancer. 2007 Jan 19;6:9.

⁶Mutational loss of PTEN induces resistance to NOTCH1 inhibition in T-cell leukemia.
Palomero T, Sulis ML, Cortina M, Real PJ, Barnes K, Ciofani M, Caparros E, Buteau J, Brown K, Perkins SL, Bhagat G, Agarwal AM, Basso G, Castillo M, Nagase S, Cordon-Cardo C, Parsons R, Zuniga-Pflucker JC, Dominguez M, Ferrando AA.
Nat Med. 2007 Oct;13(10):1203-10.

Oncomine 4.3 | Data (March 2010)