They say beauty is in the eye of the beholder. For me, well done immunolocalization images demonstrate scientific artistry. Last week I was pleased to find this paper, Global 5-hydroxymethylcytosine content is significantly reduced in tissue stem/progenitor cell compartments and in human cancers Oncotarget Sept. 2, 2011 (Haffner et al), posted on the Dr. Denise O’Keefe lab blog (E3 member).
This research showed that fully differentiated adult tissues had the highest levels of 5-hmC, while stem cells had very low levels. There was uniform loss of 5hmC in cancer tissues, matched to normal adult tissues. Reductions in 5-hmC can occur independently of 5mC in solid tumors. Global loss of 5-hmC could be an early event of carcinogenesis.
The work came from Dr. Yegnasubramanian’s lab group at John Hopkins University. He was kind enough to answer a few of my basic questions.
1. Was your lab surprised by the adult tissue 5-hmC results?
Yes, we were quite surprised and intrigued by the results. Some very elegant recent studies had shown that 5-hmC levels were high in embryonic stem cells and very early in embryogenesis and appeared to decrease in subsequent differentiation (e.g. see Ito S et al. (2010) Nature, Role of Tet proteins in 5mC to 5hmC conversion, ES-cell self-renewal and inner cell mass specification) . We initially suspected that a similar trend may hold in stem/progenitors cells later in embryogenesis and in adult tissues as well. Interestingly, we found just the opposite…
2. You have shown that 5-hmC is lost independently from 5-mC in cancer cells. Do you believe that the primary way this happens would be through oncogenic metabolites, rather than mutation of Tet genes?
The mechanism by which 5-hmC is lost in cancer genomes for the majority of cancer types is largely unknown. Recent reports have shown that inactivating mutations in Tet enzymes could certainly be a primary mode of 5-hmC loss in those cancers, such as a subset of AML, that harbor such mutations (e.g. see Ko M et al. (2010) Nature, Impaired hydroxylation of 5-methylcytosine in myeloid cancers with mutant TET2). Similarly, recent work has shown that oncogenic metabolites produced by gain-of-function mutations in IDH enzymes could potentially lead to inhibition of Tet enzymes, which can be inhibited by these metabolites (e.g. see Figurea ME et al. (2010) Cancer Cell, Leukemic IDH1 and IDH2 mutations result in a hypermethylation phenotype, disrupt TET2 function, and impair hematopoietic differentiation.). However, the majority of cancers of the types we examined are not known to harbor such mutations, and the mechanism by which these cancers have low 5hmC in the genome is essentially unknown.
3. Would you say that IHC of 5-hmC could potentially be more useful than 5mC in clinical testing? How would the biomarker information be richer ?
This is difficult to speculate on without further testing. However, for the cancers that we examined, the degree of 5hmC loss is certainly more pronounced than 5mC loss. I suspect that there will also be some cancers, such as seminomas, where there is already profound loss of 5mC (see Netto GJ et al. (2008) Mod Pathol, Global DNA hypomethylation in intratubular germ cell neoplasia and seminoma, but not in nonseminomatous male germ cell tumors), and such cancers will likely also show significant loss of 5hmC. The clinical utility of either IHC stain in different cancer types needs to be explored more thoroughly.
We are all keenly following the progress on identifying functional roles of 5-hmC – especially in the context of oncology. Thanks again to Dr. Yegnasubramanian’s lab for giving us the “big picture” on 5-hmC.
Haffner MC, Chaux A, Meeker AK, Esopi DM, Gerber J, Pellakuru LG, Toubaji A, Argani P, Iacobuzio-Donahue C, Nelson WG, Netto GJ, De Marzo AM, & Yegnasubramanian S (2011). Global 5-hydroxymethylcytosine content is significantly reduced in tissue stem/progenitor cell compartments and in human cancers. Oncotarget PMID: 21896958