Scientists have identified transcription intermediary factor 1γ (TIF1γ) as a tumor suppressor in hematopoietic cells. The team suggests changes in expression of the protein could represent a biomarker for measuring response to demethylating agents and other chromatin structure modifiers such as HDAC inhibitors in development for treating chronic myelomonocytic leukemia (CMML).
The researchers, led by Jean-Noël Bastie, Ph.D., and Laurent Delva, Ph.D., at Inserm UMR 866, the University of Burgundy, found that deletion of the TIF1γ gene in the hematopoietic tissues of experimental mice led to the animals developing an age-dependent myeloproliferative disorder that mimicked the essential characteristics of human CMML. They separately found that TIF1γ was almost undetectable in leukemic cells of 35% of CMML patients evaluated and that demethylating agents restated its expression.
The results are published in the Journal of Clinical Investigation. The paper is titled “Transcription intermediary factor 1γ is a tumor suppressor in mouse and human chronic myelomonocytic leukemia.”
TIF1γ affects human hematopoietic progenitor cell response to cytokines of the TGF-β superfamily through various mechanisms, the authors write. To further explore the role of TIF1γ in hematopoiesis, Drs. Bastie and Delva’s team examined the effects of hematopoietic tissue-targeted deletion of theTif1g gene in mice.
They found that knockout mice younger than six months old did not display any macroscopic or blood peripheral abnormalities but did show changes in the proportions of hematopoietic progenitors. The proportion of granulocyte/monocyte progenitors was increased by about 400%, at the expense of common myeloid progenitors and megakaryocyte-erythroid progenitors.
Conversely, knockout mice older than six months developed a CMML-like myeloproliferative disease with monocytic features. When bone marrow cells were taken from four month old knockout mice and transplanted into control animals, the recipients also developed the same myeloproliferative disease.
The researchers also tested whether Tif1γ could regulate expression of Csf1r (M-CSFR), Csf3r (G-CSFR), and Csf2ra (GM-CSFR), three genes known to participate in monocyte or granulocyte differentiation. They found that knockout mice had significantly reduced expression of Csf1r specifically, which they suggest could account for the altered production of peritoneal macrophages observed in the knockout animals.
Moving on to humans, the team subsequently found the TIF1γ transcript was almost undetectable in peripheral blood monocytes sorted from 21 of the 60 patients (35%) with CMML. Immunocytochemical analysis confirmed decreased expression of the protein in monocytes and neutrophils of these patients. Low levels of TIFIG in cells from these patients correlated strongly with Csf1r expression, which had previously been identified in the knockout mouse cells.
Interestingly, sequencing of all the TIF1G exons in monocytes from the 66 CMML patients failed to identify any mutation in the coding sequence, whatever the levels of TIF1γ mRNA and protein levels were expressed. Rather, they found that the TIF1G promoter of CMML patients expressing low levels of TIF1γ was methylated, while none of the DNA from either normal human monocytes or from CMML monocyte sample with normal TIF1G mRNA levels was methylated
This promoter hypermethylation appeared to be corrected on administration of demethylating agents. Exposing CMML peripheral blood monocytes with initially downregulated TIF1G expression to decitabine for three days restored TIF1γ expression. Gene expression was also reestablished in the peripheral blood monocytes of a CMML patient who responded to decitabine in vivo.
The researchers say cytosine methylation in the TIF1G promoter sequence, which they had identified in the monocytes of these patients before decitabine treatment, was no longer detected after seven cycles of decitabine, which correlated with Tif1γ mRNA re-expression.