Molecular and Functional Characterization Of The Novel Protein-Coding Gene Tihl (Translocated in Hodgkin’s Lymphoma) in Hematopoiesis

Cell cycling is a tightly regulated process involving the structured expression modulation of various regulatory genes. This process is crucial for the maintenance of cell survival/proliferation in both normal and malignant hematopoietic cells. We have previously described the highly expressed CIITA-BX648577 gene fusion (Steidl C. et al., Nature 2011), involving the novel gene locus BX648577/FLJ27352 /hypothetical LOC 145788/C15orf65 and the Class II Transactivator (CIITA) in the Hodgkin’s lymphoma cell line KM-H2. While CIITA is well known to be involved in the regulation of immune responses, specifically through regulation of the Major Histocompatibility Complex (MHC)-II, nothing is known about the expression and function of the BX648577 locus.

The objective of the current study was to (I) study RNA and protein expression of the putative full length gene encoded by the BX648577 (TIHL) gene locus, and (II) study its biological function in normal and malignant hematopoietic cells, including its effects on cell proliferation, clonogenicity and cell death.

We detected robust endogenous TIHL RNA and protein expression in a variety of healthy and malignant hematopoietic cell types using quantitative real-time polymerase chain reaction (qRT-PCR) and western blot analysis using a TIHL-specific antibody. At the functional level, we found that ectopic expression of the highly conserved full length TIHL protein in human NB4 leukemia cells and murine hematopoietic progenitor HPC-7 cells leads to enhanced clonogenicity and increased proliferative capacity with significant increases in the percentage of cells in S-phase of the cell cycle. Furthermore, we observed more aggressive leukemia and decreased survival of NSG mice following retro-orbital transplantation of TIHL-expressing compared to empty control-expressing NB4 cells. Interestingly, although we did not observe a change in the rate of cell proliferation or colony forming ability following TIHL overexpression in the ATRA-resistant cell line NB4.306, there was a significant alteration in its cell cycle distribution, with an increase in the fraction of cells in S-phase. The increase in S-phase cells was confirmed by 5-ethynyl-2’-deoxyuridine (EdU) incorporation assays and flow cytometry. Similarly, knockdown of TIHL using 2 independent lentiviral shRNAs, led to a significant decrease in the growth of both NB4 and acute myeloid leukemia KG1a cells in both suspension cultures and semi-solid media. Although we observed slightly increased apoptosis upon TIHL downregulation, the changes could be more significantly attributed to a decrease in the percentage of cells in S-phase within 2-3 days after transduction with the lentiviral shRNAs.

Finally, in silico analysis of the TIHL promoter identified various predicted transcriptional regulators of TIHL, the majority of which are cell cycle specific transcription factors including Nuclear Receptor Subfamily 5 Group A Member 1 (NR5A1), the ets domain transcription factors ELF5 and ELF1, and Glioma-Associated Oncogene Homolog 1 (GLI-1).

Our findings thus far strongly support a novel role for TIHL in cell cycle regulation/modulation in both normal and malignant hematopoiesis. Future directions include gene expression studies to identify downstream targets of TIHL following overexpression and knockdown and co-immunoprecipitation coupled to mass spectrometry analysis will be used to identify direct interacting protein partners of this novel gene.