Investigating the Role of the SETDB1 Target TANGO6 (TMCO7) in Leukemia

Despite advancements in AML therapies over the last 2 decades, 1 out of 3 patients diagnosed with AML succumb to their disease. The mainstay of treatment remains systemic chemotherapy, however better understanding of the genetic drivers in AML has generated new therapeutic opportunities. Targeted therapies have emerged to more precisely tailor leukemic treatment based on genetic and epigenetic drivers. We previously demonstrated that SETDB1, a H3K9 methyltransferase involved in heterochromatin regulation, negatively regulates the expression of proleukemic genes Hoxa9 and Meis1. Further, overexpression of SETDB1 correlates with improved overall AML patient survival and extends leukemic disease latency in mouse models (Ropa et al. Haematologica 2020). Here we describe a novel SETDB1 target, TANGO6, and aim to explore its role in leukemogenesis.

We mined our genome wide expression and epigenetic datasets generated upon overexpression of SETDB1 to identify targets that may be important for leukemic progression (Ropa et al. Haematologica 2020). TANGO6 was found to be transcriptionally and epigenetically regulated by SETDB1, with little known about its role in leukemia. We performed a targeted CRISPR-Cas9 mediated knock out of TANGO6 to investigate its importance in several human leukemic cell lines: MV4;11, K562 and KOPN8. We established the above cell lines with Cas9 constitutively expressed and introduced sgRNAs targeting TANGO6 by lentivirus. We then performed a competitive growth assay using flow cytometry to assess cell fitness on days 3, 14, 10, 21. We observed that loss of TANGO6 resulted in decreased fitness across all three leukemic cell lines compared to the negative control. This phenotype was more striking in the K562 and KOPN8 cell lines.

In lieu of having a reliable antibody to visualize changes in protein expression, we assessed mutational changes in the DNA sequence to evaluate efficiency of our CRISPR/Cas9 system. Genomic DNA was prepared from sg TANGO6 transduced K562 cells on days 5 and 10. The targeted region of TANGO6 was PCR amplified and TOPO cloned for direct Sanger sequencing. 92% (12 out of 13 samples) showed a mutation involving the target sequence. Of these mutations, ~67% resulted in frameshift mutations leading to early truncation of the protein. The remaining mutations involved deletions ranging from 1 to 28 amino acids, with one sample resulting in two single point mutations conferring a change of leucine to serine and lysine to arginine. These mutations may or may not affect the folding of the protein. This data demonstrates we are successfully targeting TANGO6, which produces the biological phenotype in our competitive growth assay.

To further understand the potential importance of TANGO6 in human leukemia, we performed qPCR on 5 leukemic cell lines (2 MLL-rearranged AML cell lines, 2 B-lymphoblastic cell lines, and 1 CML cell line). Our data showed that TANGO6 was expressed at higher levels across all leukemic cell lines compared to normal CD34+ hematopoietic stem/progenitor cells, consistent with a role in leukemogenesis.

Our preliminary data suggests that TANGO6 plays an important role in leukemogenesis. Ongoing work focuses on validating this hypothesis using in vivo leukemic mouse models and protein localization studies. Given the lack of available information on human TANGO6, characterizing its location and mechanism is essential to understanding its role in disease. Our work has established the need for further exploration of TANGO6 as a potential therapeutic target for AML.