Six6 Is A Tal1-Regulated Transcription Factor In T-Cell Acute Lymphoblastic Leukemia

Acute lymphoblastic leukemia (ALL) is the most common type of cancer in pediatric patients. In these patients, 10-12% of leukemias are of T-cell lineage (T-ALL), but the incidence increases by age and is two to three times higher among adolescents and adults. T-ALL patients have done worse than B-ALL historically, but in the last decade they have obtained rather similar event-free survival rates at primary therapy, but at relapse the prognosis is poor. T-ALL can be subdivided into separate groups based on the expression of transcription factors such as TAL1, LMO1/2 and HOX family. However, this classification has not translated into more personalized treatments. Better knowledge would improve understanding of the development and maintenance of T-ALL and possibly lead to targeted therapies. We recently compiled a large gene expression dataset that utilized a uniform platform (Affymetrix HG U133 Plus 2.0) across 36 hematological and lymphoid malignancies and healthy tissues (Mehtonen et al. 2019, Polonen et al. 2019). This dataset includes 1,008 healthy samples and 4,418 leukemia samples of which 1,713 are acute myeloid leukemia, 1,304 precursor B-ALL, 801 chronic lymphocytic leukemia, 385 T-ALL, and 215 chronic myeloid leukemia. By utilizing this dataset, we searched for disease or subtype-specific transcriptional regulators and observed a high expression of SIX homeobox 6 (SIX6) gene in T-ALL. SIX6 is a transcription factor that normally functions in the retina of developing and adult eye, but only few studies have associated it with cancer. Expression of SIX6 was highest in the TAL1 subgroup of T-ALL. Global nuclear Run-On sequencing (GRO-seq) of multiple T-ALL cell lines revealed an enhancer upstream of SIX6, whose activity correlated with expression of SIX6. Coinciding with this specific enhancer region, ChIP-seq analysis showed a binding of TAL1 and GATA3 transcription factors. Silencing of TAL1 and GATA3 expression significantly repressed expression of SIX6 (n = 4, p-value = 0.029, data from Sanda et al. 2012), thus lending further support of a direct regulation of SIX6 by TAL1. Co-expression of SIX6 together with the MYC oncogene in a Rag2:mMYC T-ALL zebrafish model did not affect leukemia progression. Clinical relevance of SIX6 gene expression was also studied in a patient cohort from TARGET dataset (n = 261), and a high expression of SIX6 was associated with an unfavorable trend in overall survival among T-ALL patients (p-value = 0.12). In conclusion, SIX6 is a T-ALL-related transcription factor which is regulated by TAL1 and GATA3 transcription factors, and associates with less favorable prognosis, thus warranting further mechanistic studies in leukemia. References: Mehtonen et al. Nucleic Acids Res. 2019:26;47(13) Polonen et al. Cancer Res. 2019:15;79(10) Sanda et al. Cancer Cell. 2012:22(2):209-21 Citation Format: Saara Laukkanen, Laura Oksa, Atte Nikkila, Juha Mehtonen, Petri Polonen, Mari Lahnalampi, Merja Heinaniemi, Olli Lohi. SIX6 is a TAL1-regulated transcription factor in T-cell acute lymphoblastic leukemia [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 3588.