Fifty years ago, a child diagnosed with acute lymphoblastic leukemia (ALL), the most common type of pediatric cancer, had little to no chance of survival. Today, those odds have increased dramatically thanks to tremendous advances in chemotherapy and other treatments. Cure rates for this type of leukemia can reach as high as 90 percent. Yet, there is one subgroup of pediatric ALL that is still very therapy resistant, T-Acute Lymphoblastic Leukemia (T-ALL).
T-ALL is an uncommon, though aggressive, subclass of ALL. In children, this particular leukemia accounts for approximately 15% of all cases of ALL. Unfortunately, its response rate to standard chemotherapy treatment is much lower. Like other types of leukemia, T-ALL is a cancer of the blood cells and it originates in the bone marrow. Leukemia groupings are organized by the specific type of white blood cell affected. ALL commonly affects B lymphocytes, while T-ALL affects T lymphocytes or T cells. Both are major cellular components of the adaptive immune response system; however, T lymphocytes participate in cell-mediated immunity, whereas B cells are involved in humoral immunity concerning antibodies.
Why T-ALL resists chemotherapy is not fully understood. However, previous studies have implicated epigenetic aberrations, such as DNA hypermethylation, as possible factors affecting a patient’s response to chemotherapy. In addition, elevated expression of the ASNS gene (asparagine synthetase) has been linked to L-asparaginase (L-Asp) resistance, a common chemotherapy drug used to treat ALL. ASNS expression, incidentally, is low in B-lineage ALL where the use of this drug has proven to be quite effective.
Taking these factors into account, a research team at the University of Bologna, Bologna, Italy conducted a study to investigate whether DNA hypermenthylation plays a role in the sensitivity of T-ALL to L-Asp. They also tested the effectiveness of using Decitabine, a demethylating agent, in combination with L-Asp on modifying sensitivity. Their results were published in BioMed Research International.
In the study, various T-ALL cell lines grouped by molecular genetic subcategories were treated with increasing concentrations of L-Asp. Cell lines displaying high sensitivity to L-Asp treatment and low ASNS expression appeared to be connected to hypermethylation of the ASNS promoter. The reverse was true for T-ALL cell lines with lower sensitivity to L-Asp.
To identify the ASNS gene silencing mechanism in T-ALL cell lines, the research team performed DNA methylation analysis on bisulfite treated DNA using EpiGentek’s Methylamp Whole Cell Bisulfite Modification Kit. This method of determining DNA methylation status involves converting cytosine to uracil while leaving 5-methylcytosine (5-mC) intact. After bisulfite modification, the promoter region of the ASNS gene was amplified with specific primers and followed by DNA sequencing. The results revealed complete methylation of CpG islands in ASNS promoter in those cell lines with low expression of ASNS. The cell lines with higher ASNS expression did not display this epigenetic signature and showed a completely unmethylated promoter region, thus demonstrating that ASNS expression was dependent on the methylation status of the promoter.
The researchers wanted to further correlate ASNS expression with promoter methylation, as well as assess the effect of a demethylating agent on L-Asp sensitivity. They added Decitabine (5-aza-2′-deoxycytidine) to the cell lines in combination with increasing concentrations of L-Asp. Interestingly, they discovered a much improved L-Asp sensitivity in the cell lines with hypermethylation of ASNS promoters. This amplified effect was not expected and suggests an alternative role of hypermethylation at the promoter region of ASNS in the cellular response to L-Asp.
The scientists, while impressed with their findings, stressed that other studies are necessary to further explain the molecular mechanisms triggering this type of outcome. They indicated that the results are “probably linked to a larger set of methylated genes in T-ALL subset. In particular, methylation status may not be restricted to ASNS gene and the reactivation of many genes such as oncosuppressors could have an important role in the synergic cytotoxic effect induced by Decitabine.”
How these results may play out in a clinical setting is unknown. But it is encouraging to think that DNA methylation inhibitors, like Decitabine – now a star in epigenetic therapy – could be successfully used in the treatment of cancer, as well as other diseases. While this study is only preliminary, the role of demethylating agents in combination with chemotherapy needs to be further analyzed.
Source: Salvatore Serravalle, Salvatore N. Bertuccio, Annalisa Astolfi, Fraia Melchionda, and Andrea Pession, Synergistic Cytotoxic Effect of L-Asparaginase Combined with Decitabine as a Demethylating Agent in Pediatric T-ALL, with Specific Epigenetic Signature BioMed Research International, vol. 2016, Article ID 1985750, 6 pages, 2016. doi:10.1155/2016/1985750.