Acute Lymphocytic Leukemia (ALL), a cancer of the blood, is the most common childhood malignancy. The survival rate of children with ALL has risen significantly from 4% in the 1960's, to 80-90% today. However, the average child with ALL receives chemotherapy for 2.5 years, and the long-term effects of these treatments are unknown. In order to understand the effects of chemotherapy on these patients' normal cells, the Finette laboratory isolates non-tumor T-cells from patients' blood and studies the reporter gene, HPRT. This gene is working in every cell of the body, so the mutations found are reflective of changes in DNA throughout the body. Research in the Finette laboratory has found that children treated with chemotherapy for ALL have significantly elevated HPRT mutant frequencies (Mfs) (30-1300 fold higher), compared to controls. To investigate the cause of these elevated Mfs, Kendall studied defects in DNA mismatch repair (MMR) using microsatellite instability (MSI) analysis, the types of HPRT mutations, as well as the extent of cell division in the mutant T cell isolates, in children treated for ALL at diagnosis, during chemotherapy, and post-chemotherapy, compared to healthy controls.
The MMR system normally repairs mispaired DNA bases following cell division. When MMR fails cells accumulate mutations in their DNA. MSI analysis is a tool for detecting defects in MMR by analyzing changes/mutations in the size of specific DNA sequences called microsatellites. MSI analysis was performed on 167 T-cell isolates from 40 healthy children, and on 842 T-cell isolates from 50 patients treated for ALL. High-frequency MSI (MSI-high) was identified in 2 healthy children (5%) and in two of twenty ALL subjects at the time of disease recurrence (relapse) (10%). There was no statistically significant difference between the prevalence of MSI-high in patients at the time of ALL relapse and healthy children, nor between the children with ALL at other time points and healthy children. These data indicate that MMR defects, represented by MSI, are not a significant contributor to the elevated HPRT Mfs seen in children treated for ALL. However, in a small number of patients, chemotherapy may play a role in the selection of cells with defects in MMR that may have long-term clinical implications.
A total of 562 T-cell mutant isolates from 87 blood samples of 47 ALL patients were characterized for both the type of HPRT mutation, and the amount of cell division. No differences were observed in the mutational spectra in children at the time of ALL diagnosis when compared to age-matched healthy controls. However, children who received chemotherapy for ALL exhibited unique types of mutations when compared to controls. Kendall also observed an increase in the extent of cell division in the mutant cells in ALL patients as they proceeded with and completed their therapy, compared to healthy children. These data indicate specific genetic changes occur following treatment for ALL, and that further investigation is necessary to understand how these changes relate to the long-term risks for other diseases, including secondary cancers, later in life.