T Cell Receptor Gamma and Delta Gene Rearrangements in T-cell Acute Lymphoblastic Leukemia in South India and Quantitation of Minimal Residual Disease
Authors: Natarajan Sudhakar , Nancy Nirmala , Kamalalayam Rajalekshmy , Thangarajan Rajkumar 
Published: November 2010

Abstract
Background:
T cell-Acute Lymphoblastic Leukemia (T-ALL) arises by clonal proliferation of lymphoid precursors arrested at particular stage of differentiation. The incidence of T-ALL in India is 37-43% of ALL. In this study, TCR gamma (TCRG) and TCR delta (TCRD) gene rearrangements were detected in diagnostic samples of ALL. Those clonal rearrangements detected at diagnosis are used as clonal markers for the quantitation of minimal residual disease (MRD) in follow up samples.Patients and Methods: BM/PB from 54 T-ALL patients (34 pediatrics and 20 adults) at diagnosis, treated by MCP 841 protocol were studied. Median age of the patients was 13. The frequency of clonal TCRG and TCRD gene rearrangements were studied by Polymerase Chain Reaction (PCR) coupled with Heteroduplex analysis (HD). Allele Specific Oligos (ASO) was designed by sequencing the junctional region sequence of clonal TCRG and TCRD gene rearrangements. MRD was studied in 4 patients with follow up samples. Diagnostic DNA with almost 100% tumor involvement as standard was serially diluted (50ng to 5ng) in 500ng control DNA to give a final concentration of one leukemic cell in 101 (1 in 101) cells to one leuekemic cell in 105 (1 in 105) cells. The serially diluted diagnostic standard in triplicates was amplified with TaqMan probe for respective TCRG/TCRD gene rearrangements in Real-time PCR along with 500ng of follow up DNA samples at different time points (Unknown) in triplicates. Analysis was done to calculate the amount of leukemic cells in follow up samples.

Results:
Using PCR-HD analysis, TCRG gene rearrangements were detected in 37 of 54 cases (68.5%) and TCRD gene rearrangements in 16 of 54 cases (29.6%). VgI-Jg1.3/2.3 was more commonly rearranged in 29 cases (53.7%) of T-ALL; VgII-Jg1.3/2.3 in 14 cases (26%). Both VgIII-Jg1.3/2.3 and VgIV-Jg1.3/2.3 rearrangements were detected in 4 cases respectively and VgI-Jg1.1/2.1 was detected in 3 cases. Vd1-Jd1 rearrangement was detected in 9 cases (16.6%); Vd2-Dd3 in 5 cases and Dd2-Dd3 in 4 cases of T-ALL. Both TCRG and TCRD gene rearrangements were detected in 8 cases (14.8%). The junctional region in TCRG rearrangements ranged from 1nucleotide to 11nucleotide (average 7.6 nt) and in TCRD rearrangements ranged from 14 nucleotide to 42 nucleotide (average 27 nt). In patient 1, the initial leukeimia load of 1 (before treatment) was reduced to 3 leukemic cell in 104 cells at the lost follow up. In patient 2, the initial leukemia load was reduced to 3.7 in 104 cells. In Patient 3, the initial leukemia load 1 was reduced to 6 in 102 cells at end of M2 and disease relapsed at M5. In patient 4, the initial leukemia load was reduced to 1.6 leukemic cell in 104 cells.

Conclusion:
Real-time PCR experiments reached a reproducible sensitivity of detecting one leukemic cell in 104 normal cells. Real-time PCR analysis showed that Patient 3 was not all responded to treatment and it was detectable by Real-time PCR at RI1 stage itself though the disease was clinically evident only at M5 stage of treatment. Thus, monitoring the MRD using Real-time PCR will help to quantitate the accurate amount of residual leukemic load, predate relapse and assess the response to treatment.