Loss of Heterozygosity on Chromosome 6q14-q24 Confers Poor Outcome on Children and Adolescents with T-Cell Lymphoblastic Lymphoma.

Deletions of chromosome 6q have been reported in a variety of hematological malignancies. Regarding their biological and prognostic impact, however, the data available are not conclusive. Therefore the current study focused on 60 pediatric and adolescent patients all diagnosed with T-cell lymphoblastic lymphoma (T-LBL), a clearly defined histological sub-entity of NHL, and all treated uniformly according to the protocol NHL-BFM 95. Deletions of chromosome 6 were analyzed by loss of heterozygosity (LOH) analysis of 25 microsatellite markers located on chromosome 6q14-q24 and the prognostic impact of LOH on 6q was evaluated after combing the results with clinical data of the patients.

Selection criteria for this study were 1) treatment according to multicentre protocol NHL-BFM 95, 2) T-cell immunophenotype of LBL, 3) germ line control and tumor DNA available and 4) three or more informative markers for the analysis of LOH. High-molecular weight germ line DNA was extracted from blood and bone marrow smears or frozen cells. Tumor DNA was isolated from tumor touch imprints or cytospin preparations of malignant effusions or frozen tumor cells. Paired normal and tumor DNA samples from each patient were amplified by PCR, after which fragment length analysis was performed on an ABI PRISM 3100 Genetic Analyzer.

From 04/95 until 03/03 a total of 186 patients were diagnosed with T-LBL and treated according to the protocol NHL-BFM 95. The probability of event free survival (pEFS) for these 186 patients was 79±3% (median follow-up of 4.5 (0.5–9.1) years). 60 of the 186 T-LBL patients were evaluable for LOH analysis, including 19 out of the total 25 of T-LBL patients who suffered from tumor failure. Fragment length analysis comparing germ line DNA and tumor DNA was performed for all 25 microsatellite markers per patient resulting in a total of 832 successfully analyzed markers for the whole group of 60 patients. The status of these 832 results was LOH in 10%, retention of heterozygosity in 55%, homozygosity in germ line control and tumor DNA in 25% and microsatellite instability in 9% of results respectively. In 19 out of the 60 evaluable patients (32%) LOH of at least one microsatellite marker was detected. LOH was observed in 13 out of the 19 evaluable patients with tumor failure compared to 6 out of the 41 patients without tumor failure. The probability of disease free survival (pDFS) (5y) for the 19 patients with LOH was 24±11% compared to 85±6% for the 41 patients without LOH (p <0.0001). A critical region of deletion could be assigned to microsatellite markers D6S1284 and D6S1716. pDFS (5y) was 0% for patients with LOH involving these markers (n=7) compared to 82±7% for patients without LOH at these markers (n=31) (p <0.0001). We conclude that LOH on chromosome 6q14-q24, especially of markers D6S1284 and D6S1716, confers poor outcome on children and adolescents with T-LBL.