TELAML1-Positive ALL: A Discordant Genotype

Abstract

In a recent gene expression profiling study, we identified genes associated with cross-resistance to four major drugs used in childhood ALL (prednisolone, vincristine, L-asparaginase and daunorubicin).1 Remarkable was that cross-resistance to these 4 different classes of drugs was not primarily linked to defects in key-molecules of the apoptotic pathway such as caspases, inhibitors of apoptosis proteins and Bcl2-family members. In contrast, cross-resistance turned out to be linked to aberrant expression of various DNA and RNA metabolism-associated genes such as those involved in RNA transcription and processing (e.g. WAC, CTCF, TCF4, TAF5, PRPF18). These data generate new hypotheses about the cause of multiple drug resistance and therapy failure in pediatric ALL, and hence, further exploration of the cross-resistance associated genes may point to novel strategies to circumvent chemo-resistance in clinical practice.

In addition to cross-resistance to the four mentioned drugs, a so-called discordant resistance subtype was identified. About 25% of the interpatient variation for cellular resistance to the four drugs could be explained by opposing sensitivity to vincristine and L-asparaginase.1 Patients who were resistant to vincristine but sensitive to L-asparaginase had a better long-term clinical outcome compared to vincristine-sensitive/L-asparaginase resistant COALL/DCOG-treated patients. Interestingly, the vincristine-resistant/L-asparaginase-sensitive subtype was especially linked to TELAML1-positive ALL, a common genetic subtype with a favorable prognosis.1 This is concordant with previous studies showing that leukemic cells with the TELAML1 translocation were previously shown to be about 6-fold more sensitive to L-asparaginase and 1.5-fold more resistant to vincristine, compared to non-TELAML1 precursor B-lineage ALL children.2,3 The relative favorable outcome of TELAML1-positive ALL may be explained by the fact that cellular L-asparaginase sensitivity is a stronger discriminator for a favorable prognosis than vincristine sensitivity (Den Boer, unpublished results and Kaspers et al. 1997).4

The genes discriminating the discordant resistance subtype may shed new light onto discrepancies in causes of cellular drug resistance between TELAML1-positive and TELAML1-negative children with ALL. Recently, we and others have questioned the general dogma that resistance to L-asparaginase is caused primarily by overexpression of asparagine synthetase, the enzyme which opposes the action of L-asparaginase. For decades it was believed that leukemic cells were sensitive to L-asparaginase because they were thought to lack asparagine synthetase. On the contrary, TELAML1-positive cases (which are in general L-asparaginase sensitive) were shown to express 2- to 5-fold more of this enzyme compared to TELAML1-negative ALL and normal bone marrow cells.3,5 Although TELAML1-negative precursor B-lineage ALL cells have lower expression levels of asparagine synthetase, it appeared only within this group of patients that increased asparagine synthetase expression is linked to increased resistance to L-asparaginase.6 This subtype-specificity may also explain why genes linked to L-asparaginase resistance in a mixed population of human leukemia cell lines could not classify TELAML1-positive cases as L-asparaginase sensitive.7 These data emphasize that TELAML1-positive ALL is a distinct entity in which drug sensitivity and resistance may originate from different mechanisms compared to other precursor B-lineage ALL subtypes.

The question remains which genes explain the vincristine-resistance and L-asparaginase-sensitive subtype and vice versa. Using gene expression profiling, Lughthart et al identified 139 genes (and 13 cDNAs) linked to the discordant subtype.1 Genes involved in protein synthesis were particularly affected, including genes encoding ribosomal proteins (RPL-family) and translation initiation and elongation factors (e.g. EIF3, EIF4, EEF 1 and EEF2). These genes represented about 50% of all gene probes that were discriminative for this discordant subtype of ALL. The protein synthesis-linked genes were overexpressed in vincristine-sensitive/L-asparaginase resistant cases, and hence are relatively underexpressed in TELAML1-positive cases. Notably, a large overlap was found in genes predictive for discordant resistance to vincristine/L-asparaginase and those genes predictive for resistance to vincristine and L-asparaginase as single drugs in a total group of precursor B-lineage ALL, ie. 11/40 genes for vincristine and 28/35 genes for L-asparaginase.8 It needs to be further explored whether the non-overlapping genes reflect more discordant subtype-specific genes that may be unique for TELAML1-positive ALL.

In conclusion, recent studies provide evidence that the mechanisms of drug sensitivity and resistance differs between TELAML1-positive ALL (25% of ALL patients) and other precursor B-lineage ALL patients. Collectively, these studies provide new insights into the genomic basis of multi-drug resistance in childhood ALL, and point to new strategies to circumvent mechanisms of treatment failure in the most common cancer in children.

References

1. Lugthart S, Cheok MH, Den Boer ML, et al. Identification of genes associated with chemotherapy crossresistance and treatment response in childhood acute lymphoblastic leukemia. Cancer Cell. 2005;7:375-386.

2. Ramakers-van Woerden NL, Pieters R, Loonen AH, et al. TEL/AML1 gene fusion is related to in vitro drug sensitivity for L-asparaginase in childhood acute lymphoblastic leukemia. Blood. 2000;96:1094-1099.

3. Stams WAG, Den Boer ML, Beverloo HB, et al. Sensitivity to L-asparaginase is not associated with expression levels of asparagine synthetase in t(12;21)+ pediatric ALL. Blood. 2003;101:2743-2747.

4. Kaspers GJL, Veerman AJP, Pieters R, et al. In vitro cellular drug resistance and prognosis in newly diagnosed childhood acute lymphoblastic leukemia. Blood. 1997;90:2723-2729.

5. Krejci O, Starkova J, Otova B, et al. Upregulation of asparagine synthetase fails to avert cell cycle arrest induced by L-asparaginase in TEL/AML1-positive leukaemic cells. Leukemia. 2004;18:434-441.

6. Stams WAG, Den Boer ML, Holleman A, et al. Asparagine synthetase expression is linked with L-asparaginase resistance in TELAML1 negative, but not in TELAML1 positive pediatric acute lymphoblastic leukemia. Blood. 2005;in press.

7. Fine BM, Kaspers GJ, Ho M, Loonen AH, Boxer LM. A genome-wide view of the in vitro response to L-asparaginase in acute lymphoblastic leukemia. Cancer Res. 2005;65:291-299.

8. Holleman A, Cheok MH, Den Boer ML, et al. Gene-expression patterns in drug-resistant acute lymphoblastic leukemia cells and response to treatment. N Engl J Med. 2004;351:533-542.