Mechanism of thiopurine resistance in acute lymphoblastic leukemia

Acute lymphoblastic leukemia (ALL) is an aggressive lymphoid malignancy that is currently the leading cause of cancer in pediatric patients1. Despite intensified chemotherapy regimens, the cure rates of ALL only approaches 40%2. Specific mutations in the cytosolic 5’-nucleotidase II (NT5C2) gene are present in about 20% of relapsed pediatric T-ALL and 3-10% of relapsed B-precursor ALL cases3,4.

NT5C2 is a cytosolic nucleotidase that maintains intracellular nucleotide pool levels by exporting excess purine nucleotides out of the cell5.  NT5C2 can also dephosphorylate and inactivate the metabolites of the 6-thioguanine (6-TG) and 6-mercaptopurine (6-MP) commonly used to treat ALL6. Thus, relapse associated activating mutations in NT5C2 confer resistance to 6-MP and 6-TG chemotherapy. Upon allosteric activation, a disordered region of NT5C2 adopts a helical configuration (helix A) and facilitates substrate binding and catalysis (Fig. 1a)7.  Mutations in this regulatory region of NT5C2 have been modeled to strongly activate NT5C2.  However, the majority of NT5C2 mutations associated with relapsed ALL do not occur in this region.
To better understand the mechanisms by which these gain-of function NT5C2 mutations lead to increased nucleotidase activity, Dieck, Tzoneva, Forouhar and colleagues investigated additional regulatory elements that may control NT5C2 activation.  They collected crystallographic data for several mutant NT5C2 homotetramers at SSRL (NT5C2-537X D52N/D407A in active state (BL9-2), NT5C2-Q523X D52N in basal state and in active state (BL14-1) and full-length NT5C2 R39Q/D52N in basal state (BL12-2)) and used the structural information as a guide in the understanding of the mechanistic details.

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Figure (a) A ribbon diagram of the active structure of NT5C2 WT, in which the allosteric helix A (αA) is shown in dark purple. The N and C termini amino acids (S4 and S488), and the termini amino acids (L402 and R421) of the disordered region in the arm segment are also labeled. Panels b and c shows ribbon and surface (for subunit B) depictions of basal (b) and active dimers (c) of WT.