Relapse of acute promyelocytic leukemia with PML-RARα mutant subclones independent of proximate all-trans retinoic acid selection pressure

Abstract

Relapse of acute promyelocytic leukemia (APL) following alltrans retinoic acid (ATRA) therapy has been associated with the acquisition of mutations in the high-affinity ATRA binding site in PML-RARa, but little information is available about the selection dynamics of the mutation-harboring subclones. In this study, 6/18 patients treated with sequential ATRA and chemotherapy on protocol INT0129 relapsed with complete replacement of the nonmutant pretreatment APL cell population by a PML-RARa mutant subclone. Two patients relapsed in proximity of ATRA treatment; however, in four patients there was a 6-48 month hiatus between the last ATRA treatment and relapse. The mutant subclones were not detectable in samples tested X3 months before relapse at X1 in 10 2 (10 À2 ) sensitivity. In one patient, a functionally weak mutation was detected at 10 À4 sensitivity before therapy but only limited pre-relapse enrichment of the mutant subclone was observed on subsequent ATRA therapy. These results indicate that proximate ATRA selection pressure is frequently not the main determinant for the emergence of strongly dominant PML-RARa mutant subclones and suggest that APL subclones harboring PMLRARa mutations are predisposed to the acquisition of secondary genetic/epigenetic alterations that result in a growth/ survival advantage.

Valproic acid induces differentiation and transient tumor regression, but spares leukemia-initiating activity in mouse models of APL

“…However, in patients, ATRA does not lead to complete remission without accompanying chemotherapy, 16,17 and some patients may develop resistance. 18 Valproic acid (VPA) belongs to the class I HDACi, which has been widely used as an antiepileptic drug. VPA induces growth arrest, differentiation and apoptosis in many leukemic cell lines, as well as in fresh leukemic blasts.…”

PML–RARα ligand-binding domain deletion mutations associated with reduced disease control and outcome after first relapse of APL

“…In four of six patients from protocol E2491 who relapsed with LBD mutations, the APL clone harboring the mutation emerged and replaced the previously predominant wild-type PML-RARa APL cell population long after the last treatment with ATRA. 3 These observations suggested that LBD mutation-harboring clones might selectively acquire ATRA-independent mechanism(s) of increased APL cell proliferation/survival competitiveness, and that this mechanism(s) might be favored in late relapse patients. In a collaborative effort to further explore this possibility, samples from previously reported late relapse patients, that is, relapse after 44 years, 5 were not available; however, eight relapse samples from patients with a relatively long interval to first relapse (median, 1015 days after treatment on French-Belgian-Swiss APL group protocol, APL2000, or off-protocol in a similar manner), 2 were available and were analyzed (Table 1).…”

“…In patient 6, second relapse occurred despite the use of arsenic trioxide consolidation therapy, suggesting that targeting the PML region of PML-RARa by arsenic trioxide, 1 which would include the truncation mutant clone, was insufficient to prevent disease recurrence. Similarly, all three alternatively reported APL patients who relapsed with LBD deletion mutations on protocols E2491 and C9710 succumbed to their disease during the protocol follow-up period (one on ATRA, two off ATRA; see Gallagher et al 3 and Gallagher et al 4 and manuscript in preparation). Together, these observations suggest that the infrequent patients who relapse with deletion or insertion/truncation mutations in PML-RARa are at increased risk of poor clinical outcome.…”

“…6 Two of the missense mutations present in the double mutant cases, R276W and R272W (Figures 1a and c), have been redundantly reported and are associated with decreased ATRA binding, owing to disruption of the interaction of basic arginine residues with the carboxylate tail of ATRA in the RARa receptorbinding pocket by the substitution of hydrophobic tryptophan. 3,7 Although not yet analyzed, the novel N298S mutation is also situated in the binding pocket region of the RARa receptor (Figures 1c and d). Thus, the characteristics of these missense mutations are consistent with a role in producing ATRA resistance and having a selection effect associated with relapse while receiving ATRA maintenance therapy.…”

“…1 Over many years, adjustments of therapy with ATRA in combination with chemotherapy for the initial treatment of APL has achieved long-term remission rates, apparent disease cure, in X85% of new APL patients. 2 In two North American phase III clinical trials employing ATRA/chemotherapy (E2491 and C9710), mutations in the ligand-binding domain (LBD) of PML-RARa were found in 30-40% of the minority of patients who relapsed (see Gallagher et al 3 and Gallagher et al 4 and manuscript in preparation). In four of six patients from protocol E2491 who relapsed with LBD mutations, the APL clone harboring the mutation emerged and replaced the previously predominant wild-type PML-RARa APL cell population long after the last treatment with ATRA.…”