Andrea Biondi

Molecular Monitoring of Minimal Residual Disease

Continuous clinical remission (CCR) in patients with acute leukemia, either lymphoid (ALL) or myeloid (AML), is conventionally defined to be <5% of bone marrow blasts by morphologic assessment. However, this relatively liberal description can be associated with a leukemic burden of up to 1 x 1010 blasts (1). As shown in Fig. 1, the term minimal residual disease (MRD) has been used to define the lowest level of disease detectable in patients in CCR by the methods available. A number of techniques have been developed over the past 5–10 yr to complement morphology in assessing response to treatment, including immunologic, molecular, fluorescence in situ hybridization (FISH) and colony assays (2). Despite notable progress with these methods, their sensitivities vary considerably, and several critical issues must be resolved before MRD determinations can be routinely considered in clinical decision making.

IKZF1plus Defines a New Minimal Residual Disease–Dependent Very-Poor Prognostic Profile in Pediatric B-Cell Precursor Acute Lymphoblastic Leukemia

Purpose Somatic deletions that affect the lymphoid transcription factor-coding gene IKZF1 have previously been reported as independently associated with a poor prognosis in pediatric B-cell precursor (BCP) acute lymphoblastic leukemia (ALL). We have now refined the prognostic strength of IKZF1 deletions by analyzing the effect of co-occurring deletions. Patients and Methods The analysis involved 991 patients with BCP ALL treated in the Associazione Italiana Ematologia ed Oncologia Pediatrica-Berlin-Frankfurt-Muenster (AIEOP-BFM) ALL 2000 trial with complete information for copy number alterations of IKZF1, PAX5, ETV6, RB1, BTG1, EBF1, CDKN2A, CDKN2B, Xp22.33/Yp11.31 (PAR1 region; CRLF2, CSF2RA, and IL3RA), and ERG; replication of findings involved 417 patients from the same trial. Results IKZF1 deletions that co-occurred with deletions in CDKN2A, CDKN2B, PAX5, or PAR1 in the absence of ERG deletion conferred the worst outcome and, consequently, were grouped as IKZF1plus. The IKZF1plus group comprised 6% of patients with BCP ALL, with a 5-year event-free survival of 53 ± 6% compared with 79 ± 5% in patients with IKZF1 deletion who did not fulfill the IKZF1plus definition and 87 ± 1% in patients who lacked an IKZF1 deletion ( P ≤ .001). Respective 5-year cumulative relapse incidence rates were 44 ± 6%, 11 ± 4%, and 10 ± 1% ( P ≤ .001). Results were confirmed in the replication cohort, and multivariable analyses demonstrated independence of IKZF1plus. The IKZF1plus prognostic effect differed dramatically in analyses stratified by minimal residual disease (MRD) levels after induction treatment: 5-year event-free survival for MRD standard-risk IKZF1plus patients was 94 ± 5% versus 40 ± 10% in MRD intermediate- and 30 ± 14% in high-risk IKZF1plus patients ( P ≤ .001). Corresponding 5-year cumulative incidence of relapse rates were 6 ± 6%, 60 ± 10%, and 60 ± 17% ( P ≤ .001). Conclusion IKZF1plus describes a new MRD-dependent very-poor prognostic profile in BCP ALL. Because current AIEOP-BFM treatment is largely ineffective for MRD-positive IKZF1plus patients, new experimental treatment approaches will be evaluated in our upcoming trial AIEOP-BFM ALL 2017.

The Role of PAX5 in ALL

It is now widely acknowledged that, like other cancers, acute lymphoblastic leukaemia is caused by the acquisition of mutations in somatic cells. The first hit in childhood ALL has been documented to occur in most (if not all) cases before birth (Golub, 2007). The progress in genomics technology holds the promise of making the complete characterization of the ‘cancer genome’ possible by a systematic search for submicroscopical mutations. Molecular studies on recurrent chromosomal translocations in ALL have indicated that, beside the constitutive activation of tyrosine kinases, the aberrant expression of transcription factors plays a central role in the pathobiology of lymphoid leukaemia. This aberrant expression leads to abnormal proliferation and differentiation arrest of lymphoid progenitors (ONeil & Look, 2007). In some cases, recurrent chromosomal translocations generate fusion transcription factors with new functions, enabling them to target genes other than those recognized by the endogenous factors. In the past few years, the PAX5 gene has been demonstrated to be a recurrent target of genetic alterations in B-lineage ALL, both in adult and paediatric patients (Mullighan et al., 2007). The role of these aberrancies in human leukemogenesis is still, however, poorly understood (Cobaleda et al., 2007a). This chapter will review the role of PAX5 gene involvement in B-lineage ALL to outline the biological and functional effects of different genetic aberrations affecting this new master gene in leukaemia. Moreover, we will address whether PAX5 alterations are driver or passenger lesions and finally what their potential prognostic impact can be.