Minimal Residual Disease Monitoring by Quantification of Fusion Gene Transcripts In Infant with MLL-rearranged Acute Lymphoblastic Leukemia Treated by MLL-Baby Protocol
Authors: Tsaur Grigory , Popov Alexander , Nasedkina Tatyana , Kalennik Olga , Kustanovich Anatoly , Aleinikova Olga , Riger Tatyana , Yakovleva Yulia , Ivanova Anna , Shorikov Egor , Saveliev Leonid , Fechina Larisa 
Published: October 2019

Abstract
Abstract 2731

Background.
Despite many attempts worldwide treatment results for infants with MLL rearrangements and especially MLL-AF4 remain unsatisfied. MLL-Baby protocol was developed for infant acute lymphoblastic leukemia (ALL). In this treatment approach conventional chemotherapy is augmented by administration of all-trans retinoic acid (ATRA). This treatment program is successfully applied for infant ALL within Russian Federation and Republic of Belarus (L. Fechina et al, ASH 2007 #2828). Minimal residual disease (MRD) is a strong tool for risk-adapted treatment. Majority of infants carry MLL rearrangements, so in this group MRD monitoring by quantification of fusion gene transcripts (FGt) is fast, easy and cost-effective approach.

Objective.
To evaluate the prognostic significance of MRD monitoring by FGt measurements in MLL-rearranged infant ALL, enrolled into MLL-Baby study.

Methods.
Twenty three infants with defined MLL translocation partner genes who received at least two ATRA courses were included in the current study. Median of follow-up period in the observed group was 41 months. Presence of MLL rearrangements was detected by FISH and confirmed by long-distance inverse PCR (C. Meyer et al, 2005). MRD detection in bone marrow (BM) was performed by real-time quantitative PCR and qualitative nested reverse-transcriptase PCR as previously described (A. Borkhardt et al.,1994, J. van Dongen et al., 1999, N. Palisgaard et al., 1998, J. Gabert et al, 2003) with sensitivity 1E–05. MRD-negativity was defined as absence of FGt in both assays. Among 23 infants there were 13 MLL-AF4-positive patients (pts), 4 MLL-MLLT10-positive pts, 3 MLL-EPS15-positive pts, 2 MLL-MLLT1-positive pts and one MLL-MLLT3-positive patient. BM samples were obtained at the time of diagnosis, on day 15 of remission induction (time point (TP) 1), at the end of remission induction (TP2) and after each course of ATRA administration (TP3-TP9). Informed consent was obtained in all cases.

Results.
All patients were MRD-positive at TP1. At TP2 two MLL-MLLT10-positive patients became MRD-negative. At TP3 other 4 pts (3 MLL-AF4-positive and 1 MLL-MLLT1-positive) converted to MRD-negativity. By TP4 18 pts were MRD-negative, while FGt were detected in 5 pts. 2 pts became MRD-negative before protocol II (at TP9), while 3 pts never achieved MRD-negativity. Retrospectively, we compared prognostic significance of MRD at each TP. TP4 was the earliest TP when discriminative data was obtained. According to the qualitative MRD results at this TP pts were divided into MRD-positive and MRD-negative groups. The first group consisted of 18 pts with different MLL translocation partner genes, while the second group included 5 MLL-AF4-positive pts, who remained MRD-positive at TP4. Groups did not differ in age at diagnosis, sex distribution, initial WBC count, immunophenotype, type of MLL partner gene, number of blast cells at day 8 of dexamethasone prophase, BM status on day 15, CNS disease, and achievement of hematological remission at day 36. Number of relapses was significantly higher in the second group (p=0.017). Odds ratio was 20.00 with 95% CI 1.61–247.99. In the first group there were 3 relapses (in one MLL-AF4-positive case and two MLL-EPS15-positive cases) while in the second group 4 relapses occurred. Cumulative incidence of relapse for pts who achieved MRD-negativity by TP4 was 0.17, for MRD-positive pts 0.80 (p=0.005). 7-years event-free survival in the first group was 0.82±0,09, in the second group 0.20±0.17 (p=0.008) (fig 1).

Conclusions.
MRD monitoring by FGt measurements has significant prognostic value in infants with MLL-rearranged ALL treated by MLL-Baby protocol. In our series achievement of MRD-negativity by TP4 corresponds to favorable outcome in infant ALL with MLL rearrangements treated by MLL-Baby protocol. Persistence of MRD-positivity at TP4 allows to define group with high incidence of relapse.

Disclosures:
No relevant conflicts of interest to declare.