Survival Outcomes in Patients with FLT3-Itd Positive Acute Myeloid Leukaemia Relative to Biological Stratification: Smaller FLT3-Itd Clone Size Predict Better Overall Survival

FLT3 internal tandem duplication (itd) mutations are found in 25% of adult patients with acute myeloid leukaemia (AML) and are associated with an adverse prognosis. This mutation results in constitutive activation of downstream pathways.

Distinct biological subgroups can be identified based on FLT3-itd mutation type: clones with heterozygous FLT3-itd mutated/wildtype; homozygous mutated FLT3 allele; FLT3 heterozygous biallelic mutant and clones with evidence of overexpression of FLT3-itd. There is evidence to suggest that these mechanisms are important in the clonal evolution of AML cells. We sought to investigate their clinical impact.

Itd mutation analysis within exon 14 and/or exon 15 of the FLT3 gene was carried out at diagnosis by PCR of genomic DNA (gDNA) and cDNA for all new AML referrals in the region over 8 years. PCR products were identified and sized using fluorescent based fragment analysis. Allelic ratio (AR) and expression ratio (ER) (mutation: wild type ratio in gDNA and cDNA respectively) was determined from the relative peak heights. High relative expression was defined as 10 fold difference of ER/AR. Copy neutral loss of heterozygosity for the wild type allele resulting in homozygosity of the FLT3-itd mutation (acquired isodisomy (AID)) was determined by analysis of microsatellite markers along chromosome 13. Overall survival (OS) (time of diagnosis to death), event free survival (EFS) (time from diagnosis to induction failure, relapse or death) was calculated. Survival rates were estimated by the Kaplan-Meier method. Differences between the survival distributions were compared with the log-rank test.

177 patients positive for the FLT3-itd mutation were identified. Median follow-up for patients alive was 3.4 years. A separate group of 49 patients tested negative for this mutation during this period had better OS and EFS (p=0.02) compared to the patients who were FLT3-itd positive (median survival 1715 and 307 days respectively).

Patients who were FLT3-itd positive had statistically significant (p<0.05) differences in outcomes based on age, presenting white cell count, treatment intensity and cytogenetic risk. The characteristics of this group of patients are described below.

Patients with lower AR (less than/equal to 0.3) as compared to higher AR (greater than 0.3) had an improved OS (p=0.018) and EFS (p=0.02). The impact of AR on OS had borderline significance (p=0.05) when only patients treated with intensive chemotherapy were considered.

AID provides true evidence of FLT3 mutant homozygosity and was detected in 15 (142 tested) patients. In 38 patients who relapsed and had samples at these stages, 5 had developed AID, but were heterozygous (mutant/wildtype) at diagnosis. 6 patients with multiple FLT3-itd products may comprise patients who have multiple different mutant/wildtype clones but may include patients with a second, independent FLT3-itd mutation resulting in biallelic heterozygous mutations, although this cannot be confirmed. A high relative expression level of FLT3-itd was seen in 12 patients. The clinical significance of these findings is uncertain due to the small numbers.

The impact of FLT3-itd mutation and other known prognostic factors has been confirmed in a heterogeneous, real life cohort of patients.

An AR over 1 provides firm evidence of loss of the wild type allele (9 patients). AID also occurs at an AR of less than 1 because of the presence of normal cells or due to preferential amplification of the wildtype allele. Direct testing for AID is a more sensitive measure of FLT3 mutant homozygosity (detected in 14% of tested patients). The development of AID in patients who relapse may be an important mechanism by which an AML clone gains a further advantage.

Lower AR was associated with improved survival. In the context of a high blast count (median bone marrow blast 80%), this implies having a small sub-clone of FLT3-itd positive cells is advantageous to having a larger FLT3-itd clone in their population of AML cells.

No relevant conflicts of interest to declare.