Subclonality and prenatal origin of RAS mutations in KMT2A (MLL)‐rearranged infant acute lymphoblastic leukaemia

Abstract

Subclonality and prenatal origin of RAS mutations in KMT2A (MLL)-rearranged infant acute lymphoblastic leukaemia KMT2A (previously termed MLL) rearrangements (KMT2A-r) constitute the most frequent aberration in infant lymphoblastic leukaemia (iALL). Mutations in the RAS genes have previously been recognized as an important somatic abnormality in KMT2A-r-iALL (Taketani et al, 2004;Liang et al, 2006;Driessen et al, 2013;Prelle et al, 2013). Certainly, KMT2A-r in iALL are present at birth (Greaves et al, 2003). On the other hand, RAS mutations (RASmut) are suggested to be secondary and subclonal hits in the leukaemogenic pathway.In this context, our study aimed to (i) quantitatively evaluate the association between RASmut at diagnosis and the main clinical features of KMT2A-r-iALL, and (ii) analyse the mutations in matched samples [neonatal blood spots, also known as Guthrie cards (Gc), remission and/or relapse]. KRAS/NRASmut were screened by direct sequencing, and mutated cases were also subjected to pyrosequencing in order to quantify the percentage of blasts harbouring the mutation. The detailed methods performed in this study are described in the Supporting Information (Data S1).Ninety-two KMT2A-r-iALL cases were included. RASmut were found in 21Á7% of cases [KRAS (14Á1%) and NRAS (7Á6%)]. In childhood ALL, RASmut have been diagnosed in 6-21% of patients (Liang et al, 2006;Wiemels et al, 2010). Recent studies in large cohorts of KMT2A-r cases have reported frequencies of 14-15% RASmut, with an even higher frequency in t(4;11) infants (c. 25%) (Driessen et al, 2013;Prelle et al, 2013). These previous data are therefore concordant with our study.The clinical-laboratory features of KMT2A-r-iALL according to RAS status are shown in Table SI. Sixty percent of RASmut patients were ≤6 months old, 60% harboured AFF1 as the KMT2A-translocation partner gene (TPG), and 80% presented both the KMT2A-AFF1 and AFF1-KMT2A transcripts at mRNA level. Although these differences were not statistically significant, these analyses are in agreement with previous data, such as a higher frequency of RASmut among younger and t(4;11) infants (Driessen et al, 2013;Prelle et al, 2013). Furthermore, KRAS (P = 0Á033) but not NRAS (P = 0Á415) mutations were associated with high white blood cell (WBC) counts.The quantitative association between RASmut at diagnosis and clinical features was analysed by pyrosequencing the mutated clones. The median percentage of mutated clones did not differ according to age group (31% vs. 27%; Fig S1A; P = 0Á43) or KMT2A-TPG (28% vs. 33%; Fig S1C; P = 0Á89). Significantly higher percentages of mutated clones were found in cases with WBC counts >300 x 10

Involvement of the central nervous system in acute lymphoblastic leukemia: opinions on molecular mechanisms and clinical implications based on recent data

“…The CNS is involved in about 5% of MLLr BCP-ALL patients [66]. MLLr-ALL have frequent mutations in the RAS pathway [67][68][69], which as previously discussed was shown to be linked with an increased risk of CNS infiltration [44]. Furthermore, KRAS activation was shown to promote CNS infiltration of MLLr cells harboring the MLL-AF4 fusion gene [70].…”

Deciphering KRAS and NRAS mutated clone dynamics in MLL-AF4 paediatric leukaemia by ultra deep sequencing analysis

“…The capability of the MLL-AF4 fusion gene to sustain leukaemogenesis without additional genetic alterations is still not completely clear. Several works showed that BCP MLL-AF4 rearranged leukaemia is characterized by a very low rate of genetic alteration 13 14 15 and using Sanger sequencing a significant amount of data on the mutational status of RAS genes in infants and paediatric patients with MLL-AF4 leukaemia were previously obtained 23 35 36 37 . However, the percentage of RAS mutated patients showed a certain inconsistency among these studies ranging from 25 to 50% 23 35 36 37 .…”

MEK inhibition is a promising therapeutic strategy for MLL-rearranged infant acute lymphoblastic leukemia patients carrying RAS mutations

“…suggested that RAS mutations in MLL -rearranged infant ALL may not act as driver mutations and are not required for disease progression, but rather act only at disease onset [4]. Yet, our previous data clearly showed that RAS -mutant MLL -rearranged infant ALL patients are at extremely high risk of therapy failure and early death.…”