Key Points Intragenic PAX5 amplification defines a novel, relapse-prone subtype of B-cell precursor acute lymphoblastic leukemia with a poor outcome.

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

Highlights CRLF2 overexpression associates with IKZF1 deletions that lead to a dominant-negative effect and with IKZF1 plus. Paediatric patients with a high load expression of IK4 isoform presented higher CRLF2 transcript levels. CRLF2 overexpression and IKZF1 deletions conferred poorer prognosis both to paediatric patients treated with RELLA05 protocol as well as to adult patients.

Chronic myeloid leukemia (CML) is a rare disease in children. Different from that in adults, childhood CML involves transformative events occurring over a short time period. CML transformation to lymphoid blast phase (BP) is associated with copy number abnormalities, characteristic of BCR‐ABL1 positive acute lymphoblastic leukemia, but not of CML in the chronic phase. Here, we present an unusual case of CML progressing to BP in a 1.6‐year‐old child, harboring IKZF1, PAX5, CDKN2A, and ETV6 deletions at diagnosis. It remains to be addressed whether distinct mechanisms might account for CML pathogenesis in early childhood.

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