CBL mutations in infant acute lymphoblastic leukaemia

Infant acute lymphoblastic leukaemia (ALL) is relatively rare, occurring in approximately 2Æ5–5% of cases of childhood ALL (Biondi et al, 2000). Infant ALLs are much more likely to present with high leucocyte counts, hepatosplenomegaly and overt central nervous system (CNS) diseases (Taki et al, 1996). T cell phenotype is much less common in infants, while myeloid antigen co-expression and the absence of CD10 expression are more frequent in infants than in older children with ALL. When molecular techniques [such as fluorescence in situ hybridization (FISH) or Southern blot analysis] are used in addition to karyotype, MLL gene rearrangements (MLL-R) are found in 70–80% of infant ALL compared with only 2–4% of older children with ALL (Taki et al, 1996; Biondi et al, 2000). Thus, infant ALL appears to be biologically distinct from the disease in older children (more than 1 year old). In this regard, recent reports of somatic mutations of the CBL proto-oncogene in myeloid neoplasms are intriguing, because these CBL mutations were shown to result in aberrant tyrosine kinase signalling, which also leads to activation of RAS signalling pathways. So far, we and others have reported that CBL mutations occur in a variety of myeloid neoplasms, including de novo acute myeloid leukaemia (AML) (Caligiuri et al, 2007), myelodysplastic syndrome (MDS), and myeloproliferative neoplasm, especially in chronic myelomonocytic leukaemia (CMML) (Sanada et al, 2009), and juvenile myelomonocytic leukaemia (JMML) (Shiba et al, 2010). The importance of CBL mutations regarding leukaemogenesis is substantially increased. Recently, we found CBL mutation in therapy-related AML with MLL-R (Shiba et al, 2011). Interestingly, the MLL-CBL fusion gene has been reported in a de novo AML case (Fu et al, 2003), and this prompted us to search for possible CBL mutations in infant ALL with MLL-R. Because CBL mutations thus far reported were almost all clustered within exons 8–9 that encode Linker/RING finger domains (Caligiuri et al, 2007; Sanada et al, 2009; Shiba et al, 2010), we confined our mutation analysis to these exons, in which polymerase chain reaction-amplified exons 8–9 were subjected to direct sequencing using an ABI PRISM 310 Genetic Analyser (Applied Biosystems, Branchburg, NJ, USA). The study adhered to the principles of the Helsinki Declaration, and was conducted under the regulations enacted by the Ethics Board of Gunma Children’s Medical Centre. CBL gene analysis was performed in 41 infant ALL patients in which MLL-R was found in 33 patients (80Æ5%), including 15 patients with t(4;11)(q21;q23), 4 with t(9;11)(p22;q23) and 5 with t(11;19)(q23;p13.3). Median age at diagnosis was 4Æ7 months (range, 0–12 months). We also performed CBL gene mutation analysis in 28 B cell precursor (BCP)-ALL patients (age range, 1–14 years). Heterozygous mutations of the CBL gene were identified in 2 (4Æ9%) of 41 infant ALL patients, but not in older children with BCP-ALL. These were located in exon 8 (Fig 1). One patient was a 3-month-old female with t(4;11)(q21;q23) and the other patient was a 6-month-old male with t(11;19)(q23;p13.3). They were registered and treated on two Japanese infant leukaemia protocols, MLL96 and MLL98 respectively (Isoyama et al, 2002; Kosaka et al, 2004). Although strong association between CBL mutations and 11q-acquired uniparental disomy (aUPD) has been reported (Sanada et al, 2009), we did not perform the single nucleotide polymorphism array analysis due to lack of DNA. MLL-R are more frequent in younger infants; up to 90% of infant ALL less than 6 months old at diagnosis have detectable MLL-R compared with 30–50% of infant ALL aged 6–12 months (Taki et al, 1996). MLL-R ALL has a characteristic gene expression profile that significantly differs from that of non-MLL-R BCP-ALL and of AML, confirming that MLL-R ALL is a biologically unique leukaemia subtype.