A father and his son with systemic AL amyloidosis.

criminate between driver and passenger mutations, as was also shown recently in the context of FLT3. In the MOLT-4 and RPMI-8402 cell lines, we identified another transcript variant, which lacks exons 7 and 8 (nucleotides 648-1178). This results in a shift in the open reading frame with generation of a premature stop codon in exon 9. Consequently, a truncated form of JAK1 is expressed in these cell lines, lacking the entire kinase, pseudokinase and SH2 domain as well as part of the FERM domain (Figure 1G, 1H). Due to the absence of a kinase domain, no further functional studies were performed on this variant. To investigate if the 18 T-ALL cell lines were dependent on JAK1 signaling for their proliferation and survival, we examined the effect of treatment with a small molecule JAK inhibitor (JAK inhibitor I, Calbiochem, San Diego, CA, USA). This compound inhibited JAK1 autophosphorylation in control cells with an IC50 value of 100 nM (data not shown). The majority of T-ALL cell lines were completely insensitive to treatment with this inhibitor (IC50 values ≥10 μM), while other T-ALL cell lines (ALL-SIL, SUP-T1, DND-41, TALL-1) displayed an increased sensitivity with IC50 values below or around 1 μM. In none of the T-ALL cell lines was proliferation completely inhibited at a concentration of 10 μM of the JAK inhibitor (data not shown). Despite the presence of JAK1 variants in the cell lines HPB-ALL, MOLT-4 and RPMI-8402, these cell lines were not more sensitive to JAK1 inhibition, confirming that the observed variants were unlikely to contribute to JAK1 activation. As a final experiment, we knocked down JAK1 expression in the HPB-ALL and RPMI-8402 cell lines using a JAK1 siRNA (Validated Stealth siRNA, Invitrogen, Carlsbad, CA, USA), which again confirmed that these T-ALL cell lines were not dependent on JAK1 expression for their survival and proliferation (data not shown). Flex et al. reported association of hyperactive JAK1 mutants with advanced age at diagnosis in ALL patients. Unfortunately, we were not able to identify a T-ALL cell line with an activating mutation in JAK1 that could be used as a T-cell model to study JAK1 signaling and study the effect of JAK1 inhibition. As most of the cell lines we tested correspond to samples from childhood/adolescent T-ALL, the results obtained in this study are in line with the data presented by Flex et al. Furthermore, our observation that concentrations in the range of 1 μM or more of a JAK kinase inhibitor are required to inhibit T-ALL cell lines indicate that T-ALL cell lines in general are not critically dependent on JAK1 activity for their proliferation and survival. Michaël Porcu, Olga Gielen, Jan Cools, Kim De Keersmaecker VIB Department of Molecular and Developmental Genetics, VIB, Leuven; Center for Human Genetics, K.U. Leuven, Leuven, Belgium