In this issue of the
JCI, Wartman and colleagues used massive parallel DNA sequencing in an effort to perform systematic mutational analysis of the murine APL genome (
10). Until recently, whole genome sequencing of primary murine and human tumors was not feasible due to cost and the requirement for large amounts of tumor material. However, these limitations have largely been overcome due to improved sequencing technology and analytic tools (
11). Indeed, previous efforts by the current investigators using whole genome sequencing in human patients with acute myelogenous leukemia (AML) has allowed them to identify novel clinically and biologically relevant AML mutations, demonstrating the power of massive parallel sequencing (
11,
12).
Wartman et al. used an innovative strategy to find additional mutations in this APL model (
10). An inbred mouse strain was used in an attempt to reduce the number of variants, as many of the variants found in sequencing a murine genome may not be relevant to disease pathogenesis. Mice expressing the
PML-RARA transgene under the control of the murine cathepsin G promoter were backcrossed to the Black 6/Taconic background for 10 generations. These mice developed an APL-like disease with a relatively long latency (9–12 months), suggesting the acquisition of additional genetic events is required for APL development in this model. In previous human studies, tumor whole genome sequencing data were compared with sequencing data from matched germline DNA to assess whether candidate mutations were present in the germline or were bona fide somatic mutations acquired during tumorigenesis. In contrast, here the authors compared the spectrum of single nucleotide variants present in murine APL cells with a sequenced genome of the initial mouse strain. An alternative, and perhaps more discriminating, strategy might have compared the APL mutational data with DNA from littermate controls or compared the murine APL genome with hematopoietic DNA from the same mouse from an earlier time point, before APL development.
Six nonsynonymous mutations were identified and validated as being present in the APL genome; the authors then performed secondary mutational analysis of 89 additional mouse APL samples for these 6 mutations. Importantly, this approach allowed them to identify that one mutation,
Jak1 V658F, was present as a recurrent alteration in murine APL. Of note, the
Jak1 V658F mutation occurs at the homologous position to JAK2 V617, which is commonly mutated in patients with myeloproliferative neoplasms (MPNs) (
13), and has been observed previously in patients with high-risk acute lymphoblastic leukemia (ALL) (
14). They then ectopically expressed Jak1 V658F in mCG-PML-RARA bone marrow, followed by transplantation into lethally irradiated recipients, which resulted in a short latency, completely penetrant APL phenotype. In addition, they also performed high-resolution copy number analysis of the murine tumors and identified a somatic deletion of a histone demethylase, lysine (K)-specific demethylase 6A (
Kdm6a, also known as
Utx), in the same murine APL genome, a deletion also observed in human APL. However, its functional contribution to APL pathogenesis and to APL development in this mouse model has not been elucidated.
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