Response: hematopoietic defects in 12/15-lipoxygenase–deficient mice

Based on the comments and data from Taylor et al,¹  we conclude that the various 12/15-lipoxygenase–deficient (Alox15) mice exhibit more similarities than differences.

The majority of Alox15 mice in our colony and those we purchased from The Jackson Laboratory appeared healthy although they had disruptions in hematopoietic stem cell (HSC) function and alterations in numbers of progenitors.^2,3  We initially referred to these as having “chronic” myeloproliferative disease (MPD)¹  and subsequently as “asymptomatic.”^2,3  Only 15% of Alox15 mice developed severe MPD and did so at ∼ 10-12 months (Figure 1A in Middleton et al⁴ ), not at 2-3 months as Taylor et al indicated.¹  We initially referred to these mice as “in crisis” because we could adoptively transfer disease to wild-type mice.⁴ 

Taylor et al imply that the phenotype we reported was unique to our colony,¹  but this is not the case. We obtained similar results with mice from The Jackson Laboratory after housing them in our facility.^2-4  Furthermore, in 2007 The Jackson Laboratory provided confirmation of the phenotype: “We examined spleens from 3 males and 3 females at 12 weeks of age of both mutant and wild-type mice, a similar number of mutant retired breeders and twice as many retired wild-type animals. At both ages, splenic weights of mutants were highly significantly heavier… . We also … found differences in the proportions of cell types.” (Anthony Nicholson, University of Massachusetts, personal communication). Dr Shaoguang Li also studied Alox15 mice at The Jackson Laboratory and has since established a colony at the University of Massachusetts. These mice also have increased splenic weights (2- to 3-fold), disruptions in blood and bone marrow composition, and defective HSC function (A. Nicholson, S. Lin, personal communication, March/April 2012).²  Therefore, there are in fact no major discrepancies between the “asymptomatic” mice we studied and The Jackson Laboratory and University of Massachusetts colonies.

Alox15 colonies described by Taylor et al share many of these features: (1) mortality rates in Berlin were similar, although it was not indicated whether mortality was associated with MPD; and (2) they report splenomegaly is not apparent at 2 months, consistent with our data showing splenomegaly developed to varying degrees in mice beginning at 8 weeks. Splenic weights in our 10- to 12-week-old asymptomatic mice (up to 0.2-0.3 g) were similar to other colonies (up to 0.29 g) at 7 months (and perhaps earlier as only data for 2 and 7 months were provided).

The use of different methods may explain the apparent disparities with regard to the hematopoietic defects. Furthermore, loss of compartmentalization in spleen was seen only in moribund mice. Alterations in splenic architecture in asymptomatic mice, which we showed were due to increased erythropoiesis,²  were more subtle. It will be of interest to determine whether splenomegaly in the other colonies is due to similar changes.

A small percentage of our mice developed progressive MPD. Thus, further studies are required to determine whether disease progression is recapitulated in a subset of mice in other colonies once sufficient numbers of mice in other colonies are observed to 1 year and are subjected to similar analyses. If disease progression is unique to mice in our facility, it might suggest that environmental or infectious agents can play a role in the progression to the more severe phenotype in a subset of mice in the context of 12/15-lipoxygenase deficiency. It is important to stress, however, that the defects we reported in the majority of Alox15 mice that are asymptomatic do not in our opinion appear to be unique to the Wistar colony, and these mice remain a valuable tool for defining the role of 12/15-lipoxygenase in hematopoiesis.