Timing is everything: an inducible model for neurofibromatosis type 2 in zebrafish

Neurofibromatosis type 2 (NF-2) is an autosomal-dominant disorder caused by variants in the NF2 tumor suppressor gene. The disorder is often associated with benign tumors in the nervous system, including vestibular schwannomas, meningiomas and ependymomas. About half of NF-2 patients have de novo variants, and many of these cases exhibit mosaicism. Clinically, NF-2 is managed with surgery, chemotherapy and radiation therapy as the primary treatments. However, patients often suffer from multiple, sometimes inoperable, tumors and may have to undergo multiple surgeries over their lifetime. Exploring the molecular pathways underlying the pathogenesis of NF-2 might help to establish more efficient targeted therapies. To accomplish this, it is essential to model the disease in systems that enable experimental manipulation and drug screening.

So far, however, the research community has faced a lack of suitable animal models that accurately replicate the full spectrum of human NF-2. In this issue of DMM, Rajan et al. present a novel genetic model of NF-2, leveraging the advantages of zebrafish. Their transparency, ex utero development, rapid growth, suitability for generating stable transgenic lines and relatively low-cost husbandry make zebrafish an excellent model for studying disease mechanisms. In the fish, the homolog of human NF2 is nf2a/b. The authors first demonstrated that zebrafish nf2a and nf2b paralogs are expressed in the neural crest, cranial mesenchyme, meninges, melanoblasts and Schwann cells during early development, as well as in adult Schwann cell precursors, with nf2b emerging as the predominant paralog and nf2a exhibiting generally low expression levels.

Using CRISPR/Cas9, the authors then created a zebrafish line, in which ubiquitous double knockout of nf2a and nf2b can be achieved in an inducible manner at a desired time point through heat-shock-induced Cas9 expression. This elegant model allows researchers to circumvent the detrimental defects associated with nf2a/b disruption at very early developmental stages. Analysis of nf2a/b knockout embryonic fish revealed increased proliferation of meningeal cells, an elevated number of Schwann cells and hyperpigmentation – phenotypes that correspond with those observed in cell types affected in human NF-2 patients. To assess whether NF-2-associated benign tumors develop in this model, nf2a/b knockout was induced in adult fish, which indeed resulted in Schwannomas and meningiomas, along with cataracts and pigmentation changes. Thus, thanks to the inducibility of nf2a/b loss in the model described by Rajan et al., the molecular pathways and the cellular cell types and stages involved in the various features of NF-2 can be readily assessed at different timepoints of life. Furthermore, the model will enable the research community to not only gain new insights into disease mechanisms but also screen for therapeutic drugs for NF-2.