The findings relating to nSMase2 regulation of the skeleton took on greater significance when, in an independent study, a mouse model of osteogenesis and dentinogensis imperfecta (the fro/fro mouse) was attributed to a mutation of the SMPD3 gene (Aubin et al., 2005). The phenotype of the fro/fro mouse is characterized by smaller size at birth, multiple fractures in long bones and ribs, with severely undermineralized bones but normal cartilage growth. Moreover, tooth and alveolar bone abnormalities linked to hypomineralization were also observed. In 2005, the partial deletion of intron 8 and exon 9 in the smpd3 gene was identified in the fro/fro mouse. This mutation results in the deletion of the C-terminal 33 amino acids of the nSMase2 protein. Importantly, this includes the crucial catalytic residues Asp638 and His639 which thus renders the fro/fro nSMase2 catalytically inactive. As with the nSMase2 KO mouse, only residual N-SMase activity (~10%) was detected in the brain and fibroblasts of the fro/fro mouse, further emphasizing its importance as a physiological N-SMase. Taken together, this suggests that the inactivation of nSMase2 may be at least partly responsible for fro/fro mouse phenotype.
