Variable Phenotypes of Knockin Mice Carrying the M712T Gne Mutation

Abstract

GNE myopathy is a recessive adult onset, slowly progressive distal and proximal myopathy, caused by mutations in the GNE gene. The most frequent mutation in GNE myopathy patients is the Middle Eastern founder mutation M712T. We have generated Gne (M712T/M712T) knockin mice. A high mortality rate in the first generation due to renal failure was recorded (as previously described). However, the following Gne (M712T/M712T) offspring generations could be classified into 3 phenotypic categories: severe, mild and without apparent phenotype. By further crossing between mice with no apparent phenotype, we were able to establish a colony of Gne (M712T/M712T) knockin mice with a high- and long-term survival rate, lacking any renal phenotype. These mice did not present any muscle phenotype (clinical or pathological) for up to 18 months. No correlation was found between the expression of any of the two mRNA Gne isoforms in muscle and the mouse genotype or phenotype. However, the expression of isoform 2 mRNA was significantly higher in the kidney of Gne (M712T/M712T) kidney affected mice compared with control. In contrast, the expression of UPR markers Bip, Chop and of the spliced form of XBP1, was upregulated in muscle of Gne (M712T/M712T) mice compared with controls, but was unchanged in the affected kidney. Thus, Gne defects can affect both muscle and kidney in mouse, but probably through different mechanisms.

GNE myopathy: current update and future therapy

“…However, most mice died with 72 h after birth due to renal disease and showed no myopathic phenotype; ManNAc administration rescued the neonatal lethal phenotype in these mice 42. Similar results were obtained in other laboratories 43. Interestingly, the Gne M712T knock-in model developed by the Jerusalem group had a different phenotype.…”

“…Interestingly, the Gne M712T knock-in model developed by the Jerusalem group had a different phenotype. In some animals, no renal disease was observed and animals survived more than 1 year without any therapy 43. Those that died at a later age did not show muscle abnormalities.…”

Activation of the Unfolded Protein Response in Sporadic Inclusion-Body Myositis but Not in HereditaryGNEInclusion-Body Myopathy

“…The upregulation of the UPR markers in the Gne M712T/M712T knock-in mice (49), and in our experiment involving pharmacological induction of the ER-stress in our GNE-h-IBM CHMFs, confirm that myofibers with a mutated kinase domain of GNE are able to respond to the ER-stress insult; thus, those mutations do not impair UPR signaling. Accordingly, in the muscle biopsies of patients with those mutations there might be an absence of the ER-stress providing the signal for the UPR induction, or sensing of it might be diminished or insufficient.…”

“…In gastrocnemius muscle of Gne M712T/M712T knock-in mice, mRNA of the markers of UPR activation GRP78, CHOP and spliced XBP-1 was increased (49). Because the mice did not display any pathological muscle phenotype (in contrast to humans bearing the same GNE mutation), the phenomenon has been postulated to be related to a protective role of the ER-stress-induced UPR in muscle; it is possibly relevant that in severely affected kidneys of those mice, UPR activation has not been detected (49). …”

Substantial deficiency of free sialic acid in muscles of patients with GNE myopathy and in a mouse model

“…These mice, when maintained in B6 background, have been shown to develop severe kidney abnormalities and die at an early postnatal stage. However, the Gne M743T/M743T mice are viable when maintained on a mixed FVB;B6 background [33]. ).…”