Although the signal was not of genome-wide significance due to the small number of cases, the <i>SBF2</i> (also known as <i>MTMR13)</i> gene within the region of shared case homozygosity was a strong positional candidate, as 22 genetic variants in the gene have been associated with demyelinating forms of Charcot-Marie-Tooth disease in humans.
CMT4B1 patients had significantly more-severe disease than CMT4B2, with earlier onset, more-frequent motor milestones delay, wheelchair use, and respiratory involvement as well as worse MRC scores and motor CMT Examination Score components despite younger age at examination.
The fact that this mutation is found in SBF2 gene may indicate that the underlying cause of thrombocytopenia in our patient is either a new variant form of Griscelli syndrome (through the Rab GTPases action) or a variant Charcot-Marie-Tooth type 4 disease as SBF2 truncating mutations were previously identified in sufferers of this disease.
The PtdIns3P and PtdIns(3,5)P(2) 3-phosphatase myotubularin gene is mutated in X-linked centronuclear myopathy, whereas its homologs MTMR2 and MTMR13 and the PtdIns(3,5)P(2) 5-phosphatase SAC3/FIG4 are implicated in Charcot-Marie-Tooth peripheral neuropathies.
Loss of either MTMR2 or MTMR13 causes Charcot-Marie-Tooth type 4B1 and B2 neuropathy, respectively, characterized by demyelination and redundant loops of myelin known as myelin outfoldings.
Mutations in myotubularin-related protein-2 (MTMR2) or MTMR13/set-binding factor-2 (SBF2) genes are responsible for the severe autosomal recessive hereditary neuropathies, Charcot-Marie-Tooth disease (CMT) types 4B1 and 4B2, both characterized by reduced nerve conduction velocities, focally folded myelin sheaths and demyelination.
The phosphoinositide-3-phosphatase MTMR2 associates with MTMR13, a membrane-associated pseudophosphatase also mutated in type 4B Charcot-Marie-Tooth disease.
The authors report a Japanese family segregating autosomal recessive Charcot-Marie-Tooth disease (CMT) with focally folded myelin, juvenile-onset glaucoma, and a nonsense mutation of SET binding factor 2 (SBF2).
Mutations in MTMR13, a new pseudophosphatase homologue of MTMR2 and Sbf1, in two families with an autosomal recessive demyelinating form of Charcot-Marie-Tooth disease associated with early-onset glaucoma.
SBF2 is a member of the pseudo-phosphatase branch of myotubularins and was an obvious candidate for CMT4B2 by virtue of its striking homology to myotubularin-related protein 2 (MTMR2), causing another form of autosomal recessive CMT with outfoldings of the myelin sheaths.
Mutations in MTMR13, a new pseudophosphatase homologue of MTMR2 and Sbf1, in two families with an autosomal recessive demyelinating form of Charcot-Marie-Tooth disease associated with early-onset glaucoma.