Peripheral neuropathy in a family with Sandhoff disease and SH3TC2 deficiency

OBJECTIVE: To report clinical findings of peripheral neuropathy in a family with Sandhoff disease and SH3TC2 deficiency.
 BACKGROUND: Sandhoff disease is a lysosomal storage disease caused by a deficiency of beta-hexosaminidase. Affected individuals present with a wide spectrum of clinical manifestations ranging from psychomotor impairment and death in the infantile form to motor neuron disease and autonomic dysfunction in the adult form. We present a family with predominantly sensory neuropathy that was found to have Sandhoff disease and deficiency of SH3TC2.
 DESIGN/METHODS: We assessed a 46-year-old male who experienced parethesia with loss of sensation in his hands and feet for thirty years. An evaluation of his 57 year old sister showed a similar pattern of sensory, motor, and autonomic involvement, with symptom onset in her 20’s. Whole exome sequencing was performed at the NIH Intramural Sequencing Center, National Human Genome Research Institute, and the National Institutes of Health.
 RESULTS: The two affected siblings were found to have a common mutation in both HEXB and SH3TC (P417L and R954X, respectively) in addition to being heterozygous for a 16kb deletion of the HEXB gene. The two unaffected siblings both had the SH3TC mutation though neither was heterozygous for both HEXB mutations. Elevated creatine, low total hexosaminidase activity, and increased fraction of HEXA activity were observed in the affected siblings. A nerve biopsy revealed evidence of a severe demyelinating neuropathy and a reduction in SH3TC2 staining.
 CONCLUSIONS: Although the P417L mutation is common in Sandhoff disease patients, the early and severe sensory findings are not a typical presentation of the disease. Our data suggest that the SH3TC2 deficiency is modifying the HEXB phenotype. The identification of a family with mutations in both SH3TC2 and HEXB helps in our understanding of the biological processes that may be common to these two diseases.
 Study Supported by: Disclosure: Dr. Bakar has nothing to disclose. Dr. Lanman has nothing to disclose. Dr. Grunseich has nothing to disclose. Dr. Chen has nothing to disclose. Dr. Schindler has nothing to disclose. Dr. Mankodi has nothing to disclose. Dr. Traslavina has nothing to disclose. Dr. Lehky has nothing to disclose. Dr. Baker has nothing to disclose. Dr. Maragakis has received personal compensation for activities with Advanced Technologies and Regenerative Medicine LLC. Dr. Tifft has nothing to disclose. Dr. Fischbeck has nothing to disclose.


Dear Sirs,
Sandhoff disease is a lysosomal storage disease caused by a deficiency of beta-hexosaminidase. Affected individuals present with a wide spectrum of clinical manifestations, ranging from psychomotor impairment and death in the infantile form to motor neuron disease and autonomic dysfunction in the adult form [1,2]. Here, we present a family with predominantly sensory neuropathy that was found to have both Sandhoff disease and deficiency of SH3TC2.
A 46-year-old man had weakness, atrophy, and paresthesia with loss of sensation in his hands and feet for 30 years (Fig. 1a). He also had severe diarrhea several times per week, decreased sweating over his feet, and episodes of lightheadedness. On examination, he was found to have marked upper and lower extremity weakness and muscle atrophy distally, with the legs more severely affected (Fig. 1b). Although hyperreflexia was found in the upper extremities, reflexes in the lower extremities were reduced. Electromyography showed evidence of chronic neurogenic changes, and nerve conduction studies showed normal motor conduction velocities (patient 40-67 m/s, sibling 39-58 m/s). However, sensory nerve conduction studies were non-responsive in the proband and either nonresponsive or reduced in amplitude and velocity (41 m/s) in the sibling. Autonomic testing (QSWEAT, Heart Rate Response to Deep Breathing, Valsalva, Tilt Table) and quantitative sensory testing (Vibratory Detection Threshold and Cooling Detection Threshold) showed evidence of autonomic dysfunction consistent with a severe small unmyelinated fiber and large myelinated fiber sensory neuropathy ( Fig. 1d) [3]. Laboratory testing for acquired causes of neuropathy was negative. An evaluation of the proband's 57-year-old sister showed a similar pattern of sensory, motor, and autonomic involvement, with symptom onset in her 20s. Both individuals did not show evidence of macular abnormalities; however, mild splenomegaly was found in the sister (span 14.2 cm).
After negative genetic testing (Athena Diagnostics Inc.) of ten candidate neuropathy genes (PMP22, Cx32, MPZ, PMP22, EGR2, NFL, PRX, GDAP1, LITAF, MFN2), whole exome sequencing (NISC/NHGRI/NIH) of the proband identified a common 1250C[T mutation (P417L) in HEXB and a previously reported [4] 2860C[T mutation (R954X) in SH3TC2, which was confirmed by Sanger sequencing (GeneDx). Further testing through multiplex ligation-dependent probe amplification (MLPA, GeneDx) showed that the proband is also heterozygous for a 16-kb deletion that includes exons 1-5 of the HEXB gene. The three variants were also confirmed in the affected sister, and although two of the unaffected sisters were found to have the SH3TC2 mutation, neither have both HEXB mutations. MR spectroscopy of the superior cerebellar vermis detected elevated creatine in the proband (Fig. 1c). Serum testing in both affected siblings showed low total hexosaminidase activity of 1.4 and 1.0 nmol/min/mL (normal 10.4-23.8) and an increased fraction of HEXA activity at 91 and 100 % (normal 56-80 %). A sural nerve biopsy from the proband showed evidence of a severe mixed axonal and demyelinating neuropathy ( Fig. 1e-i) and a reduction in SH3TC2 immunoreactivity (Fig. 1j).
The adult form of beta-hexosaminidase deficiency is characterized by upper and lower motor neuron dysfunction, dysautonomia, and ataxia. The amino acid change in HEXB at position 417 is a common gene mutation in adult Sandhoff disease patients [5]. The early and severe presentation of sensory findings in both of these siblings is not a typical presentation of the disease [6-8]. The nonsense mutation observed in this family is predicted to reduce levels of SH3TC2, consistent with the pattern detected on peripheral nerve staining.
Given the unusual presentation with pronounced sensory neuropathy in this HEXB family and the evidence of myelin defects on nerve biopsy, not explained by Sandhoff disease alone, it is plausible that the deficiency in SH3TC2 may be modifying the HEXB phenotype. SH3TC2 is localized to the endosome, and disease-causing missense mutations mistarget the protein away from this location and result in a demyelinating neuropathy [9]. It is possible that the accumulation of GM2 ganglioside in HEXB-deficient endosomes and lysosomes may be preferentially damaging to tissues that also have a deficiency of SH3TC2.
Acknowledgments We thank Elizabeth Hartnett for her help with arranging the subject visits and evaluations and Dr. Andrea C. Corse for her help in acquiring tissue from Johns Hopkins University. This work was supported by intramural funding from the National Institute of Neurological Disorders and Stroke, NIH.

Conflicts of interest
The authors declare that they have no conflict of interest.
Ethical standard All human and animal studies have been approved by the appropriate ethics committee and have therefore been performed in accordance with the ethical standards laid down in the 1964 Declaration of Helsinki and its later amendments.
Informed consent All persons gave their informed consent prior to their inclusion in the study.
Open Access This article is distributed under the terms of the Creative Commons Attribution License which permits any use, distribution, and reproduction in any medium, provided the original author(s) and the source are credited.  Fig. 1 Family pedigree and phenotypical evaluations in the proband. a Family pedigree. The arrow indicates the proband. Three of the proband's sisters were also heterozygous for the SH3TC2 mutation, but only the two affected individuals had compound heterozygous mutations in HEXB and the SH3TC2 mutation. A paternal uncle was diagnosed with amyotrophic lateral sclerosis (ALS j Staining with an antibody for SH3TC2 shows a reduction in staining in the patient compared to control. Bar 50 lm for (f-j)