muscle atrophy

Sandhoff Disease

Clinical Characteristics
Ocular Features: 

Retinal ganglion cells are rendered dysfunctional from the toxic accumulation of intra-lysosomal GM2 ganglioside molecules causing early visual symptoms.  These cells in high density around the fovea centralis create a grayish-white appearance.  Since ganglion cells are absent in the foveolar region, this area retains the normal reddish appearance, producing the cherry-red spot.  Axonal decay and loss of the ganglion cells leads to optic atrophy and blindness. 

Systemic Features: 

Sandhoff disease may be clinically indistinguishable from Tay-Sachs disease even though the same enzyme is defective (albeit in separate subunits A and B that together comprise the functional enzymes).  The presence of hepatosplenomegaly in Sandoff disease may be distinguishing. The infantile form of this lysosomal storage disease seems to be the most severe.  Infants appear to be normal until about 3-6 months of age when neurological development slows and muscles become weak.  Seizures, loss of interest, and progressive paralysis begin after this together with loss of vision and hearing.  An exaggerated startle response is considered an early and helpful sign in the diagnosis.  Among infants with early onset disease, death usually occurs by 3 or 4 years of age.   

Ataxia with spinocerebellar degeneration, motor neuron disease, dementia, and progressive dystonia are more common in individuals with later onset of neurodegeneration.  The juvenile and adult-onset forms of the disease also progress more slowly.  

Genetics

Sandhoff disease results from mutations in the beta subunit of the hexosaminidase A and B enzymes.  It is an autosomal recessive disorder caused by mutations in HEXB (5q13). 

Tay-Sachs disease (272800) can be clinically indistinguishable from Sandoff disease and they are allelic disorders.  However, the mutation in Tay-Sachs (272800) is in HEXA resulting in dysfunction of the alpha subunit of hexosaminidase A enzyme. 

Pedigree: 
Autosomal recessive
Treatment
Treatment Options: 

No specific treatment is available beyond general support with proper nutrition and maintainence of airways.  Anticonvulsants may be helpful in some stages.  Gene therapy in fibroblast cultures has achieved some restoration of  hexosaminidase A activity in Tay-Sachs disease and may have potential in Sandhoff disease as well. 

References
Article Title: 

Charcot-Marie-Tooth Disease with Glaucoma

Clinical Characteristics
Ocular Features: 

Optic atrophy can be an ocular manifestation of CMT disease, especially in the X-linked forms, but this variant is the only one in which early-onset glaucoma is a feature.  It may begin at birth in some patients who have features of congenital glaucoma such as buphthalmos, while in other family members, including juveniles, only elevated intraocular pressures were reported.  Optic nerve damage seems to occur rapidly.

Systemic Features: 

This is a sensorineural disease of myelination that causes a polyneuropathy with muscular weakness and sensory deficits.  CMT4B2 is characterized by abnormal myelin sheath folding.  Symptoms of lower limb weakness and evidence of muscle atrophy commonly appear in the middle of the first decade with progression to upper limb involvement.  Areflexia follows with development of pes cavus and hammertoes.  Motor nerve conduction velocities may be severely reduced and muscle biopsies show severe loss of myelinated fibers and focal myelin sheath folding.

Genetics

This seems to be an autosomal recessive disorder although only a few families have been reported.  Homozygous mutations in the SBF2 gene (sometimes called MTMR13) (11p15.4) were found in these CMT families with early-onset glaucoma (604563).  This gene codes for SET binding factor 2 important to the normal development of the trabecular meshwork.  Not all SBF2 mutations cause glaucoma though.  Of course, it is possible that the occurrence of glaucoma is incidental and not part of CMT4B2 at all.

A clinically similar neurological condition without glaucoma, CMT4B1 (601382), has been reported to be caused by a mutation in MTMR2 located at 11q22 (601382). 

Pedigree: 
Autosomal recessive
Treatment
Treatment Options: 

Little is known about the natural history of the glaucoma in this condition but it occurs early and severe visual loss seems to be common.  Early diagnosis and vigorous treatment are important.  The neurological disease requires a multidisciplinary approach with physical therapists, neurologists, orthopedic surgeons and the use of prostheses.

References
Article Title: 

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

Azzedine H, Bolino A, Taieb T, Birouk N, Di Duca M, Bouhouche A, Benamou S, Mrabet A, Hammadouche T, Chkili T, Gouider R, Ravazzolo R, Brice A, Laporte J, LeGuern E. 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. Am J Hum Genet. 2003 May;72(5):1141-53.

PubMed ID: 
12687498

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