Lysosomal enzymes in ataxia: discovery of two new cases of late onset hexosaminidase A and B deficiency (adult Sandhoff disease) in French Canadians

A case refort of Sandhoff disease

Sandhoff disease is a rare autosomal recessive metabolic disease presenting bilateral optic atrophy and a cherry red spot in the macula. This case report presents the characteristics of a patient with Sandhoff disease as assessed by ophthalmic, neuroimaging, and laboratory procedures. Ophthalmologic examination revealed that the patient could not fixate her eyes on objects nor follow moving targets. A pale optic disc and a cherry red spot in the macula were seen in both eyes. Low signal intensity at the thalamus and high signal intensity at the cerebral white matter were noted in a T2-weighted brain MR image. A lysosomal enzyme assay using fibroblasts showed the marked reduction of both total beta-hexosaminidases, A and B. Based on the above clinical manifestations and laboratory findings, we diagnosed the patient as having Sandhoff disease.

Adult Sandhoff's disease: R505Q and I207V substitutions in the HEXB gene of the first Japanese case

We describe a 31-year-old Japanese man with adult Sandhoff s disease presenting as spinocerebellar degeneration. There was a marked cerebellar atrophy on MRI, and proliferation of abundant PAS-positive foamy macrophages in the rectal mucosa. The activities of total beta-Hex, beta-Hex A, and beta-Hex B in leucocytes of the patient were 14%, 15%, and 6% of control values, respectively. However, oligosacchariduria or ultrastructural storage materials in liver tissue were nil. Direct sequencing of cDNA and genomic DNA, and restriction digestion revealed two different homozygous base substitutions in the HEXB gene: the G1514-->A substitution (R505Q) and the A619-->G substitution (1207V). The parents were consanguineous. His healthy mother, an enzymatic heterozygous carrier, was homozygous for 1207V, but heterozygous for R505Q mutation. Thus, the patient is probably homozygous for both base substitutions and a R505Q mutation may be linked to the phenotype of adult Sandhoff's disease.

Early and severe sensory loss in three adult siblings with hexosaminidase A and B deficiency (Sandhoff disease)

Three siblings in their sixth and seventh decade with hexosaminidase A and B deficiency (adult form of GM2-gangliosidosis, variant O) developed early and severe sensory loss in addition to chronic motor neuron disease and cerebellar ataxia. Prominent mechanoallodynia was a manifesting symptom in two siblings. It is suggested that sensory deficits are due to a central-peripheral dying back axonopathy. The early and dominant sensory disturbances extend the clinical range of GM2-gangliosidosis.

Molecular basis of an adult form of Sandhoff disease: substitution of glutamine for arginine at position 505 of the beta-chain of beta-hexosaminidase results in a labile enzyme

Sandhoff disease is a lysosomal storage disorder characterized by accumulation of GM2 ganglioside due to mutations in the beta-chain of beta-hexosaminidase. Hexosaminidase activity is negligible in infantile Sandhoff disease whereas residual activity is present in juvenile and adult forms. Here we report the molecular basis of the first described adult form of Sandhoff disease. Southern analysis of chromosomal DNA indicated the absence of chromosomal deletions in the gene encoding the beta-chain. Northern analysis of RNA from cultured fibroblasts demonstrated that at least one of the beta-chain alleles was transcribed into normal-length mRNA. Sequence analysis of the entire cDNA prepared from poly-adenylated RNA showed that only one point mutation was present, consisting of a G-->A transition at nucleotide position 1514. This mutation changes the electric charge at amino acid position 505 by substitution of glutamine for arginine in a highly conserved part of the beta-chain, present even in the slime mold Dictyostelium discoideum. The nucleotide transition generated a new restriction site for DdeI, which was present in only one of the alleles of the patient. Reverse transcription of mRNA followed by restriction with DdeI resulted in complete digestion at the mutation site, demonstrating that the second allele was of an mRNA-negative type. Transfection of COS cells with a cDNA construct containing the mutation but otherwise the normal sequence resulted in the expression of a labile form of beta-hexosaminidase. These results show that the patient's is a genetic compound, and that the lability of beta-hexosaminidase found in this form of Sandhoff disease is based on a single nucleotide transition.

Structure and distribution of an Alu-type deletion mutation in Sandhoff disease

Sandhoff disease is a recessively inherited lysosomal storage disease resulting from a deficiency of beta-hexosaminidase activity. The enzyme occurs in two major forms, beta-hexosaminidase A, composed of an alpha- and beta-subunit and beta-hexosaminidase B, composed of two beta-subunits. Both isozyme activities are deficient in Sandhoff disease, owing to mutations of the HEXB gene encoding the common beta-subunit. We have cloned and fully characterized a deletion at the HEXB gene from fibroblasts of a patient with the infantile form of Sandhoff disease. The deletion removes approximately 16 kb of DNA including the HEXB promoter, exons 1-5 and part of intron 5. It most likely arose from recombination between two Alu sequences, with the breakpoints occurring at the midpoint between the left and right arms in each case and regenerating an intact Alu element in the deletion sequence. The deletion allele accounts for 27% of the Sandhoff mutant alleles we analyzed. Two cell lines were shown to be homozygous for the deletion and both had the infantile form of the disease. Four additional patients were compound heterozygotes with other mutations, all of whom displayed a different clinical phenotype. Finally, the mutant allele was present in different ethnic backgrounds, suggesting that it may have been subject to genetic drift.

Sandhoff disease mimicking adult-onset bulbospinal neuronopathy

A 32 year old male is described with an onset of upper limb postural tremor in adolescence followed by muscle cramps. Progressive proximal amyotrophy and weakness in the limbs developed late in the third decade. Examination disclosed, in addition, bilateral facial weakness and mild dysarthria. Enzyme studies revealed hexosaminidase A and B deficiency, indicating a diagnosis of Sandhoff disease. Intra-axonal membranocytoplasmic bodies were present in a rectal biopsy. The presentation, which resembled that of X-linked bulbospinal neuronopathy, widens the clinical spectrum for disorders related to G(M2) gangliosidosis.

Neurophysiological study in chronic GM2 gangliosidosis (hexosaminidase A and B deficiency), with motor neuron disease phenotype

We report the electrophysiological investigation of two adult cases with GM 2 gangliosidosis with hexosaminidase A and B deficiency. Superficial peroneal biopsy was obtained from one patient. The electrophysiological alterations of the peripheral nervous system were fasciculations, signs of collateral reinnervation and loss of motor units, decrease in sensory potential amplitude and increase in distal motor latency. Increase in N9-N13 interpeak latency of the somatosensory evoked potentials and an increase I-V interpeak latency of the brain-stem auditory potentials were evident in both cases. Visual evoked potentials were normal. Nerve biopsy showed a severe loss of myelinated fibers, especially of those with the largest diameter, with no signs of segmental demyelination, or remyelination. A tentative interpretation of our findings is given.

Adult onset motor neuronopathy in the juvenile type of hexosaminidase A and B deficiency

Two sisters presented with progressive muscle cramps, as well as wasting and weakness of the legs with onset after age 20. They also showed intention tremor of the upper extremities and dysarthria starting during the first decade. The older patient also had fasciculations; the younger, hyperreflexia. Total plasma beta-hexosaminidase (Hex) activity with 4-methylumbelliferyl-acetyl-glucosamine as substrate was reduced to 1.4% and 2.7% of the control in the 2 patients, respectively. Hex A activity measured by 4-methylumbelliferyl-N-acetylglucosamine-6-O-sulphate as substrate was 9.9% and 12.8% of the mean control value in the 2 patients, respectively. Hex B activity was undetectable in both patients. Leukocyte total Hex activity was 7-8% of normal; residual Hex A activity in the 2 patients was 17.8% and 16.3% of normal controls, respectively. Fibroblastic residual Hex A activity in the 2 patients was 9.6% and 22% of normal mean value, respectively. Appendiceal ganglion cells contained membranous cytoplasmic bodies in the younger patient. Thin layer chromatography of the appendiceal extract from one patient (III/2) showed a marked increase of GM2 ganglioside, and some increase of GM3 ganglioside. Northern blots performed on fibroblast cell lines from both patients for the demonstration of alpha and beta locus messenger RNA showed no difference between patients and control. These patients have a rare form of adult-onset progressive motor neuron disease presumably due to abnormal beta subunits, causing severe deficiency of both Hex A and Hex B. The phenotypic expression of this disease is similar to motor neuron disease due to alpha locus mutations, which suggests that the Hex A deficiency, even though only a partial one, may be the important pathogenic factor.