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SANDHOFF DISEASE[edit]

DESCRIPTION[edit]

Sandhoff disease is an inherited lipid storage disorder that progressively destroys nerve cells (neurons) in the brain and spinal cord. It is a lysosomic genetic disorder. It is cused by defiency of the enzyme beta-hexsoaminidase, which results in the harmful accumulation of certain fats(lipids) in the brain and other organs of the body. It is a severe form of tay-sachs disease, the incidence of which had been particularly high in people of Eastern European and Ashkenazi Jewish descent, but Sandhoff disease is not limited to any ethnic group.

Other Names:

Beta-hexosaminidase-beta-subunit deficiency

GM2 gangliosidosis

Sandhoff’s jatzkewitz disease

TYPES[edit]

There are three types of Sandhoff disease, classic infantile, juvenile, and adult late onset.Each form is classified by the severity of the symptoms as well as the age at which the patient shows these symptoms.

SYMPTOMS[edit]

Infants with the classic form of Sandhoff disease typically appear normal until the age of 3 to 6 months when their development slows and muscles used for movement weaken. Affected infants typically lose motor skills such as turning over, sitting, and crawling. They may also develop an exaggerated startle reaction to loud noises. As the disease progresses, children with Sandhoff disease may experience seizures, vision and hearing loss, intellectual disability, and paralysis. An eye abnormality called a cherry-red spot, which can be identified through an eye examination, is characteristic of this disorder. Some affected children also have an enlarged liver and spleen, frequent respiratory infections, or bone abnormalities.Children with the severe infantile form of Sandhoff disease usually live only into early childhood.

Forms of Sandhoff disease where the symptoms develop after infancy are very rare. Signs and symptoms can begin in childhood, adolescence, or adulthood and are usually milder than those seen with the infantile form. Characteristic features include muscle weakness, loss of muscle coordination (ataxia) and other problems with movement, speech problems, and mental illness. These signs and symptoms vary widely among people with late-onset forms of Sandhoff disease.

The Human Phenotype Ontology (HPO) provides the following list of features that have been reported in people with this condition. Much of the information in the HPO comes from Orphanet, a European rare disease database. If available, the list includes a rough estimate of how common a feature is (its frequency). Frequencies are based on a specific study and may not be representative of all studies.

Signs and Symptoms	Approximate number of patients (when available)
Abnormality of glycosphingolipid metabolism	Very frequent (present in 80%-99% of cases)
Abnormality of movement	Very frequent (present in 80%-99% of cases)
Ataxia	Very frequent (present in 80%-99% of cases)
Blindness	Very frequent (present in 80%-99% of cases)
Cherry red spot of the macula	Very frequent (present in 80%-99% of cases)
Failure to thrive	Very frequent (present in 80%-99% of cases)
Hearing impairment	Very frequent (present in 80%-99% of cases)
Kyphosis	Very frequent (present in 80%-99% of cases)
Macrocephaly	Very frequent (present in 80%-99% of cases)
Motor deterioration	Very frequent (present in 80%-99% of cases)

CAUSES[edit]

Inheritance[edit]

Sandhoff disease is inherited in an autosomal recessive pattern, which means both copies of the gene in each cell have mutations. The parents of an individual with an autosomal recessive condition each carry one copy of the mutated gene, but they typically do not show signs and symptoms of the condition.

Mutations in the HEXB gene cause Sandhoff disease. The HEXB gene provides instructions for making a protein that is part of two critical enzymes in the nervous system, beta-hexosaminidase A and beta-hexosaminidase B. These enzymes are located in lysosomes, which are structures in cells that break down toxic substances and act as recycling centers. Within lysosomes, these enzymes break down fatty substances, complex sugars, and molecules that are linked to sugars. In particular, beta-hexosaminidase A helps break down a fatty substance called GM2 ganglioside.

Mutations in the HEXB gene disrupt the activity of beta-hexosaminidase A and beta-hexosaminidase B, which prevents these enzymes from breaking down GM2 ganglioside and other molecules. As a result, these compounds can accumulate to toxic levels, particularly in neurons of the brain and spinal cord. A buildup of GM2 ganglioside leads to the progressive destruction of these neurons, which causes many of the signs and symptoms of Sandhoff disease.

Because Sandhoff disease impairs the function of lysosomal enzymes and involves the buildup of GM2 ganglioside, this condition is sometimes referred to as a lysosomal storage disorder or a GM2-gangliosidosis.

CLINICAL FEATURE[edit]

Clinical heterogeneity[edit]

Spence et al. (1974) described a case of clinically, histologically, and chemically typical Sandhoff disease in a black male. Total hexosaminidase activity in the blood was 20 to 24% of normal (compared with the usual value of less than 5%), whereas in the liver the level was less than 2% of normal. This may be an allelic variant of Sandhoff disease.

In a 10-year-old male with progressive cerebellar ataxia and psychomotor retardation, Wood and MacDougall (1976) found almost complete absence of total hexosaminidase activity in serum, leukocytes, and cultured skin fibroblasts. In spite of disparate clinical findings, this disorder may be allelic to the classic infantile form of Sandhoff disease in view of the similarity of the enzyme deficiency. Studies of residual hexosaminidase isozymes in the juvenile and infantile forms suggested that the defects may be different allelic modifications of the beta subunit common to Hex-A and Hex-B (Wood and MacDougall, 1976). Wood (1978) found no complementation of Sandhoff and juvenile Sandhoff cells, suggesting allelism.

Johnson and Chutorian (1978) found a new form of hexosaminidase deficiency characterized clinically by mild, juvenile-onset, slowly progressive cerebellar ataxia, and macular cherry red spots. Hexosaminidase B appeared to be absent, resulting in a relative increase in Hex-A in screening tests. They suggested that this condition may be due to a mutation allelic to that for Sandhoff disease.

In one of its mutant forms, Hex-A deficiency can lead to late-onset, progressive motor neuron disease. Cashman et al. (1986) presented a case demonstrating that the same is true for Hex-B deficiency. Their female patient had a progressive motor neuron syndrome that began at age 7 years and was characterized by dysarthria, muscle wasting, fasciculations, and pyramidal tract dysfunction. Rectal biopsy at age 24 showed membranous cytoplasmic bodies in submucosal ganglion cells.

PROGNOSIS[edit]

The prognosis for individuals with Sandhoff disease is poor. Death usually occurs by age 3 and is generally caused by respiratory infections.

DIAGNOSIS[edit]

Sandhoff disease can be diagnosed by performing an enzyme assay to determine activity of the hexosaminidase A and B enzymes. Affected individuals have absent or reduced activity of both enzymes. Molecular genetic (DNA) testing is available to determine the specific gene mutation that is present in the beta subunit of the Hexosaminidase B gene and confirm the diagnosis.Diagnosis is performed to identify individuals who have a and may be at risk for having a child or other family members with the same mutation. usually do not have symptoms related to the gene mutation. Carrier testing is typically offered to individuals who have family members with a genetic condition, family members of an identified carrier, and individuals in ethnic or racial groups known to have a higher carrier rate for a particular condition.

There two types of carrier screening tests: and biochemical. Sandhoff carrier screening is available through DNA or biochemical testing. DNA carrier screening detects specific known mutations that are ‘looked’ for in the test. A negative DNA carrier result does not eliminate an individual's chances of being a carrier because of the possibility of carrying an unknown mutation or one not ‘looked’ for in the test. Biochemical testing, also called an assay, detects the level of the enzyme(s) in question in the blood. Enzyme assays can be done using serum or (white blood cells). Serum is typically the standard test, but leukocyte testing is recommended when the person being tested is pregnant, on birth control pills or taking any medications that affect because all of these situations can potentially interfere with the accuracy of the serum test. DNA carrier screening may be recommended if the results of the biochemical test are uncertain.

Sandhoff disease is associated with deficiencies of both hexosaminidase A (hex A) and hexosaminidase B (hex B) enzyme activity. Carriers of Sandhoff disease have reduced (but adequate) amounts of both hex A and hex B. While most hex A assays are performed to identify Tay-Sachs carriers, the test also can also identify individuals that are carriers of Sandhoff disease. Looking at the total hexosaminidase activity in combination with the percent of hex A activity present can aid in determining whether an individual is a carrier of Sandhoff disease. Typically, a decreased amount of total hexosaminidase activity along with an increase in the proportion of hex A activity in leukocytes is suggestive of a Sandhoff carrier.[5] In contrast, Tay-Sachs carriers have a decrease in the amount of hex A activity. When the hex A enzyme result indicates that an individual is a possible Sandhoff carrier, the next step is typically to offer carrier testing to the individual's partner. If the partner is negative, the risk for the couple to have a child affected with the disorder is very significantly decreased. If the partner is also a possible carrier, more comprehensive testing may be offered.

TREATMENT[edit]

Treatment of Sandhoff disease is symptomatic and supportive and includes nutritional and respiratory therapy. Anticonvulsants may be prescribed to temporarily control seizures. Death from respiratory infection usually occurs by age three for the infantile form of the disease.

Genetic counseling is recommended for affected individuals and their families.

AFFECTED AREA[edit]

Sandhoff disease is a very rare disorder that affects males and females in equal numbers. This disorder occurs in people of many different ethnic backgrounds. Sandhoff disease may be more common in the Creole population of northern Argentina, the Metis Indians in Saskatchewan, Canada and individuals of Lebanese ancestry.