Sandhoff Disease

1. Sandhoff Disease Presented by: Megan Frantz

2. History Sandhoff disease is named for Konrad Sandhoff, a German chemist who first described Sandhoff in Life Science in 1968. Sandhoff, like its near twin Tay-Sachs, is a progressive neurological autosomal recessive genetic disorder that appears in three forms: Classic Infantile, Juvenile and Late Onset or Chronic Sandhoff.

3. What is Sandhoff Disease? Sandhoff disease is a rare, genetic, lipid storage disorder resulting in the progressive deterioration of the central nervous system. It is caused by a deficiency of the enzyme beta-hexosaminidase, which results in the accumulation of certain fats (lipids) in the brain and other organs of the body. Sandhoff disease is a severe form of Tay-Sachs disease--which is prevalent primarily in people of Eastern European and Ashkenazi Jewish descent--but it is not limited to any ethnic group. Onset of the disorder usually occurs at 6 months of age.

4. What are the symptoms? Neurological symptoms may include motor weakness, startle reaction to sound, early blindness, progressive mental and motor deterioration, macrocephaly (an abnormally enlarged head), cherry-red spots in the eyes, seizures, and myoclonus (shock-like contractions of a muscle). Other symptoms may include frequent respiratory infections, doll-like facial appearance, and an enlarged liver and spleen.

6. Cherry red spot

7. Classic Infantile Sandhoff Classic Infantile Sandhoff is the most common form of this rare disease and is characterized by very little to no Hexosaminidase-A (Hex-A) and Hexosaminidase-B (Hex-B) enzyme activity. Motor weakness begins in the first 6 months of life and is progressive. Death usually occurs at about 3 years of age.

8. Juvenile Sandhoff Disease Individuals who have low levels of Hex-A and Hex-B, have a slower onset of symptoms and progression of disease, compared to those with Classic Infantile Sandhoff. Within each form of Sandhoff disease, there is a range of severity and each person’s experience with the disease is distinctive. In children with Juvenile Sandhoff, Hex-A enzyme activity is extremely low but not as low as in children with Classical Infantile Sandhoff. Children with the Juvenile form of the disease usually develop symptoms around 5 years of age. Though the course of the disease is slower, end stages generally occur in late adolescence. Death usually occurs within the first 15 years due to other complications, such as respiratory infection.

9. Late-Onset Sandhoff Disease Late-Onset Sandhoff disease is a rare form of Sandhoff disease Because only a low level of Hex-A and Hex-B is functional in patients with Late-Onset Sandhoff, the symptoms typically presents in adolescence, with dysarthria, proximal (trunk) muscle weakness, tremor and ataxia. Muscle cramps, especially in the legs at night, and fasciculations (muscle twitching) are common. Late-Onset Sandhoff differs from the juvenile form primarily in its impact of intelligence, which is minimal in patients with Late-Onset Sandhoff.

10. What is a ganglioside? Gangliosides are acidic glycosphingolipids; they contain oligosaccharides with terminal, charged N-acetyl neuraminic acids (NANA). Depending on the number of NANA sugars, gangliosides are designated M, D, T, Q (e.g., GM2 ). Glycosphingolipids, or glycolipids, are composed of a ceramide backbone with a wide variety of carbohydrate groups (mono- or oligosaccharides) attached to carbon 1 of sphingosine.

11. Synthesis of glycosphingolipids Glycosphingolipids are synthesized by membrane-bound glycosyltransferases which exist as multienzyme complexes. Synthesis reactions occur in the Endoplasmic Reticulum and Golgi apparatus. The first step in their synthesis is the condensation of a molecule of palmitoyl-CoA with the amino acid serine to form 3-ketosphinganine, catalyzed by the enzyme 3-ketosphinganine synthase. The ketone group is reduced to an alcohol by the enzyme 3-ketosphinganine reductase, resulting in sphinganine. The sphinganine is acylated by fatty acyl-CoA to yield N-acylsphinganine. The palmitoyl group can then be oxidized to a double bond to form ceramide. Ceramide can combine with phosphatidylcholine to form sphingomyelin, which is the major lipid component of the myelin membrane, or it can add glucosyl groups onto the alcohol to form glycosphingolipids, which are important in cell-cell recognition and intracellular communications.

12. The biosynthesis of globosides and GM gangliosides.

13. Degradation of glycosphingolipids A series of acid hydrolases participate in the degradation of glycosphingolipids. Degradation of most substrates occurs by the stepwise activity of a series of hydrolases, with each step requiring the action of the previous hydrolase to modify the substrate, so that the substrate can be further degraded by the next enzyme in the pathway. If one step in the process fails, further degradation ceases and the partially degraded substrate accumulates. The GM2-activator binds GM2 and helps expose it to the surface of the membrane. The GM2-activator-GM2 complex can then bind hexosaminidase A, an aß dimer that hydrolyzes N-acetylgalactosamine from GM2 at the lipid-water interface.

14. Hexosaminidase A The Hex A gene, located on chromosome 15, codes for the alpha subunit of the hexosaminidase A enzyme which is necessary for breaking down GM2 gangliosides in nerve cells. When there is a mutation in the coding for beta subunit of the hexosaminidase A it does not function properly and leads to an accumulation of GM2 which is toxic and eventually causes cell death. Sandhoff is characterized by loss of function of both the alpha and beta subunit of hexosaminidase A enzyme. Within lysosomes, beta-hexosaminidase A forms part of a complex that breaks down a fatty substance called GM2 ganglioside.

15. Hexosaminidase B  Sandhoff is caused by a mutation in the Hex B gene on chromosome 5. The Hex B gene codes for part of two essential nervous system enzymes: the beta subunit of hexosaminidase A and the beta subunit of hexosaminidase B. When there is a mutation in the coding for beta subunit of hexosaminidase A and the beta subunit of hexosaminidase B both enzymes do not function properly and lead to an accumulation of GM2 which is toxic and eventually causes cell death. Tay-Sachs is characterized by loss of function of only the alpha subunit of the hexosaminidase A enzyme.

16. Treatment To date, there is no cure or effective treatment for Sandhoff disease. However, there is active research being done in many investigative laboratories in the U.S. and around the world exploring a range of therapeutic approaches– primarily in a Sandhoff mouse model, an animal model for Sandhoff disease. For the first time in the history of the disease there currently are clinical trials testing the potential of a substrate reduction drug (miglustat) in all three forms of Sandhoff and Tay-Sachs, with the Late Onset trial having started in 2002. The uses of enzyme replacement therapy to provide the Hex-A and Hex B that is missing in babies with classic infantile or significantly reduced in children and adults with Sandhoff disease has been explored but presents serious obstacles. Stem cell transplantation using umbilical cord blood is an investigational procedure attempted with a small number of very young children, but to date there is not enough information for specific results about reversing or slowing damage to the central nervous system in this group with Sandhoff disease.

17. Testing Since Sandhoff is a recessive genetic disorder, both parents must be carriers for Sandhoff disease in order to have a child with Sandhoff disease. Enzyme assay- Measurement of enzyme activity with particular substrate. The assay involves the use of an artificial hexosaminidase substrate, 4-methylumbelliferyl-ß-D-N-acetylglucosamine, which yields a fluorescent product upon hydrolysis. Fluorescence of the product, indicates hexosaminidase activity. DNA analysis looks for particular or known mutations in the genome or genetic make up. Pre-implantation Genetic Diagnosis (PGD) - Tests early-stage embryos produced through in vitro fertilization (IVF) for the presence of a variety of genetic conditions. One cell is extracted from the embryo in its eight-cell stage and analyzed. Embryos free of conditions that would cause serious disease can be implanted in a woman's uterus and allowed to develop into a child.

18. Simple serum assay reaction

19. Summary While Hex-A and Hex-B differ in the kinds of subunits they contain, both play a role in the degradation of GM2 gangliosides Individuals with Sandhoff are unable to degrade these GM2 gangliosides and the gangliosides accumulate in special compartments of the cells called lysosomes. Since the accumulation of GM2 gangliosides is at the root of Sandhoff disease, it is classified as a GM2 gangliosidosis – to distinguish it from other lysosomal storage diseases

20. Summary (cont.) The severity of Sandhoff disease depends on the amount of residual enzyme that is produced. Children with virtually no hexosaminidase activity will have the infantile (acute onset) form of the disease. The infantile form is the most severe and, unfortunately, the most common. Those born with a small amount of hexosaminidase activity will have the juvenile, subacute form. Those with still more activity will have a later onset adult (chronic) form of Sandhoff disease. The juvenile and adult forms of Sandhoff disease occur later and tend to be much more variable in their clinical features. The amount of residual enzyme, and therefore the clinical course, is determined by the specific mutation(s) in the ß-subunit of Hex-A.