The molecular genetics of Lafora progressive myoclonus epilepsy.

摘要

Lafora disease (LD) is one of the most severe forms of adolescent-onset progressive myoclonus epilepsy. Increasing seizures are paralleled with a cognitive decline towards dementia, and death usually occurs within 10 years of onset. Endoplasmic reticulum-associated depositions of starch-like glycogen molecules (known as polyglucosans, which accumulate to form Lafora bodies) are observed in brain, liver, skeletal muscle and heart. The first disease-causing gene, EPM2A, was identified by our laboratory in 1998 and at the time this gene accounted for approximately 45% of mutations in LD families. The EPM2A translational product, a protein named laforin, contains a carbohydrate-binding domain and a dual-specificity phosphatase domain. Towards understanding the pathogenesis of LD, I generated a transgenic mouse carrying a construct that allowed overexpression of laforin. The vector was designed such that it contained an inactivating mutation that would generate a product capable of trapping its substrate. This experimental strategy was successful and Lafora bodies formed in brain, liver, and muscle. Importantly, laforin was localized to polyglucosans establishing a direct association between the disease-defining storage product and disease protein. These findings suggest that laforin is involved in the monitoring and prevention of polyglucosan formation. To further understand the cellular mechanism that is defective in LD, I sought to identify additional proteins that are involved in the LD pathway. Since LD demonstrates genetic heterogeneity, I was able to map a second disease-causing gene; EPM2B, to chromosome 6p22 and study the protein it encodes. The EPM2B gene encodes a putative E3 ubiquitin ligase, which we named malin. Nineteen sequence variations predicted to cause deleterious effects on the protein product were found to co-segregate with LD in 37 families. The discovery of the malin protein has led us towards a new area of investigation, namely how ubiquitination plays a role in LD.; The findings described in this thesis represent significant advancements made towards a more complete understanding of the disease mechanism associated with LD. Many new areas of investigation have been revealed, which will lead to new experiments designed to further understand the pathway that protects against Lafora body production and epilepsy in LD.