Glycogen is the sole carbohydrate storage molecule found in mammalian cells and plays an important role in cellular metabolism in nearly all tissues, including the brain. Defects in glycogen metabolism underlie the glycogen storage diseases (GSDs), genetic disorders with variable clinical phenotypes depending on the mutation type and affected gene(s). Lafora disease (LD) is a fatal form of progressive myoclonus epilepsy and a non-classical GSD. LD typically manifests in adolescence with tonic-clonic seizures, myoclonus, and a rapid, insidious progression. Patients experience increasingly severe and frequent epileptic episodes, loss of speech and muscular control, disinhibited dementia, and severe cognitive decline; death usually ensues in the second decade of life. LD, like one- third of all epilepsy disorders, is intractable and resistant to antiseizure drugs.
A hallmark of LD is the accumulation of intracellular, insoluble carbohydrate aggregates known as Lafora bodies (LBs) in brain, muscle, and other tissues. LBs are a type of polyglucosan body, an insoluble aggregate of aberrant glycogen found in some GSDs and neurodegenerative disorders. Like most GSDs, LD is an autosomal recessive genetic disorder. Approximately 50% of LD patients carry mutations in the epilepsy, progressive myoclonus 2A (EPM2A) gene encoding laforin, a glycogen phosphatase. Remaining patients carry mutations in EPM2B, the gene that encodes malin, an E3 ubiquitin ligase. Laforin and malin play important roles in glycogen metabolism. In the absence of either enzyme, glycogen transforms into an insoluble, hyperphosphorylated and aberrantly branched polysaccharide reminiscent of plant starch. This abnormal polysaccharide precipitates to form LBs and has pathological consequences in the brain.
Since a definitive LD diagnosis requires genetic testing, whole exome sequencing has been increasingly used to diagnose LD. As a result, numerous cases of more slowly progressing or late-onset LD have been discovered that are associated with missense mutations in EPM2A or EPM2B. Over 50 EPM2A missense mutations have been described. These mutations map to many regions of the laforin X-ray crystal structure, suggesting they produce a spectrum of effects on laforin function. In the present work, a biochemical pipeline was developed to characterize laforin patient mutations. The mutations fall into distinct classes with mild, moderate or severe effects on laforin function, providing a biochemical explanation for less severe forms of LD.
LBs drive LD pathology. As a result, LBs and glycogen metabolism have become therapeutic targets. Since LBs are starch-like, and starch is degraded by amylases, these enzymes are potential therapeutics for reducing LB loads in vivo. However, amylases are normally secreted enzymes. Degradation of intracellular LBs requires a cell-penetrating delivery platform. Herein, an antibody-enzyme fusion (AEF) technology was developed to degrade LBs in vitro, in situ in cell culture, and in vivo in LD mouse models. AEFs are a now putative precision therapy for LD, potentially the first therapeutic to provide a significant clinical benefit.
Prior to this work, LD was considered a homogenous disorder and treatments were only palliative. The data herein support a spectrum of clinical progression, a potential therapy for LD, and mechanistic insights into LD pathophysiology. This work illustrates how personalized medicine, both in diagnosis and treatment, can be achieved through basic biochemical approaches to human disease.
| Identifer | oai:union.ndltd.org:uky.edu/oai:uknowledge.uky.edu:biochem_etds-1041 |
| Date | 01 January 2019 |
| Creators | Brewer, Mary Kathryn |
| Publisher | UKnowledge |
| Source Sets | University of Kentucky |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | Theses and Dissertations--Molecular and Cellular Biochemistry |
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