FINAL DIAGNOSIS: GLYCOGEN STORAGE DISEASE TYPE II, INFANTILE-ONSET (POMPE DISEASE)
Glycogen storage disease type II is an autosomal recessive disease involving GAA, the gene that encodes acid alpha-glucosidase. This lysosomal 76/70 kDa enzyme is responsible for cleaving the alpha-1, 4- and alpha-1, 6-glucosidic bonds that join glucose monomers in the glycogen polymer. Reduced activity of alpha-glucosidase leads to accumulation of lysosomal glycogen in various tissues, most notably cardiac, skeletal, and smooth muscles, as well as neurons of the anterior horn and brainstem motor nuclei . Because only 1-3% of glycogen is degraded via lysosomes, neither energy depletion nor hypoglycemia is seen in this disease . J.C. Pompe first described the infantile form of the disease in 1932, and in 1963 it was the first disease categorized as a lysosomal storage disorder. In addition to "Pompe disease," glycogen storage disease type II is also known as acid maltase deficiency because maltose was once used as a test substrate in activity assays. It is categorized as both a lysosomal storage disorder and a glycogen storage disorder , and holds the distinction of being the most severe of all the glycogen storage diseases .
Glycogen storage disease type II involves progressive muscle weakness in the trunk, lower limbs, and diaphragm. However, there is a wide spectrum of clinical manifestations, which is due to variable residual alpha-glucosidase activity, which in turn is due to a variety of mutations (over 150 described to date) scattered across the 20 KB gene on chromosome 17q25 [1, 4]. The mutations include missense, nonsense, frameshift, and splice site alterations, producing anything from no enzyme at all to normal amounts of enzyme with reduced activity . Because the enzyme undergoes extensive posttranslational modifications, any alteration of those events can further modify enzyme activity . The most severe form, infantile-onset, is the variant synonymous with Pompe disease. This subtype typically has less than 1% residual alpha-glucosidase activity and is characterized by hypotonia, cardiomegaly, hypertrophic cardiomyopathy, hepatomegaly, feeding problems, loss of motor development milestones, and respiratory difficulties. About half of such patients also have macroglossia. Congestive heart failure, respiratory failure, and/or aspiration pneumonia are the most frequent causes of death, which usually occurs within 1 year . The incidence of this variant is about 1 in 138,000 , and is as high as 1 in 14,000 among African Americans . If the defective alpha-glucosidase enzyme retains about 1.5-5% activity, the patient develops juvenile-onset disease, which has a slower course and only rarely involves cardiac muscle. Approximately 15-40% enzyme activity results in adult-onset disease. The disease course has an even slower progression than the juvenile variant, with some patients surviving into late adulthood . Both of these later-onset variants have a combined incidence of 1 in 57,000, and there is considerable overlap between the two. The total combined incidence of glycogen storage disease type II is 1 in 40,000. Patients have a normal range of intelligence, regardless of variant .
The diagnosis of glycogen storage disease type II can take years in the juvenile and adult variants because the symptoms overlap with many other conditions (e.g. myasthenia gravis, polymyositis, Becker's muscular dystrophy). Infantile-onset Pompe disease is often mistaken for Werdnig-Hoffman disease, Krabbe disease, congenital muscular dystrophy, and benign congenital hypotonia, among others . A characteristic finding on electrocardiogram is a PR interval shortening, as well as QRS complex enlargement [2, 4]. Postmortem exams provide valuable information and can greatly assist the diagnosis based upon the gross findings of cardiomegaly and hepatomegaly, as well as the presence of PAS-positive, diastase-sensitive glycogen in the muscles, liver, and neurons. PAS analysis of frozen sections can also be helpful, as glycogen is often extracted from tissues during processing for paraffin embedding. Electron microscopy is necessary, however, to verify the presence of membrane-bound glycogen in the vacuolated cells, indicative of intra-lysosomal glycogen.
From a laboratory perspective, elevated creatine phosphokinase levels are almost always seen in glycogen storage disease type II of all variants, which makes CPK a good screening test. Because many other myopathies can elevate CPK, though, it is not adequate for diagnosis. The gold standard for diagnosis is the alpha-glucosidase enzyme activity assay, in which cells from the patient are cultured and the cellular lysates are harvested to provide the enzyme. Its ability to hydrolyze a synthetic substrate, 4-methylumbelliferyl-a-D-glucoside, into glucose and 4-methylumbelliferone at a pH of 4.5 is quantifiable because the latter product excites at 365 nm and emits at 450 nm. Thus, when the substrate is in excess, the fluorescence intensity at 450 nm is proportional to enzyme activity. The most favored cells for this assay are fibroblasts from a skin biopsy. Whole leukocytes contain alpha-glucosidase and are easily obtained with a blood sample, but are not reliable because they also express neutral maltase, which has enough activity at the assay's acidic pH to inflate fluorescence levels and produce false-negative results. Prenatal testing of the enzyme is available using uncultured chorionic villus cells. Newer methods of directly measuring alpha-glucosidase protein in urine, plasma, and blood spots are being developed, but have not yet replaced the cell culture enzyme assay . Muscle biopsies also can be used for assessment of enzyme activity and can demonstrate glycogen vacuoles, but are needlessly invasive and not as sensitive as the fibroblast enzyme assay .
A recently described entity, Danon disease, closely mimics glycogen storage disease type II both clinically and histologically. Danon disease is caused by an X-linked mutation in the lysosome-associated membrane protein 2, which is involved in targeting proteins to the lysosome . Because Danon disease does not involve alpha-glucosidase, the alpha-glucosidase enzyme assay can be used to differentiate between the two. Carriers for glycogen storage disease type II cannot be reliably detected via the enzyme assay, though they usually show reduced alpha-glucosidase activity .
Glycogen storage disease type II has no treatment other than supportive/palliative care, and the course is relentless. Diets high in protein and alanine, designed to reduce glycogen synthesis, do not slow the disease, and bone marrow transplantation has not worked. In recent years, however, cell surface receptors have been discovered that facilitate the endocytosis of lysosomal enzymes to target tissues. This has led to successful, albeit expensive, recombinant enzyme replacement therapy (ERT) of certain lysosomal storage disorders such as Gaucher disease and Fabry disease. Clinical trials are ongoing for Pompe disease, with promising results . If ERT proves successful in Pompe disease, then it will be of the utmost importance to recognize and diagnose this disease early, especially in the infantile variant where the window of opportunity for treatment is smaller. Thus, Pompe disease may someday join diseases like phenylketonuria, hypothyroidism, and maple syrup urine disease in the screening panel routinely done on all neonates.
Contributed by Craig Horbinski, MD, PhD, Charleen T. Chu, MD, PhD, David Finegold, MD, and Mohamed A. Virji, MD, PhD