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Genetic disease involving the Human alpha glucosidase gene
Stephen Perritt, Ben Ernest, and Elizabeth Johnson
UTK and GST
Funded by NIH, ISMD Award # 1R25GM086761-01
Pompe’s Disease
Pompe disease, also termed glycogen storage disease type II or acid
maltase deficiency, is an inherited lysosomal storage disorder with an
estimated frequency of 1 in 40,000 births [1]. The disease is
characterized by a total or partial deficiency of the enzyme acid αglucosidase. This enzyme is needed to break down glycogen that is
stored within the lysosome, a cytoplasmic organelle involved in
cellular recycling and tissue remodeling. Deficiency of acid αglucosidase leads to accumulation of lysosomal glycogen in virtually
all cells of the body, but the effects are most notable in muscle. The
pathologic mechanisms by which glycogen accumulation eventually
causes muscle malfunction are not fully understood. Muscle wasting
in Pompe disease has been explained by increased tissue breakdown
by autolytic enzymes released from ruptured lysosomes [5]. Another
hypothesis is that glycogen-filled lysosomes and clusters of noncontractile material disturb the myofibrillar morphology and thus the
transmission of force in the muscle cells [6].
Mutations
At present more than 200 different mutations in the acid αglucosidase gene are known.
Autosomal Resessive Genetic Disorder
The enzyme deficiency in Pompe disease is caused by pathogenic mutations in the acid
α-glucosidase gene (GAA) located on chromosome 17. The mode of inheritance is
autosomal recessive.
Autosomal recessive diseases follow the mode of inheritance described by the pedigree
graphic below.
A novel mutation, C118t, in exon 2 of the acid -glucosidase gene has
been found in an infant with glycogen storage disease type II. This
mutation is predicted to result in protein truncation. The phenotype
was that of the severe infantile form of the disorder with lack of motor
development, but with eye regard, social smile and vocalization.
The acid α-glucosidase gene (GAA) encodes a precursor protein of 952 amino acids
with 7 N-linked
carbohydrate chains. The first 37 N-terminal amino acids constitute a signal-peptide
directing entry into the
lysosomal / secretory pathway. The signal-peptide and an additional 19 amino acids
are cleaved off before the
precursor protein leaves the trans Golgi network (Moreland, et al., 2005; Wisselaar, et
al., 1993). The apparent
molecular mass of the intracellular (C) precursor is approximately 110 kD (C110).
Transport from the trans Golgi
network to the lysosomes is mannose 6-phosphate receptor mediated (Hasilik and
Neufeld, 1980a; Hasilik and
Neufeld, 1980b; Reuser, et al., 1985). Some newly formed acid α-glucosidase does
not reach the lysosomes but is
secreted into the culture medium (M) and has a similar molecular mass of
approximately 110 kD (M110). The
intra-cellular pool of precursor molecules is further modified by controlled proteolytic
processing prior to or just
after entry into the lysosomes. This results in the formation of larger 95 kD (C95), 76
kD (C76), 70 kD and smaller
molecular species (< 20kD) (Hasilik and Neufeld, 1980b; Moreland, et al., 2005;
Reuser, et al., 1985; Wisselaar, et
al., 1993). The larger and smaller fragments are held together by S-S bridges, but are
visible as separate species
upon polyacrylamide gel electrophoresis under denaturing circumstances (SDS-PAGE)
followed by Western
blotting (Moreland, et al., 2005).
The quantity and quality of these molecular species, as expressed in cells transfected
with mutant compared to
normal forms of recombinant human acid α-glucosidase cDNA and as visualized by
amount and mobility in SDSPAGE,
are in part informative for the severity of GAA sequence variations. The activity of acid
α-glucosidase in
these transfected cells provides more direct information on the functional effect of the
mutation. Both types of
assay were performed to analyze the impact of sequence variations.
Future Treatments
Gene Therapy
Chaperone
A patient has two pathogenic mutations in the acid α-glucosidase gene, one on each
chromosome. Basically, the nature of the mutations in the acid α-glucosidase gene and
the combination of mutant alleles determine the level of residual lysosomal acid αglucosidase activity and primarily the clinical phenotype of Pompe disease. Although
exceptional cases have been described, in general a combination of two alleles with fully
deleterious mutations leads to virtual absence of acid α-glucosidase activity and to the
severe classic infantile phenotype. A severe mutation in one allele and a milder mutation
in the other result in a slower progressive phenotype with residual activity up to 23% of
average control activity. In these patients genotype and enzyme activity are not always
predictive of the age at onset and the progression of the disease. For example, patients
with the common c.-32-13T>G mutation, combined with a fully deleterious mutation on
the other allele, all show significant residual enzyme activity and a protracted course of
disease, but onset of symptoms varied from the 1st year of life to late adulthood.
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References
1. Ausems MG, Verbiest J, Hermans MP, et al. Frequency of glycogen storage disease type II in The
Netherlands: implications for diagnosis and genetic counselling. Eur J Hum Genet 1999; 7:713-716.
2. De Duve C, Pressman BC, Gianetto R, Wattiaux R, Appelmans F. Tissue fractionation studies. 6.
Intracellular distribution patterns of enzymes in rat-liver tissue. Biochem. J. 1955; 60:604-617.
3. Hers HG. a-Glucosidase deficiency in generalized glycogen storage disease (Pompe's disease). Biochem.
J. 1963; 86:11-16.
4. Hirschhorn R, Reuser AJ. Glycogen storage disease type II; acid alpha-glucosidase (acid maltase)
deficiency. In: Scriver CR, Beaudet AL, Sly W, Valle D (eds) The metabolic and molecular bases of inherited
disease. New York: McGraw-Hill, 2001: p 3389-3420.
5. Umpleby AM, Wiles CM, Trend PS, et al. Protein turnover in acid maltase deficiency before and after
treatment with a high protein diet. J Neurol Neurosurg Psychiatry 1987; 50:587-592.
6. Drost MR, Hesselink RP, Oomens CW, Van der Vusse GJ. Effects of non-contractile inclusions on
mechanical performance of skeletal muscle. J Biomech 2005; 38:1035-1043.