Pompe's disease - RePub - Erasmus Universiteit Rotterdam

Chapter 1
Diagnosis
Clinical Diagnosis
A thorough clinical examination may raise the suspicion of Pompe’s disease, but is not
suffi cient to establish the diagnosis. Additional laboratory tests, such as the measurement of
serum CK, ALAT, ASAT and LDH, which levels are usually increased, may be helpful in setting
the diagnosis (82, 92, 93). An EMG may reveal abnormal patterns with repetitive discharges
and positive waves as a sign of muscle pathology (94-96).
Microscopic sections of the muscle show “purple” membrane bound deposits of glycogen
within the muscle fi bres if the material is properly processed, fi xed with glutaraldehyde and
stained with Periodic Acid Schiff (PAS). The glycogen is washed out if microscopic sections are
not properly handled and a lacework pattern appears (97, 98).
Glycogen storage is more pronounced in infantile than in the late onset forms of the disease,
as is the rate of progression. In particular, in patients with an adult onset of disease the
glycogen accumulation may vary between fi bers and muscle groups. Some muscle biopsies of
mildly affected patients may show no microscopical abnormalities, and the diagnosis can be
missed. A defi nite diagnosis for Pompe’s disease can be made by demonstrating a defi ciency
of α-glucosidase activity or occurrence of a pathogenic mutation in each of the two acid aglucosidase
alleles.
Enzyme diagnosis
Acid α-glucosidase hydrolyses the α-1,4 and α-1,6 glycosidic linkages of glycogen to release
glucose. The activity of the enzyme can be measured with the natural substrate glycogen or
the synthetic substrate 4-Methylumbelliferyl-α-D-glucopyranoside. For diagnostic purposes
the enzyme activity is preferably measured in fi broblasts or muscle tissue.
Determination of acid α-glucosidase activity in leukocytes is not recommended because
neutral α-glucosidases interfere with the measurement causing an overlap
of the enzyme activity levels between patients and controls (99, 100). Patients with infantile
Pompe’s disease have a complete enzyme defi ciency whereas patients with the late onset form
of the disease have residual activities up to 25% of normal. In general the clinical subtype
correlates well with the level of residual enzyme activity, but unusually low acid α-glucosidase
activities have been reported in a few cases of adult onset disease. Also non-classical infantile
and childhood Pompe’s disease cannot always be distinguished from the classic infantile type
of disease on the basis of enzyme activity. In addition, there are mutations which affect the
activity for the natural substrate more than for the artifi cial substrate (62, 101). Carrier
detection is unreliable via enzyme activity measurement.
DNA diagnosis
The acid α-glucosidase gene (GAA) was mapped on chromosome 17, region q25.3 (102). It is
approximately 20 kb long and contains 20 exons. The gene is transcribed and spliced into an
m-RNA of 3.6 kb with 2856 bp of coding sequence (103). The acid α-glucosidase polypeptide
contains 952 amino acids (104, 105). The mutations are equally distributed over all the
coding exons (2-20). Deletions, insertions, missense and splice site mutations were found.
More than 100 mutations are presently known in the public domain (www.Pompecenter.nl).
The most common mutation world-wide is IVS1(-13T−>G). It causes aberrant splicing of the
fi rst coding exon (exon 2) in 80-90% of the splicing events.
Some mutations specifi cally occur in certain ethnic groups. For example, the deletion of
exon 18 is quite common in the Caucasian Dutch sub-population and in the southern part of
Italy, while the delT525 mutation in exon 2 is common in Western Europe and in the French-
Canadian population (106). Both mutations lead to a total defi ciency of acid α-glucosidase.
The C1935A (exon 14) transition is a frequent mutation in Taiwanese and Chinese populations
and also leads to a total defi ciency of enzyme activity (107).
Different strategies can be taken for mutation analysis. Certain subgroups of patients can be
screened fi rst for the presence of frequent mutations, but otherwise it is quicker to sequence
the whole gene. The fi nding of a known mutation is immediately informative, but new
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