Pompe disease (Glycogen storage disease type II) is
a lysosomal storage disorder caused by deficiency of the lysosomal enzyme
acid-a-glucosidase
(GAA) [1]. It is an autosomal recessive disorder with a 25% recurrence
risk in each subsequent pregnancy of a heterozygous carrier couple, who
have a previously affected child [2].
Prenatal diagnosis of Pompe disease is conventionally
done through measurement of GAA enzyme activity in uncultured chorionic
villus tissue and less often in cultured amniocytes. However, with the
advancements in molecular genetic techniques, full gene sequencing and/or
targeted mutation analysis have now become available and prenatal
diagnosis through targeted mutation analysis of fetal tissue has become
feasible [3].
We report two families wherein prenatal diagnosis for
Pompe disease was attempted and discuss pertinent issues.
Case Report
Case 1: A non-consanguineous couple with two
previous children affected with Pompe disease, presented for prenatal
diagnosis of the third pregnancy. CVS was done at 13.4 weeks of gestation
and the GAA enzyme level was 26 nmol/h/mg (reference range 101-305 nmol/h/mg)
and 60 nmol/h/mg (reference range 140-280 nmol/h/mg) respectively, from
two different laboratories. As the enzyme assay results were rather
ambiguous, with the levels being clearly reduced but not altogether
deficient, it was decided that targeted mutation analysis should be
performed before conveying the results to the family. The proband’s DNA
stored in the DNA bank was procured, with the informed consent of the
parents. Whole gene sequencing of the GAA gene identified 2 mutations in
the proband – c.1465G>A (p.Asp489Asn) in exon 10 and c.1799G>A
(p.Arg600His) in exon 13. Targeted mutation analysis of the CVS DNA showed
absence of both mutations. The family was therefore counseled that the
fetus was neither affected nor a carrier of Pompe disease. Three months
after delivery, the baby was found to be normal on clinical examination
and her blood GAA assay showed an enzyme level of 100 nmol/h/mg (reference
range 86 – 296 nmol/h/mg).
Case 2: A non-consanguineous family with two
previous affected children presented to us for prenatal diagnosis of the
fourth pregnancy. CVS was performed at 11 weeks of gestation. CVS enzyme
activity was reported to be 13.3 nmol/hr/mg (reference range 132-525 nmol/h/mg)
and 56 nmol/h/mg (reference range-140-280 nmol/h/mg) respectively, from
two different laboratories. Whereas one GAA value was very low and
suggestive of an affected fetus, the other value suggested that the fetus
might just be an unaffected carrier. To resolve this diagnostic dilemma,
molecular genetic testing of fetal tissue was planned. As the proband’s
DNA was not available, GAA gene sequencing was done on the parents’
DNA. One mutation was identified in each parent: c.1962_1964delAGA
(p.Glu655del) in exon 14 in the father and c.1465G>A (p.Asp489Asn) in exon
10 in the mother. Targeted mutation analysis of the CVS DNA revealed
presence of only the paternal mutation (c.1962_1964delAGA). The parents
were accordingly counseled that the fetus was only a heterozygous carrier
and was thus not likely to be affected with Pompe disease. Ten months
after birth, the baby was ascertained to be normal through telephone
interview and communication.
Discussion
The infantile form of Pompe disease is a uniformly
lethal condition. Though enzyme replacement therapy is now available, it
is costly, not readily accessible, and its long term benefits remain to be
proven. Hence, an accurate prenatal diagnosis in early gestation for
families with a previously affected child is essential [3].
If the fetus is affected with the classic infantile
onset form of Pompe disease, the CVS enzyme level is expected to be
grossly reduced and is usually found to be <1% of the normal reference
range [1]. However, as for other lysosomal storage disorders, enzyme assay
for prenatal diagnosis of Pompe disease has its limitations [4]. The
enzyme assay results can at times be equivocal and difficult to interpret,
as illustrated by the two cases described here.
Enzyme assay is a demanding technique requiring a great
deal of accuracy and strict quality control measures. Maternal tissue
contamination of the CVS sample can compound the difficulties and give an
erroneous result. As has been reported for blood based GAA analysis,
enzyme activity may be influenced by variations in temperature and
humidity during transport and storage of the sample [5].
Mutation analysis for Pompe disease requires whole gene
sequencing of the GAA gene, because more than 250 mutations
spanning the whole gene are known and barring a few mutations which occur
commonly in certain Dutch and Canadian populations, no definite ethnic
preponderance of any particular mutation exists [2]. If the proband’s
mutations can be identified, targeted mutation analysis of the fetal DNA
would provide unequivocal evidence of the fetal disease status and would
be the gold standard for prenatal diagnosis [3]. However, molecular
genetic testing may not be applicable as a prenatal diagnostic tool for
all cases because GAA gene sequencing is a costly technique and is
being done only in a few centers world-wide (not presently being done in
any center in India) and it can be applied only if the proband’s mutations
are known/ identified.
These cases illustrate the fact that at times, enzyme
assay could provide equivocal results and a decision to terminate a
pregnancy based on these results alone could turn out to be erroneous.
Therefore, wherever feasible, enzyme assay results should be confirmed by
targeted mutation analysis for the prenatal diagnosis of Pompe disease, as
also for all other lysosomal storage disorders.
Acknowledgement: ICMR, New Delhi.
Contributors: SP conceived the paper and along with
RP, analyzed data, reviewed literature and drafted the manuscript. CR and
DB helped with genetic analysis, interpreted their results and contributed
to the final draft.
Funding: None. Competing interests: None
stated.
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