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Letters |
1-Antitrypsin Deficiency as a Result of Compound Heterozygosity for the Z and MHeerlen Alleles
Departments of
1
Clinical Chemistry, and
2
Pulmonary Diseases, Canisius Wilhelmina Hospital, Weg door Jonkerbos 100, NL-6532 SZ Nijmegen, The Netherlands
aAuthor for correspondence. Fax 31-24-3658671; e-mail m.d.metz{at}cwz.nl.
To the Editor:
The specificity of many common rapid molecular techniques for
only one or a few mutations may produce confusion when a rare mutation
is present. We describe a patient in whom the
MHeerlen allele of the AAT gene for
the protease inhibitor 
1-antitrypsin (AAT) was
missed by widely used molecular genotyping (1)(2)(3)(4) and
phenotyping methods for mutations in the AAT gene. The
majority of these assays were developed to detect the S and Z mutations
(Glu264
Val and Glu342Lys, respectively), which are the most
prevalent mutations in a Caucasian population (5).
A 40-year-old woman with a serum AAT of 0.3 g/L (reference interval, 0.8–2.0 g/L; Beckman Array), suffering from dyspnea and a familial history of emphysema was genotyped as PIMZ. However, isoelectric focusing (IEF) analysis (Blood Transfusion Service, Amsterdam, The Netherlands) revealed only the Z isoform. Hence she was considered to be a ZZ phenotype, in agreement with her low serum AAT and her early emphysema. The only sibling/sister of the propositus was also affected by pulmonary dysfunction and was also genotyped as PIMZ with a ZZ phenotype. The sister’s children had a normal phenotype, M; their genotype was MM. However, based on the phenotype of their mother, the children were expected to have at least a partial Z phenotype. We decided to check for the presence of other mutations in the AAT gene that may account for the difference in results between the two tests.
DNA sequence analysis (ABI PRISM 3700) of the amplicon generated to
test the presence of the Z allele revealed the presence of the
MHeerlen mutation (Pro369Leu), which was
described previously by Hofker et al. (6). Hence, the
patient’s actual genotype was PIMHeerlenZ. The
MHeerlen mutation is thought to encode for the
synthesis of a misfolded protein that is degraded in the endoplasmic
reticulum of hepatocytes without being secreted. This explains the
absence of a M-like protein in IEF gels. The low serum AAT (0.3 g/L) is
in fact the result of expression of only the Z allele. Both the
MHeerlen and Z alleles could be traced back to a
previous generation of this family (Table 1
); the MHeerlen allele, but not the Z
allele, also appeared in later generations. Both children of the
propositus’ sister carried the MHeerlen allele
and a nonmutated M allele, as verified by sequence analysis. Therefore,
the father must have at least one nonmutated M allele. In all cases,
serum AAT concentrations corresponded with expected protein expression
of the respective genotypes, assuming that the
MHeerlen allele does not significantly contribute
to serum AAT concentrations (Table 1
). Because the presence of the
MHeerlen allele fully accounted for the
described phenotypes, no efforts were undertaken to look for other
mutations in the AAT gene.
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Obviously, none of the routine tests [PCR-restriction fragment length polymorphism (RFLP) analysis and IEF] detected the cause of the partial AAT deficiency in the propositus, although the phenotypic IEF analysis was in agreement with her diminished pulmonary function. Genotyping for S and Z alleles has become a routine laboratory test. However, it is important to realize that these tests might give misleading results, as shown in this case. One should always be aware of the possible presence of alleles such as MHeerlen if a standard genotypic analysis is not in agreement with serum AAT values and/or IEF analysis. Recognition and awareness of these caveats warrants reliable molecular genotyping test results. If an allelic variation such as MHeerlen is suspected in an individual, it is important to realize that there are many possible variants that may also account for discrepancies such as those reported here (7). To detect any of these variants one could use an aselective prescreening method to detect the presence of mutations per se. A variety of methods to detect single nucleotide polymorphisms and small deletions or insertions are currently available, e.g., single strand conformation polymorphism analysis, heteroduplex analysis, and enzymatic mutation detection. If an aberration is found, the exact mutation can be identified by sequence analysis. The alternative would be to sequence entire genes or coding regions. However, AAT gene mutations are all named after the places where they were initially described (8). Therefore, the geographic origin of an individual might give a clue to the nature of the possible variation in question. In fact, it turned out that the grandparents of the propositus used to live in the town of Heerlen.
To our knowledge, the combination of a Z and a MHeerlen allele has not been described before now. As can be expected from the AAT concentration, the pulmonary function in the two sisters is affected to a degree similar to that seen in Pi ZZ individuals. One can only speculate about potential liver involvement at a young age because deposition of AAT in hepatocytes does not seem to complicate MHeerlen homozygosity. The risk of juvenile cirrhosis in Pi ZZ individuals, as a result of deposition of AAT in inclusion bodies, is well known. Compound heterozygotes for Z and MHeerlen might have an intermediate risk for liver involvement.
References
1-antitrypsin deficiency. Clin Chem 1993;39:2157-2162.[Abstract]
-1-proteinase inhibitor deficiency alleles Pi*Z and Pi*S by DNA analysis. Eur J Clin Chem Clin Biochem 1996;34:761-764.[ISI][Medline]
[Order article via Infotrieve]
1-antitrypsin Z and S mutations. Clin Chem 1997;43:403-404.1-antitrypsin deficiency alleles Pi*Z and Pi*S by real-time fluorescence PCR and melting curves. Clin Chem 1999;45:1872-1875.-1 antitrypsin deficiency. Ann Clin Biochem 1997;34:230-246.
Leu substitution in codon 369 of the -1-antitrypsin deficiency variant PI MHeerlen. Hum Genet 1989;81:264-268.[Medline]
[Order article via Infotrieve]
1-antitrypsin deficiency. Chest 1997;111:123S-128S.The following articles in journals at HighWire Press have cited this article:
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  J. Prins, B. B. van der Meijden, R. J. Kraaijenhagen, and J. P.M. Wielders Inherited Chronic Obstructive Pulmonary Disease: New Selective-Sequencing Workup for {alpha}1-Antitrypsin Deficiency Identifies 2 Previously Unidentified Null Alleles Clin. Chem., January 1, 2008; 54(1): 101 - 107. [Abstract] [Full Text] [PDF]
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