Testing

Analysis of urinary glycosaminoglycans (heparan sulfate and dermatan sulfate) was the earliest method available for diagnosis of MPS I and remains a useful preliminary investigative test. However, definitive diagnosis is now established by enzyme assays using fluorogenic substrates specific for α-L-iduronidase.[1],[2],[3]

Cultured fibroblasts, leukocytes or plasma are generally used, the choice of which depends on the preference of the testing laboratory; some diagnostic laboratories post procedures for sample preparation and shipment on their websites. Accurate testing is critical to ascertain the diagnosis, as other mucopolysaccharide diseases exhibit clinical features similar to those of MPS I.

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Carrier testing

Families with a history of MPS I frequently request testing for carrier status. Unfortunately, to date the analysis of α-L-iduronidase enzyme activity does not provide definitive carrier information. This is related to the fact that there is considerable overlap between the normal and heterozygous ranges and that pseudodeficiency of α-L-iduronidase has been reported.[3] Thus, there are problems associated with interpreting results of enzyme levels in the general population. Moreover, even if one can accurately determine the carrier status of a relative of an MPS I individual, a determination of the carrier status of the unrelated spouse is not possible. Therefore the value of carrier testing is limited with respect to the purpose of identifying couples who may be at risk of having an affected child.

As noted earlier, the a priori risk of being a carrier (i.e. having a defective allele) in the general population is 1:160. Unfortunately, the heterogeneity of mutations that underlie MPS I and the technologies available to assess gene mutations do not currently allow for carrier detection by molecular methods.[3]

Prenatal diagnosis

Prenatal diagnosis is routinely carried out on cultured cells from amniotic fluid or chorionic villus biopsies using the same enzyme assay that is used for α-L-iduronidase in cultured fibroblasts or leukocytes. Some difficulty has been reported with MPS I prenatal diagnosis because of the low levels of α-L-iduronidase present in normal chorionic villi, but this may be overcome by taking appropriate precautions.[4] The results obtained from uncultured materials may require confirmation with cultured cells. Measurement of glycosaminoglycans in amniotic fluid (or α-L-iduronidase activity) is complicated by the high glycosaminoglycan excretion of fetuses. Measurement of radiolabeled (35S)-glycosaminoglycan accumulation by cultured cells,[5] while not practical, may be particularly useful for establishing cases of pseudodeficiency. Molecular-based prenatal diagnosis can be done, if the mutations carried by the parents are known.

Molecular genetic testing and molecular diagnosis

When considering DNA-based tests one must take into account the great heterogeneity of mutations underlying MPS I. Mutant alleles need to be identified for the specific family before molecular diagnosis can be undertaken for members at risk. Many patients will likely be compound heterozygotes; thus both mutant alleles must be known for carrier testing to be helpful for the family. Once the mutant allele(s) are identified (either by the mutation itself, or by an intragenic polymorphism), molecular diagnosis may become easier and require less material, which is important for prenatal testing.[6] However, the large number of private mutations may keep mutation analysis impractical for some families. Until mutation analysis becomes more readily available, diagnosis should be established by enzyme assay. DNA-based diagnosis is the only definitive test for determining carrier status but will likely have limited value in individuals who are at low risk of being carriers.

Genotype-phenotype correlations

Genotype-phenotype correlations in MPS I are complex and further research is required before they can be clinically useful. To date, research has indicated that all nonsense mutations (including the two most common mutations, W402X and Q70X) when present in a homozygous or compound heterozygous state always confer a phenotype with severe disease and CNS involvement. [7] The clinical consequences of other types of mutations (missense, deletion, insertion and splice-site mutations) are not as straightforward and have been identified in patients with severe, intermediate, and mild disease in the homozygous and heterozygous form, even when in association with a known severe nonsense allele. Thus, prediction of phenotype in patients shown to have at least one missense, deletion, insertion, or splice-site mutation can only be made by looking at the phenotype of patients who have previously presented with the mutations.

The 2 most common mutations identified in patients with a less severe phenotype without CNS involvement are the missense and splice-site mutations R89Q and 678-7g>a, respectively. Together they account for 30%-40% of mutations in such individuals.

Established mutations represent only a fraction of known cases, and as these studies progress, it is expected that more mutations will be discovered that are null (mutations resulting in complete absence of α-L-iduronidase activity) and lead to severe MPS I disease with CNS involvement. Mutations that cause less severe forms of MPS I disease without CNS involvement are expected to be limited and mostly of the missense type.[6] Although some genotype-phenotype correlations have been established, some striking variations in disease manifestation have also been reported in affected siblings with the same mutations. In general, patients with slight residual enzyme activity will have a less severe phenotype.

References

  1. Hall, C.W., Liebaers, I., Di Natale, P., and Neufeld, E.F. (1978) Enzymatic diagnosis of the genetic mucopolysaccharide storage disorders. Methods Enzymol. 50: 439.
  2. Kresse, H., von Figura, K., Klein, U., Glossl, J., Paschke, E., and Pohlmann R. (1982) Enzymatic diagnosis of the genetic mucopolysaccharide storage disorders. Methods Enzymol. 83: 559.
  3. Neufeld EF, Muenzer J. The Mucopolysaccharidoses. In: Valle D, Beaudet AL, Vogelstein B, Kinzler KW, Antonarakis SE, Ballabio A, Gibson K, Mitchell G. eds. The Online Metabolic and Molecular Bases of Inherited Disease New York, NY: McGraw-Hill; 2014. http://ommbid.mhmedical.com/content.aspx?bookid=971§ionid=62642135. Accessed April 11, 2017.
  4. Young, E.P. (1992) Prenatal diagnosis of Hurler disease by analysis of α-L-iduronidase in chorionic villi. J Inherit Metab Dis. 15: 224.
  5. Fratantoni, J.C., Hall, C.W., and Neufeld, E.F. (1968a) Hurler and Hunter syndromes: Mutual correction of the defect in cultured fibroblasts. Science. 162: 570.
  6. Neufeld EF, Muenzer J. The Mucopolysaccharidoses. In: Valle D, Beaudet AL, Vogelstein B, Kinzler KW, Antonarakis SE, Ballabio A, Gibson K, Mitchell G. eds. The Online Metabolic and Molecular Bases of Inherited Disease New York, NY: McGraw-Hill; 2014. http://ommbid.mhmedical.com/content.aspx?bookid=971§ionid=62642135. Accessed April 11, 2017.
  7. Scott, H.S., Bunge, S., Gal, A., Clarke, L.A., Morris, C.P., and Hopwood, J.J. (1995) Molecular genetics of mucopolysaccharidosis type I: Diagnostic, clinical, and biological implications. Hum Mutat. 6: 288.