Three apparent pseudo-deficiency alleles in the IDUA gene identified by newborn screening. L. M. Pollard1, S. R. Braddock2, K. M. Christensen2, D. J. Boylan2, L. D. Smith3, B. A. Heese3, A. M. Atherton3, C. E. Lawson3, M. E. Strenk3, M. Willing4, L. Manwaring4, T. C. Wood1 1) Biochemical Genetics Laboratory, Greenwood Genetic Center, Greenwood, SC; 2) Division of Medical Genetics, SSM Cardinal Glennon Children's Medical Center, St. Louis, MO; 3) Division of Genetics, Children's Mercy Hospitals and Clinics, Kansas City, MO; 4) Department of Pediatrics, Washington University School of Medicine, St. Louis, MO.
Pseudo-deficiency has been reported for several lysosomal enzymes in which a phenotypically unaffected patients cells demonstrate deficient activity for a specific enzyme using an artificial substrate in vitro. Examples of this phenomenon are rare because they are typically identified in family members being evaluated for carrier status. The recent initiation of newborn screening for several lysosomal storage disorders has resulted in testing large numbers of unaffected individuals for these conditions. Our laboratory has received samples from 14 infants with an abnormal newborn screen for alpha-iduronidase, the enzyme responsible for Hurler syndrome (MPS I), for diagnostic enzyme analysis using a 4-methylumbelliferone (4-MU) substrate. Of these 14 cases, five had normal alpha-iduronidase activity (> 6 nmol/hr/mg) and one is affected with MPS I. However, the remaining eight infants had alpha-iduronidase activities below normal, but near or above the affected cut-off (2 nmol/hr/mg). Molecular analysis of these patients revealed three common sequence alterations in IDUA: c.235G>A (p.A79T), c.246C>G (p.H82Q) and c.965T>A (p.V322E). Three African American infants are homozygous for p.A79T, which has only been observed in African Americans, with an allele frequency of 2.8%. Two African American infants are heterozygous for both p.A79T and p.V322E; the latter is predicted to be damaging by both PolyPhen and SIFT, and has an allele frequency < 1% in both African and European Americans. Additionally, one African American infant is heterozygous for p.A79T and a p.D223N change, which is only detected in African Americans, with an allele frequency <1%. Finally, p.H82Q, with an allele frequency <1% in both European and African Americans, was detected in two infants: homozygous in a Caucasian infant, and heterozygous in compound with p.V322E in a biracial infant. It is too early to rule out MPS I in these patients based on clinical features; however, of the six who have had urine studies performed, all had normal chromatography/electrophoresis. We will perform enzyme analysis for these eight patients using a tandem mass spectrometry substrate to determine if the proposed pseudo-deficiency is specific to the 4-MU substrate used in our diagnostic assay. These findings indicate that pseudo-deficiency is a common phenomenon for alpha-iduronidase, which could have a negative impact on newborn screening for MPS I.
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