Laboratory Issues

One of the issues encountered in sequence-based clinical testing is the interpretation of novel sequence variations, particularly alleles of uncertain pathogenic significance. A valuable resource for laboratories and referring clinicians is provided by the American College of Medical Genetics (ACMG), which has issued recommended standards for interpretation of sequence variations (ACMG Laboratory Practice Committee Working Group 2000; policy statement available at http://www.acmg.net/).25 Another issue pertaining to Rett syndrome DNA testing at the current time is the value of two-tier testing (e.g., sequencing followed by dosage analysis) to provide comprehensive mutation analysis of the MECP2 gene. Identification of the mutation in the proband facilitates prenatal testing in subsequent pregnancies. The majority of MECP2 point mutations are new mutations of paternal origin, with low recurrence risk. Prenatal testing is recommended for fetuses of mothers who are identified to carry a point mutation or large rearrangement in the MECP2 gene. In cases where the mother is not a carrier, prenatal testing may be sought for parental reassurance due to rare reports of gonadal mosaicism. The current focus on point mutation analysis

by DHPLC analysis. Elution profiles of MECP2 exon 4PCR products corresponding to the wild-type (red and green) and the mutant (yellow) sequences are shown. (b) Identification of the corresponding MECP2 nonsense mutation by DNA sequencing. A single base change of C to T at nucleotide 880 is identified (N), which predicts an arginine to stop substitution at residue 294 of the MeCP2 protein (880C^T, R294X).

within the coding region by sequence analysis does not rule out potential mutations in regulatory elements or other important noncoding regions of the MECP2 gene. Ideally, barriers to increased uptake of MECP2 clinical testing may be addressed by improvements in genotype-phenotype correlations, assay design, and clinical services. With the accumulation of additional genetic information and the development of improved DNA technologies, appropriate enhancements can be incorporated into clinical testing of the MECP2 gene for Rett syndrome.

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