Diagnosis of New Mutation Diseases Using the Polymerase Chain Reaction

Abstract

Duchenne muscular dystrophy (DMD) and the Lesch-Nyhan (LN) syndrome are human genetic diseases that arise from heterogeneous mutations at the dystrophin and the hypoxanthine phosphoribosyltransferase (HPRT) loci, respectively.^1,2 The genes that are deficient in the two diseases are located on the X-chromosome but have many other different features (Figure 1). While the DMD gene spans more than two megabases of DNA, is split into more than sixty exons and has a mature mRNA that is approximately 14 kb in length, the HPRT gene is just 44 kb in length³ and produces a 1.6-kb mature mRNA. Dystrophin is expressed in muscle and brain and is not detectable in other tissues.^4,5 In contrast, the HPRT gene product is ubiquitously expressed⁶ Examination of the molecular basis of LN and DMD by Southern analysis reveals that the spectrum of mutations in the two disorders is also different. Approximately 60% of DMD mutations are associated with DNA deletions⁷ while only 15% of LN patients have abnormal Southern patterns that indicate DNA deletions or other gross rearrangements.⁸ Approximately 80% of LN cases also produce HPRT mRNA that appears normal by Northern analysis, suggesting a preponderance of single DNA base changes⁹ As an additional difference, several informative DNA sequence polymorphisms have been identified within the dystrophin gene^7,10 while there are few genetic markers that are both informative and closely linked to HPRT.