Similar splice-site mutations of the ATP7A gene lead to different phenotypes: Classical Menkes disease or occipital horn syndrome

More than 150 point mutations have now been identified in the ATP7A gene. M ost of these mutations lead to the classic form of Menkes disease (MD), and a few lead to the milder occipital horn syndrome (OHS). To get a better un derstanding of molecular changes leading to classic MD and OHS, we took adv antage of the unique finding of three patients with similar mutations but d ifferent phenotypes. Although all three patients had mutations located in t he splice-donor site of intron 6, only two of the patients had the MD pheno type; the third had the OHS phenotype. Fibroblast cultures from the three p atients were analyzed by reverse transcriptase (RT)-PCR to try to find an e xplanation of the different phenotypes. In all three patients, exon 6 was d eleted in the majority of the ATP7A transcripts. However, by RT-PCR amplifi cation with an exon 6-specific primer, we were able to amplify exon 6-conta ining mRNA products from all three patients, even though they were in law a bundance. Sequencing of these products indicated that only the patient with OHS had correctly spliced exon 6-containing transcripts. We used two diffe rent methods of quantitative RT-PCR analysis and found that the level of co rrectly spliced mRNA in this patient was 2%-5% of the level found in unaffe cted individuals. These findings indicate that the presence of barely detec table amounts of correctly spliced ATP7A transcript is sufficient to permit the development of the milder OHS phenotype, as opposed to classic MD.