No abstract

BackgroundMenkes disease (MD) is an X-linked, fatal neurodegenerative disorder of copper metabolism, caused by mutations in the ATP7A gene. Thirty-three Menkes patients in whom no mutation had been detected with standard diagnostic tools were screened for exon duplications in the ATP7A gene.MethodsThe ATP7A gene was screened for exon duplications using multiplex ligation-dependent probe amplification (MLPA). The expression level of ATP7A was investigated by real-time PCR and detailed analysis of the ATP7A mRNA was performed by RT-PCR followed by sequencing. In order to investigate whether the identified duplicated fragments originated from a single or from two different X-chromosomes, polymorphic markers located in the duplicated fragments were analyzed.ResultsPartial ATP7A gene duplication was identified in 20 unrelated patients including one patient with Occipital Horn Syndrome (OHS). Duplications in the ATP7A gene are estimated from our material to be the disease causing mutation in 4% of the Menkes disease patients. The duplicated regions consist of between 2 and 15 exons. In at least one of the cases, the duplication was due to an intra-chromosomal event. Characterization of the ATP7A mRNA transcripts in 11 patients revealed that the duplications were organized in tandem, in a head to tail direction. The reading frame was disrupted in all 11 cases. Small amounts of wild-type transcript were found in all patients as a result of exon-skipping events occurring in the duplicated regions. In the OHS patient with a duplication of exon 3 and 4, the duplicated out-of-frame transcript coexists with an almost equally represented wild-type transcript, presumably leading to the milder phenotype.ConclusionsIn general, patients with duplication of only 2 exons exhibit a milder phenotype as compared to patients with duplication of more than 2 exons. This study provides insight into exon duplications in the ATP7A gene.

Disease-causing variants in ATP7A lead to two different phenotypes associated with copper deficiency; a lethal form called Menkes disease (MD), leading to early death, and a much milder form called occipital horn syndrome (OHS). Some investigators have proposed that an ATP7A transcript missing exon 10 leads to a partly active protein product resulting in the OHS phenotype. Here, we describe an individual with OHS, a biology professor, who survived until age 62 despite a splice site mutation, leading to skipping of exon 15. ATP7A transcripts missing exon 10, or exon 15 preserve the reading frame, but it is unknown if either of these alternative transcripts encode functional protein variants. We have investigated the molecular consequence of splice site mutations leading to skipping of exon 10 or exon 15 which have been identified in individuals with OHS, or MD. By comparing ATP7A expression in fibroblasts from three individuals with OHS (OHS-fibroblasts) to ATP7A expression in fibroblasts from two individuals with MD (MD-fibroblasts), we demonstrate that transcripts missing either exon 10 or exon 15 were present in similar amounts in OHS-fibroblasts and MD-fibroblasts. No ATP7A protein encoded from these transcripts could be detected in the OHS and MD fibroblast. These results, combined with the observation that constructs encoding ATP7A cDNA sequences missing either exon 10, or exon 15 were unable to complement the high iron requirement of the ccc2Δ yeast strain, provide evidence that neither a transcript missing exon 10 nor a transcript missing exon 15 results in functional ATP7A protein. In contrast, higher amounts of wild-type ATP7A transcript were present in the OHS-fibroblasts compared with the MD-fibroblasts. We found that the MD-fibroblasts contained between 0 and 0.5% of wild-type ATP7A transcript, whereas the OHS-fibroblasts contained between 3 and 5% wild-type transcripts compared with the control fibroblasts. In summary these results indicate that protein variants encoded by ATP7A transcripts missing either exon 10 or exon 15 are not functional and not responsible for the OHS phenotype. In contrast, expression of only 3-5% of wild-type transcript compared with the controls permits the OHS phenotype.