Occipital Horn Syndrome as a Result of Splice Site Mutations in ATP7A. No Activity of ATP7A Splice Variants Missing Exon 10 or Exon 15

Abstract

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.

Deep intronic variant causes aberrant splicing of ATP7A in a family with a variable occipital horn syndrome phenotype

“…Analysis of RNA from V:2 showed a higher proportion of canonical ATP7A transcripts (13.9 %) compared to the cases reported previously ( Yasmeen et al, 2014 ; Møller et al . ), with the majority of the non-canonical transcripts featuring exon 5 skipping, rather than pseudoexon inclusion, as a result of the ATP7A c.1544-872C>G variant.…”

“…The proportions of canonical transcripts in the less severely affected patients were 11 % and 2 %, suggesting that the proportion of canonical transcripts required to prevent the onset of the severest phenotypes lies below a 2 % threshold. This hypothesis was supported by the subsequent investigation of five patients with splice-site variants leading to exon skipping ( Møller et al .). In one patient with Menkes disease, 0.5 % of transcripts were canonical, which increased to 3.5 % canonical transcripts in patents with OHS.…”

Genetic diversity, tissue-specific expression, and functional analysis of the ATP7A gene in sheep

“…Menkes is an infantile, fatal, inherited copper deficiency characterized by progressive neurological damage that eventually causes death. A mild form of Menkes disease is called occipital angle syndrome (OHS) ( Moller et al, 2021 ; Yasmeen et al, 2014 ), which showed a symptom of exophytic bone warts ( De Feyter et al, 2023 ) that is similar to the bony core of the horn. So we suspect ATP7A can have an important role in the ovine horn.…”