Abstract
Wilson disease (WD) is an autosomal recessive disease of copper excess due to pathogenic variants in the ATP7B gene coding for a copper-transporting ATPase. We present a 5-year-old girl with the homozygous frame shift variant NM_000053.3: c.19_20del in exon 1 of ATP7B (consecutive exon numbering with c.1 as first nucleotide of exon 1), detected by whole-exome sequencing as a secondary finding. The variant leads to a premature termination codon in exon 2. The girl exhibited no WD symptoms and no abnormalities in liver biopsy. ATP7B liver mRNA expression was comparable to healthy controls suggesting that nonsense-mediated mRNA decay (NMD) could be bypassed by the mechanism of translation reinitiation. To verify this hypothesis, a CMV-driven ATP7B minigene (pcDNA3) was equipped with the authentic ATP7B 5′ untranslated region and a truncated intron 2. We introduced c.19_20del by site-directed mutagenesis and overexpressed the constructs in HEK293T cells. We analyzed ATP7B expression by qRT-PCR, northern and western blot, and examined protein function by copper export capacity assays. Northern blot, qRT-PCR, and western blot revealed that c.19_20del ATP7B mRNA and protein is expressed in size and amount comparable to wild-type. Copper export capacity was also comparable to wild-type. Our results indicate that c.19_20del in ATP7B is able to bypass NMD by translation reinitiation, demonstrating that the classification of truncating variants as pathogenic without additional investigations should be done carefully.
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Acknowledgements
We thank Claudia Davenport for manuscript proofreading.
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This work was supported by the Deutsche Forschungsgemeinschaft (DFG, Cluster of Excellence REBIRTH; ‘From Regenerative Biology to Reconstructive Therapy’ EXC 62).
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Stalke, A., Pfister, ED., Baumann, U. et al. Homozygous frame shift variant in ATP7B exon 1 leads to bypass of nonsense-mediated mRNA decay and to a protein capable of copper export. Eur J Hum Genet 27, 879–887 (2019). https://doi.org/10.1038/s41431-019-0345-1
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DOI: https://doi.org/10.1038/s41431-019-0345-1
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