ClinVar

Description

The c.815-27T>C intronic alteration consists of a T to C substitution 27 nucleotides before coding exon 10 in the NUBPL gene and is predicted to affect the native branch site. Based on data from the Genome Aggregation Database (gnomAD) database, the NUBPL c.815-27T>C alteration was observed in 0.35% (992/280082) of total alleles studied (including 10 homozygotes), with a frequency of 1.24% (310/24978) in the European (Finnish) subpopulation. The [c.166G>A (p.G56R); c.815-27T>C] complex allele has been reported in the homozygous state, compound heterozygous state, and confirmed in trans with a second pathogenic allele in multiple unrelated patients with mitochondrial encephalomyopathy (Calvo, 2010; Kevelam, 2013). Emerging evidence is suggestive that the NUBPL [c.166G>A (p.G56R); c.815-27T>C] complex allele is likely pathogenic when these alterations are in cis; however, the clinical significance of these alterations in isolation remains uncertain. The c.815-27T nucleotide is highly conserved in available vertebrate species. Functional analysis of cultured fibroblasts from a patient bearing the [c.166G>A (p.G56R); c.815-27T>C] complex allele in trans with a complex rearrangement, as well as a control patient who was heterozygous for only the c.815-27T>C alteration, demonstrated an aberrant RT-PCR pattern with three distinct transcripts: wild-type, a lengthened transcript resulting from a cryptic splice site which introduces an additional 72 bp and results in a frameshift (p.G272Vfs*11), as well as a truncated transcript generated due to exon 10 skipping resulting in a frameshift (p.D273Qfs*31) (Tucker, 2012). Analysis by qRT-PCR and Western blot showed that the heterozygous control with only the c.815-27T>C alteration had reduced mRNA expression (74%) and protein expression (59%) compared to wild type controls, and the patient with the [c.166G>A (p.G56R); c.815-27T>C] complex allele and complex rearrangement on the other allele had more significant reduction in mRNA expression (15%) and undetectable protein expression (Tucker, 2012). No defective function was identified when the protein with the G56R missense change was over-expressed (Tucker, 2012). The p.D273Qfs*31 transcript is completely non-functional in yeast assays (Wydro, 2013; Maclean, 2018). In silico splice site analysis predicts that the c.815-27T>C alteration will weaken the native splice acceptor site and will result in the creation or strengthening of a novel splice acceptor site. Based on the available evidence, this alteration is classified as likely pathogenic.