Late-onset respiratory failure due to TK2 mutations causing multiple mtDNA deletions

Abstract

Mutations in nuclear genes involved in the maintenance of mitochondrial DNA (mtDNA) are associated with an extensive spectrum of clinical phenotypes, manifesting as either mtDNA depletion syndromes or multiple mtDNA deletion disorders.(1.)

Figure. Clinical and molecular genetic characterization

(A) Clinical presentation includes mild ptosis despite previous corrective surgery, subtle progressive external ophthalmoplegia, and facial weakness. The patient has marked wasting of proximal musculature, including the sternocleidomastoids. Neck flexion and extension were weak, resulting in head drop. (B) Sequential COX-SDH histochemistry demonstrates focal COX deficiency affecting a number of fibers. (C) [i] gDNA TK2 sequencing reveals 2 novel heterozygous variants—c.103C>T (p.Gln35*) and c.582G>T (p.Lys194Asn); [ii and iii] allele-specific cDNA analysis confirms both variants are allelic; [iv] wild-type reference sequence. (D) Assessment of individual COX-positive and COX-deficient fibers reveals high levels of clonally expanded MTND4 deletion in the majority of COX-deficient fibers, while only 2 COX-positive fibers have mitochondrial DNA (mtDNA) deletions quantified at levels of >60% mutation load. This profile in individual fibers is consistent with a diagnosis of multiple mtDNA deletions. (E) Assessment of mtDNA copy number in individual COX-positive and COX-deficient fibers shows no evidence of quantitative mtDNA copy number loss, as comparable mtDNA levels are noted in both groups of fibers. (F) Biochemical assessment of TK2 activity in the Escherichia coli model reveals a marked decrease in activity due to the p.Lys194Asn substitution (38 ± 2% of controls; n = 4), confirming pathogenicity.