Molecular basis of infantile reversible cytochrome c oxidase deficiency myopathy

Abstract

Childhood-onset mitochondrial encephalomyopathies are usually severe, relentlessly progressive conditions that have a fatal outcome. However, a puzzling infantile disorder, long known as 'benign cytochrome c oxidase deficiency myopathy' is an exception because it shows spontaneous recovery if infants survive the first months of life. Current investigations cannot distinguish those with a good prognosis from those with terminal disease, making it very difficult to decide when to continue intensive supportive care. Here we define the principal molecular basis of the disorder by identifying a maternally inherited, homoplasmic m.14674T>C mt-tRNA(Glu) mutation in 17 patients from 12 families. Our results provide functional evidence for the pathogenicity of the mutation and show that tissue-specific mechanisms downstream of tRNA(Glu) may explain the spontaneous recovery. This study provides the rationale for a simple genetic test to identify infants with mitochondrial myopathy and good prognosis.

Figure 1

Pedigrees of three families suggested a maternal inheritance. RFLP analysis for m.14674T>C in the family of Patient 14 revealed that the mutation is homoplasmic in all maternal individuals and absent in the fathers. The two brothers, at ages 8 and 10 years, showed signs of a mild residual myopathy. P14 (III:1) has a myopathic face and both patients (III:1, III:2) show scapular winging.

Figure 2

Histochemical staining of the muscle biopsy from Patient 7 at 3 months of age (A and B) and 9 years (C and D) and of the muscle from his asymptomatic mother (E and F). The early biopsy confirmed severe mitochondrial myopathy with RRF (A) and even more COX-negative fibres (B). These changes significantly improved but did not completely disappear at 9 years of age (C and D). The muscle biopsy from the mother revealed a few COX-negative (F), SDH hyper-reactive (E) fibres (stars). A, C: Gomori trichrome; B, D, F: COX; E: SDH stain. Magnification: A, B 100×, C, D, E, F 200×.

Figure 3

(A) Northern blot for mt-tRNA^Glu showed severely decreased steady-state levels in skeletal muscle of Patient 11 at 1 month of age and slightly higher, but still significantly decreased levels, after clinical recovery at 9 years of age, as well as in muscle biopsies of her asymptomatic mother (M). Data from skeletal muscle are not shown for Patients 7 and 14. Myoblasts of Patient 14 showed steady-state levels similar to late biopsies and to biopsies of a healthy mother. Patient mt-tRNA^Glu steady-state levels normalized to mt-tRNA^Leu(UUR) levels are expressed relative to normalized control levels. (B) Immunoblotting showed decreased levels of the mitochondrial COXI, COXII and complex I (NDUFB8) subunits in the biopsy of Patient 11 at 1 month of age, which returned to normal at 9 years of age, and was also normal in her asymptomatic mother. The same subunits were slightly increased in myoblasts of Patient 14.

Figure 4

(A) Schematic representation of m.14674T>C in mt-tRNA^Glu, F: forward sequence, R: reverse sequence. (B) ³⁵S methionine pulse-labelling in myoblasts of Patients 7 and 14 revealed normal mitochondrial translation.