Figure 1. Comparison of COX/SDH histochemistry with complex I, IV and porin immunofluorescence.

COX/SDH and quadruple immunofluorescence were performed in serial muscle sections from patients: P1, P2, P11 and P17. Fluorescent detection was used to visualise: complex IV subunit I (COX-I) - green (488 nm), complex I subunit (NDUFB8) - purple (647 nm) and porin (mitochondrial mass) - red (546 nm). P1 (LRPPRC mutations) shows widespread COX deficiency whereas P2 (m.4175G > A MT-ND1 mutation) shows widespread NDUFB8 deficiency and preserved COX activity. Both P11 and P17 show mosaic COX deficiency. Selected muscle fibres demonstrate: (1) COX deficiency (COX-deficient fibres) with absent COX-I immunoreactivity, (2) decreased COX activity (COX-intermediate fibres) and decreased level of COX-I abundance, (3) normal COX activity (COX-positive fibres) and COX-I level but absent NDUFB8 immunoreactivity, (4) apparently normal COX activity and low level of COX-I immunoreactivity. All fibres highlighted show down-regulated levels of NDUFB8 abundance. P1 is a paediatric case and thus has smaller fibre size than the other cases. Scale bars measure 50 μm.