Role of electron microscopy in the diagnosis of mitochondrial cytopathies

Endothelial ultrastructural alterations of intramuscular capillaries in infantile mitochondrial cytopathies: "mitochondrial angiopathy"

Electron microscopy (EM) is a reliable method for diagnosing mitochondrial diseases in striated muscle biopsy in infancy. Ultrastructural alterations in mitochondria of myofibers are well documented, but there are few studies of endothelial involvement in intramuscular capillaries. Quadriceps femoris biopsies of five representative infants and toddlers, ages neonate to 3.5 years, were performed because of clinical and laboratory data consistent with mitochondrial disease without mitochondrial DNA (mtDNA) mutations and likely with nuclear DNA mutations. Pathological studies included histochemistry, EM, respiratory chain enzymatic assay and mtDNA sequencing and deletion/duplication analysis. EM demonstrated frequent and severe alterations of mitochondria in capillary endothelium. The most constant changes included: either too few or fragmented cristae; stacked and whorled cristae; paracrystallin structures that often were large and spheroid with stress fractures; closely apposed membranes of granular endoplasmic reticulum surrounding mitochondria with loss of the normal intervening layer of cytoplasm; long narrow, thin looped microvilli extending into the lumen; and thick microvilli containing large, abnormal mitochondria. We conclude that mitochondrial cytopathies in early life exhibit more severe ultrastructural alterations in the endothelium than in myofibers and that paracrystallin body structure differs, perhaps due to less rigid surrounding structures. This distribution may explain the frequent lack of prominent histochemical and biochemical abnormalities in muscle biopsies of young patients. Endothelial changes do not distinguish the genetic defects. Vascular involvement in brain contributes to cerebral lesions and neuronal death by impairment of molecular and nutrient transport and ischemia; endothelium in muscle may reflect similar changes.

Impairment of mitochondrial tRNAIle processing by a novel mutation associated with chronic progressive external ophthalmoplegia

We report a sporadic case of chronic progressive external ophthalmoplegia associated with ragged red fibers. The patient presented with enlarged mitochondria with deranged internal architecture and crystalline inclusions. Biochemical studies showed reduced activities of complex I, III and IV in skeletal muscle. Molecular genetic analysis of all mitochondrial tRNAs revealed a G to A transition at nt 4308; the G is a highly conserved nucleotide that participates in a GC base-pair in the T-stem of mammalian mitochondrial tRNA(Ile). The mutation was detected at a high level (approx. 50%) in muscle but not in blood. The mutation co-segregated with the phenotype, as the mutation was absent from blood and muscle in the patient's healthy mother. Functional characterization of the mutation revealed a six-fold reduced rate of tRNA(Ile) precursor 3' end maturation in vitro by tRNAse Z. Furthermore, the mutated tRNA(Ile) displays local structural differences from wild-type. These results suggest that structural perturbations reduce efficiency of tRNA(Ile) precursor 3' end processing and contribute to the molecular pathomechanism of this mutation.

Electron microscopic findings in levator muscle biopsies of patients with isolated congenital or acquired ptosis

OBJECTIVE

Systemic mitochondriopathies as chronic progressive external ophthalmoplegia (CPEO) are frequently associated with ptosis. We investigated whether mitochondrial abnormalities in the levator muscle are also found in patients with isolated congenital or acquired ptosis showing no other signs of mitochondrial cytopathy.

METHODS

Biopsies of levator muscle were taken during surgery from 24 patients with isolated congenital (group 1) or early-onset acquired ptosis (group 2). All patients were given a thorough clinical examination before and after surgery. Ultrathin muscle sections were examined by transmission electron microscopy. The findings were compared with biopsies from five patients with CPEO (positive control) and two patients with traumatic ptosis or pseudoptosis (negative control).

RESULTS

The mean levator function equalled 7.3 mm (range 4-10 mm) in group 1 and 12.8 mm (range 9-15 mm) in group 2. Eight out of 11 patients in group 1 and eight out of 13 patients in group 2 were found to have mitochondrial alterations such as megamitochondria, mitochondrial matrix alterations and abnormal cristae, similar to CPEO. Within group 1 and 2, no significant clinical differences were found between patients with and without mitochondrial abnormalities.

CONCLUSION

Mitochondrial alterations were found in a surprisingly large proportion of levator biopsies from patients with isolated congenital or early-onset acquired ptosis. There was no statistically significant correlation between mitochondrial alterations and levator function. Our findings suggest that the ultrastructural assessment of mitochondria in the eyelid muscle is a valuable tool, and may guide further biochemical and mutation screening tests that will help to understand the etiopathology of this disease.