Radboud Centre for Mitochondrial Medicine Pediatric MRI score

Phenotype-Genotype Analysis Based on Molecular Classification in 135 Children With Mitochondrial Disease

OBJECTIVES

Over the past decades, mitochondrial disease classification has been mainly based on molecular defects. We aim to analyze phenotype-genotype correlation of mitochondrial disorders according to molecular classification.

METHODS

In this cohort study, we identified 135 individuals diagnosed with mitochondrial disorders, and all patients were divided into four subgroups based on molecular functions: the Respiratory Chain group (including subunits and assembly proteins in the respiratory chain), the Protein Synthesis group (including mitochondrial RNA metabolism, mitochondrial translation), the mitcohindrial DNA (mtDNA) Replication group, and the Others group (including cofactors, homeostasis, substrates, and inhibitors).

RESULTS

We found that in China, patients with the mtDNA variant constituted a large percentage of mitochondrial disease and were associated with a male preponderance in the Respiratory Chain group, whereas those in the Protein Synthesis group showed a relatively later onset and higher serum lactate level. In contrast, patients with nuclear DNA variants were younger at onset, with no specific lactate or cranial imaging features, especially in the Others group, which contained several mitochondrial diseases with corresponding treatment.

CONCLUSION

The mtDNA was recommended to detect first in patients with typical lactate and cranial imaging features. A broader consideration and detection are necessary for a better prognosis in an atypical patient.

High predictive value of brain MRI imaging in primary mitochondrial respiratory chain deficiency

Background

Because the mitochondrial respiratory chain (RC) is ubiquitous, its deficiency can theoretically give rise to any symptom in any organ or tissue at any age with any mode of inheritance, owing to the twofold genetic origin of respiratory enzyme machinery, that is, nuclear and mitochondrial. Not all respiratory enzyme deficiencies are primary and secondary or artefactual deficiency is frequently observed, leading to a number of misleading conclusions and inappropriate investigations in clinical practice. This study is aimed at investigating the potential role of brain MRI in distinguishing primary RC deficiency from phenocopies and other aetiologies.

Methods

Starting from a large series of 189 patients (median age: 3.5 years (8 days–56 years), 58% males) showing signs of RC enzyme deficiency, for whom both brain MRIs and disease-causing mutations were available, we retrospectively studied the positive predictive value (PPV) and the positive likelihood ratio (LR+) of brain MRI imaging and its ability to discriminate between two groups: primary deficiency of the mitochondrial RC machinery and phenocopies.

Results

Detection of (1) brainstem hyperintensity with basal ganglia involvement (P≤0.001) and (2) lactate peak with either brainstem or basal ganglia hyperintensity was highly suggestive of primary RC deficiency (P≤0.01). Fourteen items had a PPV>95% and LR+ was greater than 9 for seven signs. Biallelic SLC19A3 mutations represented the main differential diagnosis. Non-significant differences between the two groups were found for cortical/subcortical atrophy, leucoencephalopathy and involvement of caudate nuclei, spinothalamic tract and corpus callosum.

Conclusion

Based on these results and owing to invasiveness of skeletal muscle biopsies and cost of high-throughput DNA sequencing, we suggest giving consideration to brain MRI imaging as a diagnostic marker and an informative investigation to be performed in patients showing signs of RC enzyme deficiency.