Neuropathological hallmarks in autopsied cases with mitochondrial diseases caused by the mitochondrial 3243A>G mutation

Abstract

The mitochondrial (m.) 3243A>G mutation is known to be associated with various mitochondrial diseases including mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes (MELAS). Their clinical symptoms have been estimated to occur with an increased mitochondrial DNA (mtDNA) heteroplasmy and reduced activity of oxidative phosphorylation (OXPHOS) complexes, but their trends in the central nervous system remain unknown. Six autopsied mutant cases and three disease control cases without the mutation were enrolled in this study. The mutant cases had a disease duration of 1-27 years. Five of six mutant cases were compatible with MELAS. In the mutant cases, cortical lesions including a laminar necrosis were frequently observed in the parietal, lateral temporal, and occipital lobes; less frequently in the frontal lobe including precentral gyrus; and not at all in the medial temporal lobe. The mtDNA heteroplasmy in brain tissue samples of the mutant cases was strikingly high, ranging from 53.8% to 85.2%. The medial temporal lobe was preserved despite an inhospitable environment having high levels of mtDNA heteroplasmy and lactic acid. OXPHOS complex I was widely decreased in the mutant cases. The swelling of smooth muscle cells in the vessels on the leptomeninges, with immunoreactivity (IR) against mitochondria antibody, and a decreased nuclear/cytoplasmic ratio of choroidal epithelial cells were observed in all mutant cases but in none without the mutation. Common neuropathological findings such as cortical laminar necrosis and basal ganglia calcification were not always observed in the mutant cases. A high level of mtDNA heteroplasmy was observed throughout the brain in spite of heterogeneous cortical lesions. A lack of medial temporal lesion, mitochondrial vasculopathy in vessels on the leptomeninges, and an increased cytoplasmic size of epithelial cells in the choroid plexus could be neuropathological hallmarks helpful in the diagnosis of mitochondrial diseases.

Keywords: MELAS; choroidal epithelial cell swelling; mitochondrial 3243A>G mutation; mitochondrial vasculopathy; mtDNA heteroplasmy; stroke-like episodes.

FIGURE 1

Common neuropathological findings of cases with the m.3243A>G mutation. (A, G) Superficial vacuolation of the cerebral cortex in case M5. A high magnification view demonstrated type II astrocytes (arrowhead) and various sizes of vacuolations surrounding neurons (arrows). (B) Laminar necrosis of the cerebral cortex in case M3. (C, D) Necrotic lesions of the basal ganglia in case M1 (C) and thalamus in case M4 (D). (E) Ischemic lesion of the cerebellum in case M4, showing loss of granule cells and Purkinje cells with Bergmann gliosis. (F) Necrosis of the cerebellum in case M4. (H) Capillary mineralization in the putamen of case M3. (I) Coarse mineralization in the globus pallidus of case M3. (J, N, R) Mitochondrial vasculopathy showing swollen smooth muscle cells of the tunica media with small cytoplasmic vacuolations and mitochondria immunoreactivity in case M4. (K, O) Normal artery in case C1. (L, P) Swollen epithelial cells of the choroid plexus in case M4. (M, Q) Normal epithelial cells of the choroid plexus in case C1. (S–U) Electron microscopic images. Smooth muscle cells of the tunica media had many cytoplasmic vacuolations, presumably compatible with the ballooned mitochondria in case M1. (A–M): HE staining; (N‐Q): immunohistochemistry against mitochondria. Bar: 5 mm for (C), 1 mm for (D, F), 200 μm for (A, B, E), 100 μm for (I), 50 μm for (J, K, N, O), 20 μm for (G, H, L, M, P, Q), 5 μm for (S), 2 μm for (R, T), and 200 nm for (U). TI, tunica intima; TM, tunica media.

FIGURE 2

Representative mitochondrial mutation analysis, and western blot in cases with the m.3243A>G mutation. (A) ApaI digestion products were seen in the mutant case (M3), in which the mtDNA heteroplasmy was calculated as 73.2%. (B) Fragments of OXPHOS complex I in case M1 were decreased compared to those in C1.

FIGURE 3

Heatmap visualization in cases with or without the m.3243A>G mutation. Each case was placed in ascending order of brain weight. In the mutant cases, cortical lesions were frequently observed in the lateral temporal, parietal, and occipital lobes, and the mtDNA heteroplasmy was entirely high. White down arrow symbols showed less than 60% of mtDNA heteroplasmy. OXPHOS complex I was decreased in the mutant cases compared to the control cases.

FIGURE 4

Projected brain images in cases with or without the m.3243A>G mutation. Mean values in each brain region were projected into the brain images. The medial temporal lobe in the mutant cases was preserved despite an inhospitable environment with high mtDNA heteroplasmy and lactic acid. OXPHOS complex I was widely decreased in the mutant cases.