Neuropathological and molecular studies in Alpers' syndrome

Abstract

Mitochondrial disease is a significant burden on human health, at least 16.5 in 100,000 individuals are at risk of developing a mitochondrial DNA disorder (Schaefer et al., 2008). Early-onset mitochondrial disease affects young children and can have devastating consequences. One such disease, Alpers’ syndrome, is a rare, autosomal recessive, early-onset neurological condition, which was first described in 1931 (Alpers 1931), characterised by refractory seizures, developmental delay, ataxia, visual abnormalities, and liver dysfunction and failure (Huttenlocher et al., 1976). It is primarily attributable to mutations in the gene POLG which encodes the only known polymerase to replicate mitochondrial DNA (Hance et al., 2005). POLG mutations lead to secondary defects of mitochondrial DNA (mtDNA), including deletion and depletion. There is a poor prognosis for patients diagnosed with Alpers’ syndrome with no effective treatments currently available and little research into this rare condition. The research presented here aims to understand the mechanisms leading to the neuropathological characteristics and clinical features in 12 patients with Alpers’ syndrome and sex-/age-matched controls. Four brain areas were investigated; three areas frequently reported to show involvement in Alpers’ syndrome and one area that is less frequently described. A battery of immunohistological stains and antibodies was used to assess the cohort. Neuropathological features identified and quantified included severe neuronal loss, widespread astrogliosis, reduced mitochondrial mass, white matter abnormalities, and microglial activation. The most severe neuropathology occurred in the posterior regions of the brain, yet all areas investigated exhibited pathology to different degrees. The expression of key respiratory chain proteins was quantified in neurons from different brain regions and revealed wide variation within and between individuals. There was a clear deficiency of complexes I and III in neurons, with a milder deficiency of complex IV. Assessment of mtDNA content in neurons revealed prominent mtDNA depletion, with a subset of neurons in older patients displaying high levels of mtDNA deletion. This study shows clear evidence of respiratory chain deficiency of complexes I and III. This occurs most severely in the posterior region of the brain, correlating with more neuropathological changes in these regions. MtDNA depletion is a central feature of DNA damage in neurons likely driving the development of respiratory chain deficiency. These findings provide a clear insight into the mechanisms of respiratory chain deficiency leading to the neuropathology and clinical features in patients with Alpers’ syndrome.