Using mice heterozygous for a null mutation in the Ube3a gene, in which the mutant gene was contributed by the mother, we have observed a partial mitochondrial respiratory complex III defect in the brains of Ube3a m-\p+ mice. This mitochondrial enzyme defect is associated with abnormal brain mitochondrial morphology and alteration in the synaptic vesicle density.
While the complex III defect was significantly reduced for the whole brain, it was not significantly reduced when the hippocampus or cerebellum mitochondria were assayed separately. This may be explained by the observation that the paternal allele of Ube3a
is strongly inactivated in hippocampal and cerebellar neurons [11
]. Therefore, during development, those cells in which the paternal allele was fully inactivated would become complex III null, which is incompatible with life. Hence, this subset of cells would be lost prior to birth, resulting in the Angelman Syndrome phenotype. It is well established that different types of differentiated cells have different mitochondrial physiologies and express different sets of mitochondrial genes, including different isoforms of the same protein [24
]. Therefore, it is likely that some neurons and various classes of glial cells would have normal or only partially reduce the expression of the p+ allele. Therefore, in the hippocampus and cerebellum where the p+ allele inactivation would be most complete, all cells undergoing p+ inactivation would be lost during development resulting in the AS phenotype. The remaining cells would thus not be subject to p+ inactivation, such that the specific activity of the mitochondria of the remaining cells would be maximal. By contrast, in other parts of the brain where p+ inactivation would be partial, the partial complex III defect would be more apparent and thus detectable with the methods currently available. Even relatively mild alterations in mitochondrial function can result in mitochondrial morphological changes. Hence, it would be expected that a 50% reduction in complex III in the Ube3a
m-\p+ mice would result in morphological alterations in the brain mitochondria that we observed.
The mechanism by which a defect in Ube3a
could cause a mitochondrial defect is unknown. The reduced enzyme activities of the brain mitochondrial complexes in the Ube3a
deficient mice may be due to inactivation of the enzymes causing impairment of the electron transport chain. Oxidative damage could potentially preferentially affect the stability and function of the complexes containing iron-sulfur clusters (complexes II+III) in the Ube3a
deficient mice brain. However, it is clear that ubiquitination is important in maintaining the structural and functional integrity of mitochondria in yeast [8
]. Moreover, the mitochondrial ubiquitin ligase MARCH-V is important in regulating the mitochondrial dynamics of Drp1 and Fis1 in mammalian cells [16
] and the human deubiquitinating enzyme ubiquitin-specific protease 30 (USP30) is embedded in the mitochondrial outer membrane and participates in the maintenance of mitochondrial morphology [19
]. Therefore, partial defects in Ube3a
could have either primary or secondary consequences for the maintenance of mitochondrial complex III. Since the Ube3a
protein E6-AP has been shown to interact with Drosophila
High-wire which regulated synaptic growth, alteration in ubiquitin biochemistry could also explain the aberrations in synaptic morphology [3
], acting either through the mitochondria or by an independent pathway.
It is becoming increasingly clear that alterations in mitochondrial function accompany epigenomic alterations. One of the best examples is Rett syndrome (RS), which shares many characteristics with AS [13
], and is the result of mutations in the X-linked MeCP2
gene. RS patients have been documented to have brain mitochondrial structural and OXPHOS complex defects [7
] and mice in which the MeCP2
gene is inactivated have alterations in complex III gene expression and uncoupled mitochondrial [18
The hippocampus which resides in the temporal lobe has a low seizure threshold [26
]. Yashiro et al
] found that the Ube3a
deficient mice had impaired synaptic plasticity in experience-dependent neocortical development, demonstrating Ube3a
plays a critical role in the modifications of neuronal circuits. This is consistent with the observed reduction in spine density and synaptic current frequency of the pyramidal neurons of the cortex [27
]. The reduction of synaptic vesicle density in the hippocampus may also be the result of a maturational delay associated with the Ube3a
deletion and possibly contributes to the impaired neuronal circuits and also to pathophysiology of AS.
Therefore, while we lack sufficient depth of understanding of the biology of the mitochondrion to know the full nature of the interactions between the nucleus and mitochondrion, the similarities in clinical phenotypes that have been observed between patients with mitochondrial disease [24
] and patients with diseases of the epigenome [9
] are sufficiently similar as to suggest a common pathophysiological mechanism. If this proves to be true, then approaches that are being developed for mitochondrial disease therapies may be help for the more complex epigenomic diseases.