A MITOCHONDRIAL-INTERNEURONAL HYPOTHESIS OF AUTISM Abstract: Autism is a neurobehavioral disorder of unknown etiology that affects one in 68 children. We hypothesize that a major contributor to autism spectrum disorder (ASD) risk is partial mitochondrial dysfunction causing interneuron inhibition and developmental migration defects. This results in cortical neuronal excitation-inhibition imbalance. Partial mitochondrial dysfunction has been repeatedly observed in ASD. ASD patients exhibit EEG abnormalities of likely GABAergic inhibitory interneuron origin. Interneurons are highly energetic and acutely sensitive to mitochondrial inhibition and cortical excitation-inhibition imbalance is associated with ASD behavioral abnormalities. Extensive nuclear DNA (nDNA) genetic studies have identified multiple ASD-associated haploinsufficient loci, each accounting for a few cases, implying that there are over a thousand ASD loci. The mitochondrial genome consists of between one and two thousand nDNA genes plus thousands of copies of the mitochondrial DNA (mtDNA), so partial mitochondrial dysfunction can result from nDNA haploinsufficiency or deleterious mtDNA variants. We have shown that many ASD-associated nDNA haploinsufficiency variants affect mitochondrial functions, that certain mtDNA lineages (haplogroups) correlate with ASD-risk, that the ASD-associated mtDNALeu(UUR) nt 3243A>G mutation results in mitochondrial dysfunction and perturbation of expression of multiple ASD nDNA genes, that chemical and genetic inhibition of OXPHOS impacts interneuron cortical developmental migration, and that mice harboring mild mtDNA mutants exhibit ASD-associated endophenotypes including EEG abnormalities. To further test the mitochondrial defect-interneuron imbalance hypothesis we propose three specific aims. First, we will determine the mtDNA sequences of ASD patients and controls using off-target exome sequence data or direct mtDNA sequencing and correlate the mtDNA haplogroups and recent deleterious mutations with ASD risk. ASD-associated mtDNAs will then be analyzed for mitochondrial dysfunction within transmitochondrial cybrids. Second, we will analyze 16p11.2 CNV ASD cell lines for mitochondrial dysfunction and altered transcription profiles and compare the results to ASD mtDNALeu(UUR) nt 3243A>G cybrids. We will then determine how mtDNA variation affects the clinical variability associated with 16p11.2 CNVs. Finally, we will determine the effect of interneuron-specific nDNA mitochondrial gene and systemic mtDNA gene defects on cortical interneuron developmental migration and associated manifestation of ASD endophenotypes and social-behavioral aberrations. Mice with endophenotypes but non-overt ASD-like behavior will be exposed to poly (I:C)-induced in utero inflammation to determine if they are more prone to induction of ASD-like behavior. If validated by these experiments our mitochondrial defect-interneuron imbalance hypothesis can encompass virtually all of the disparate observations associated with ASD.
Autism, a neurobehavioral disorder of unknown etiology that affects 1 in 68 children, has been associated with mitochondrial dysfunction, interneuronal defects resulting in excitation-inhibition imbalance, and a complex array of genetic defects. Since the interneurons are acutely sensitive to mitochondrial inhibition and mitochondrial genetics encompasses thousands of nuclear DNA (nDNA) and mitochondrial DNA (mtDNA) genes, we hypothesize that a significant proportion of autism cases are due to partial mitochondrial defects resulting in interneuron developmental and functional aberrations. We propose to test this mitochondrial defect-interneuron imbalance hypothesis by identifying mtDNA variants that are autism risk factors, if autism patients harboring the common 16p11.2 copy number variant have mitochondrial defects, and if mice with partial nDNA or mtDNA mitochondrial defects exhibit interneuron developmental migration defects, autism- associated endophenotypes, and behavioral manifestations.
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