Many variants that influence the risk for neuropsychiatric disorders are heterozygous in the affected individual. Genomic imprinting, which is a heritable epigenetic mechanism that results in preferential expression of the maternal or paternal allele for some genes, can worsen the effects of heterozygous mutations by silencing the healthy allele, leading to expression of only the mutant allele. Canonical imprinting involves complete silencing of one parent's allele. The authors recently discovered hundreds of genes that exhibit a maternal or paternal allele bias in the brain. They refer to this as noncanonical imprinting. Using a novel method, the authors have evidence that noncanonical imprinting effects detected at the tissue level involve allele-silencing in subpopulations of neurons in the brain. They further found that noncanonical imprinting significantly influences the impact of heterozygous mutations on offspring behavior depending on the parental origin of the mutation. Thus, the results indicate the existence of highly cell-type specific imprinting effects n the brain. Currently, it is unknown which neuronal populations are involved, how specific behaviors are impacted, and whether additional genes are imprinted in specific neuron types. Noncanonical imprinting was found to influence the expression of tyrosine hydroxylase (Th) and dopa decarboxylase (Ddc), two genes essential for the synthesis of the monoamine neurotransmitters dopamine, noradrenaline and serotonin. Dysfunction of the monoaminergic systems occurs in anxiety disorders, major depression, bipolar, schizophrenia and other mental disorders. The proposed study will test the hypothesis that noncanonical imprinting causes allele- silencing effects in subpopulations of monoaminergic neurons to regulate important aspects of behavior and physiology.
In aim 1, anatomically-defined subpopulations of monoaminergic neurons that are enriched for Th and Ddc imprinting effects will be defined in the male and female brain using transgenic mice that express egfp and tdTomato reporters from the maternal and paternal alleles, respectively.
In aim 2, the function(s) of noncanonical imprinting effects influencing monoaminergic neurons will be defined using a battery of behavioral assays to compare mutant mice with maternally versus paternally inherited deletions in Th and Ddc. Finally, in aim 3, imprinting effects in monoaminergic neurons will be comprehensively profiled using single-cell RNASeq on EGFP+ neurons purified from transgenic F1 hybrid offspring. Hybrid offspring will be generated from reciprocal matings of Ddc::egfp transgenic reporter mice (Aim 1) on the C57BL/6J x CastEiJ genetic backgrounds. Polymorphic sites in the F1 hybrids will reveal allelic expression in the RNASeq data. Overall, the study will define the cellular nature and function of imprinting effects in monoaminergic neurons in the brain and establish noncanonical imprinting as an important epigenetic phenomenon that influences the function and allelic architecture of monoaminergic circuits.
This study will make a substantial contribution towards improving our understanding of the expression of maternally and paternally inherited alleles in the brain and in neurons with important roles in mental health. The proposed research is relevant to public health because it could lead to improvements in our ability to evaluate genetic risk factors for mental illness according to whether mutations are maternally or paternally derived.
