Identification of a Functional Non-coding Variant in the GABA

GABA type-A (GABA-A) receptors containing the α2 subunit (GABRA2) are expressed in most brain regions and are critical in modulating inhibitory synaptic function. Genetic variation at the GABRA2 locus has been implicated in epilepsy, affective and psychiatric disorders, alcoholism and drug abuse. Gabra2 expression varies as a function of genotype and is modulated by sequence variants in several brain structures and populations, including F2 crosses originating from C57BL/6J (B6J) and the BXD recombinant inbred family derived from B6J and DBA/2J. Here we demonstrate a global reduction of GABRA2 brain protein and mRNA in the B6J strain relative to other inbred strains, and identify and validate the causal mutation in B6J. The mutation is a single base pair deletion located in an intron adjacent to a splice acceptor site that only occurs in the B6J reference genome. The deletion became fixed in B6J between 1976 and 1991 and is now pervasive in many engineered lines, BXD strains generated after 1991, the Collaborative Cross, and the majority of consomic lines. Repair of the deletion using CRISPR-Cas9-mediated gene editing on a B6J genetic background completely restored brain levels of GABRA2 protein and mRNA. Comparison of transcript expression in hippocampus, cortex, and striatum between B6J and repaired genotypes revealed alterations in GABA-A receptor subunit expression, especially in striatum. These results suggest that naturally occurring variation in GABRA2 levels between B6J and other substrains or inbred strains may also explain strain differences in anxiety-like or alcohol and drug response traits related to striatal function. Characterization of the B6J private mutation in the Gabra2 gene is of critical importance to molecular genetic studies in neurobiological research because this strain is widely used to generate genetically engineered mice and murine genetic populations, and is the most widely utilized strain for evaluation of anxiety-like, depression-like, pain, epilepsy, and drug response traits that may be partly modulated by GABRA2 function

Identification of a Functional Non-coding Variant in the GABAA Receptor α2 Subunit of the C57BL/6J Mouse Reference Genome: Major Implications for Neuroscience Research

GABA type-A (GABA-A) receptors containing the α2 subunit (GABRA2) are expressed in most brain regions and are critical in modulating inhibitory synaptic function. Genetic variation at the GABRA2 locus has been implicated in epilepsy, affective and psychiatric disorders, alcoholism and drug abuse. Gabra2 expression varies as a function of genotype and is modulated by sequence variants in several brain structures and populations, including F2 crosses originating from C57BL/6J (B6J) and the BXD recombinant inbred family derived from B6J and DBA/2J. Here we demonstrate a global reduction of GABRA2 brain protein and mRNA in the B6J strain relative to other inbred strains, and identify and validate the causal mutation in B6J. The mutation is a single base pair deletion located in an intron adjacent to a splice acceptor site that only occurs in the B6J reference genome. The deletion became fixed in B6J between 1976 and 1991 and is now pervasive in many engineered lines, BXD strains generated after 1991, the Collaborative Cross, and the majority of consomic lines. Repair of the deletion using CRISPR-Cas9-mediated gene editing on a B6J genetic background completely restored brain levels of GABRA2 protein and mRNA. Comparison of transcript expression in hippocampus, cortex, and striatum between B6J and repaired genotypes revealed alterations in GABA-A receptor subunit expression, especially in striatum. These results suggest that naturally occurring variation in GABRA2 levels between B6J and other substrains or inbred strains may also explain strain differences in anxiety-like or alcohol and drug response traits related to striatal function. Characterization of the B6J private mutation in the Gabra2 gene is of critical importance to molecular genetic studies in neurobiological research because this strain is widely used to generate genetically engineered mice and murine genetic populations, and is the most widely utilized strain for evaluation of anxiety-like, depression-like, pain, epilepsy, and drug response traits that may be partly modulated by GABRA2 function

The Genetic Architecture of Murine Glutathione Transferases.

Glutathione S-transferase (GST) genes play a protective role against oxidative stress and may influence disease risk and drug pharmacokinetics. In this study, massive multiscalar trait profiling across a large population of mice derived from a cross between C57BL/6J (B6) and DBA2/J (D2)--the BXD family--was combined with linkage and bioinformatic analyses to characterize mechanisms controlling GST expression and to identify downstream consequences of this variation. Similar to humans, mice show a wide range in expression of GST family members. Variation in the expression of Gsta4, Gstt2, Gstz1, Gsto1, and Mgst3 is modulated by local expression QTLs (eQTLs) in several tissues. Higher expression of Gsto1 in brain and liver of BXD strains is strongly associated (P < 0.01) with inheritance of the B6 parental allele whereas higher expression of Gsta4 and Mgst3 in brain and liver, and Gstt2 and Gstz1 in brain is strongly associated with inheritance of the D2 parental allele. Allele-specific assays confirmed that expression of Gsto1, Gsta4, and Mgst3 are modulated by sequence variants within or near each gene locus. We exploited this endogenous variation to identify coexpression networks and downstream targets in mouse and human. Through a combined systems genetics approach, we provide new insight into the biological role of naturally occurring variants in GST genes

Summary of expression and QTL mapping across BXD strains for <i>Gstt2</i>.

<p>Top two panels (bar plots) show expression of <i>Gstt2</i> in each BXD strain in hippocampus and midbrain. Average log₂ expression is shown on the y-axis and unique strains are shown on the X-axis. Red and blue indicate inheritance of the paternal <i>D</i> or maternal <i>B</i> allele of <i>Gsta4</i> in each strain, respectively. Black indicates a heterozygous (likely erroneous) genotype call. Higher expression is associated with inheritance of the <i>D</i> allele. Bottom panel shows the genetic mapping results in each tissue. Association strength (LOD) is shown on the Y-axis and plotted across the genome on the X-axis (by chromosome) for hippocampus (blue) and midbrain (red). Genome-wide significance is determined by permutation (<i>n</i> = 5000) with the threshold for significance indicated as black (significant, <i>p</i> <0.05) and grey (suggestive, <i>p</i> < 0.3) horizontal lines. Expression of <i>Gstt2</i> is modulated by variants within or near its own locus on Chr 10, a cis eQTL.</p

Summary of cis modulation of expression.

<p>Summary of cis modulation of expression.</p

Summary of QTL mapping of <i>Gsto1</i> expression across BXD strains.

<p>Genetic mapping results are shown for brain and peripheral tissue. Association strength (LOD) is shown on the Y-axis and plotted across the genome on the X-axis (by chromosome) in each QTL map. Genome-wide significance is determined by permutation (<i>n</i> = 5000) with the threshold for significance indicated as black (significant, <i>p</i> <0.05) and grey (suggestive, <i>p</i> < 0.3) horizontal lines. Expression of <i>Gsto1</i> is modulated by variants within or near its own locus on Chr 19, a cis eQTL.</p

Summary of recombination and linkage disequilibrium near the <i>Gsto1</i> locus.

<p><b>(A)</b> The association score (LOD) (y-axis) for <i>Gsto1</i> expression in the hippocampus is plotted as a solid blue line across the genome (x-axis) with horizontal lines indicating significance thresholds for significant (red) and suggestive (grey) LOD values. The physical position of <i>Gsto1</i> is indicated by the orange triangle and the locations of other cis-modulated genes are shown as light purple triangles. Haplotypes are shown above the linkage map for BXD strains with strain number shown to the right and left of the haplotypes. Vertical black lines designate marker position with marker names below the haplotype map. Red and green blocks indicate a chromosomal region inherited from the maternal B6 or paternal D2 strain, respectively. Blue areas are heterozygous and grey areas are undefined meaning that more markers would be needed to pinpoint the exact recombination breakpoint. As expected for a recombinant inbred population, this region is primarily inherited as an entire haplotype block from either parental strain. (B) Pairwise correlations between markers in the region are shown. Intensity reflects correlation strength. Markers in this region are tightly linked.</p

Summary of GST ASE.

<p>Summary of GST ASE.</p

Summary of GST gene expression.

<p>Summary of GST gene expression.</p

Summary of <i>Mgst3</i> expression across BXD strains.

<p>Panels (bar plots) show expression of <i>Mgst3</i> in each BXD strain in multiple tissues. Average log₂ expression is shown on the y-axis and unique strains are shown on the X-axis. Red and blue indicate inheritance of the paternal <i>D</i> or maternal <i>B</i> allele in each strain, respectively. If only a single individual was used for expression measurements, error bars are not shown. For genetic reference populations, mapping power is derived from the number of individuals as opposed to the number of biological replicates. Higher expression is associated with inheritance of the <i>D</i> allele.</p