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Irene Whitney, Patrick Keeley, Sarra Borhanian, Benjamin Reese; Genetic Regulation of Type 3b Cone Bipolar Cell Number. Invest. Ophthalmol. Vis. Sci. 2013;54(15):2495.
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© ARVO (1962-2015); The Authors (2016-present)
Previous studies have shown that the size of retinal nerve cell populations differs between the C567BL/6J (B) and A/J (A) inbred mouse strains, including cone photoreceptors, horizontal cells, and amacrine cells. Preliminary studies showed, by contrast, minimal variation of the bipolar cell populations between these two inbred strains. To investigate this further, the present study has examined the size of the type 3b cone bipolar cell population in a panel of recombinant inbred (RI) mice, and mapped quantitative trait loci (QTL) in order to identify potential candidate genes controlling the size of this population.
A mouse monoclonal antibody to protein kinase A, regulatory subunit II beta (PKARIIbeta), was used to label type 3b cone bipolar cells in the mouse retina. Estimates of total cell number were determined for the B and A strains, their reciprocal F1 crosses and 25 RI strains derived from them. Interval mapping of type 3b total number was conducted at GeneNetwork.org to identify QTLs.
The A and B strains do not differ significantly in their type 3b population, with roughly 57,400 and 58,700 cells, respectively. The RI strains, by contrast, showed conspicuous variation by strain, the lowest strain having 50,200 cells and the highest having 60% more, with 80,500 cells. While such variation between strains is substantial, the variation within any strain is modest, with an average CoV of 0.05. The proportion of the variation between all mice that can be attributed to an effect of strain is 64%. We mapped that variation to three prominent QTLs. At two of these, on chromosomes (Chr) 3 and 13, increases in trait values correlate with the presence of B alleles, while on Chr 6, increases correlate with A alleles. The combined additive effect of these haplotypes at the three loci is 25,500 cells, accounting for a large proportion of the variation seen across strains.
Three genomic loci have been identified that modulate type 3b cone bipolar cell number, and potential candidate genes at each locus are being investigated. Interestingly, a cluster of three contactin genes resides on the Chr 6 QTL, each with genetic variants and retinal expression. While the contactin genes have recently been implicated in producing the appropriate lamination of the inner retina, they have also been shown elsewhere to play a role in neuronal differentiation and survival, both possible mechanisms for modulating neuronal number.
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