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B. E. Reese, M. A. Raven, I. E. Whitney, R. W. Williams, Y. Elshatory, L. Gan; A QTL on Chromosome 13 Contributes to the Two-fold Variation in Horizontal Cell Number in the Mouse Retina. Invest. Ophthalmol. Vis. Sci. 2008;49(13):3738.
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Size, numbers, and ratios of cell populations in the vertebrate retina are highly variable. The developmental basis for such natural variation between and within species is not yet understood, although polymorphic genes that modulate cell proliferation and survival are likely to be major factors. We have sought to identify quantitative trait loci (QTL) associated with the variation in horizontal cell number in the retina using recombinant inbred (RI) strains of mice.
Retinas were immunolabeled with antibodies to calbindin, and horizontal cell numbers were estimated from a minimum of three mice in the A/J and C57BL/6J strains (hereafter A vs B), an F1 cross, and in each of 25 RI strains of the AxB/BxA strain-set. WebQTL was used to map genomic loci associated with the variance in the phenotypic data. Isl1 conditional knockout and littermate control mice were similarly sampled.
The horizontal cell population in the A strain approximated 9,900 cells, and in the B strain it was nearly 18,700 cells, while the F1 cross had 14,500 cells. The RI strains of mice varied nearly two-fold as well, their differences in average number being independent of retinal size. We mapped a QTL to the distal tip of chromosome 13, where inheritance of B alleles increased trait values. One potential candidate within the QTL is Isl1, a LIM homeodomain transcription factor gene that plays a role in fate determination and differentiation elsewhere within the nervous system. We examined the effect of knocking out Isl1 function within the retina by crossing Six3-cre expressing mice with mice carrying a floxed allele of Isl1. Horizontal cell density was elevated by 53% in the central retina of the conditional knockout mice, where recombinase activity is greatest in these mosaic retinas.
The present results show that horizontal cell number is a highly heritable polymorphic trait. A gene or genes on chr 13 contribute about one-third of the difference between the parental strains. Isl1 resides within the QTL, and its deletion from the retina increases the number of horizontal cells, demonstrating a novel role for Isl1 during retinal development. Polymorphisms in Isl1 may therefore contribute to the natural variation in horizontal cell number in the mouse retina.
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