Heritable renal hypoplasia and frontonasal dysplasia is associated with misexpression of six2

Renal hypoplasia and frontonasal dysplasia are malformations associated with tissues derived from mesoderm. A radiation induced mouse mutant, called Brachyrrhine ( Br ) exhibits both malformations. Initial microsatellite analysis placed the mutation on distal chromsome 17 in Br . The purpose of this study was to further resolve the Br locus and determine tissue expression of candidate genes. An interspecific backcross breeding strategy was designed using 3H1 Br/+ mice crossed to Castaneus and Balb mice. Segregation analysis showed that Br was inherited as a semidominant lethal mutation when outcrossed to both inbred strains. DNA was extracted from F2 progeny and scored for recombinations. Results showed that Br localized to a region just distal to D17Mit76 that included six2 . Using this microsatellite as a genotypic marker, C3H1 x Balb Br/+ F1 mice were crossed and embryos were obtained at embryonic days 10.5–11.5. The embryos were subjected to whole‐mount in situ hybridization (WMISH) and quantitative RT‐PCR. Results showed that homozygous Br/Br animals have significantly reduced six2 expression. Quantitative RT‐PCR reflected a pattern of six2 haploinsufficiency with heterozygous mutants showing about a 50% decrease in gene dosage and homozygous mutants showing about a 10% expression. It is concluded that a mutation affecting six2 expression is responsible for mesodermal differentiation pathways in the face and kidney. Supported by R01‐ DK064752 , Meiji Yasuda Endowment, and HS‐BRIN P20 PP16467