Phenotypic Variability and Incomplete Penetrance Are Features of GDF6 Mutations

Abstract

Purpose: : To characterize the degree of phenotypic variability present in patients and two model organisms with reduced Gdf6/gdf6a function, and to begin identifying downstream targets of GDF6.

Methods: : 183 microphthalmia, anophthalmia and coloboma (MAC) patients were screened for GDF6 mutations, and the segregation of mutations determined. C57BL mice, heterozygous for a loss-of-function Gdf6 mutation, were phenotyped with retinal examinations, photography, ERG, and ocular histology. In parallel, a 44k-probe zebrafish microarray was used to identify putative downstream targets of gdf6a, together with whole mount in situ hybridizations.

Results: : Heterozygous missense mutations identified in one coloboma and two microphthalmia patients, altered evolutionarily conserved amino acid residues and were absent from >370 control chromosomes. Two of these did not arise de novo and are present in one unaffected parent of the probands, consistent with our previous findings of incomplete penetrance. Phenotypes observed in Gdf6^+/- mice were variable and included optic disc cupping (8 of 12), microphthalmia (1 of 12) and marked asymmetry (6 of 12). ERGs revealed no significant difference in a- and b-wave amplitude or latency between the wild type and heterozygous mice. Microarray analysis of gdf6a morphants demonstrated increased expression of two BMP antagonists and in situ hybridization of these and other related genes provides preliminary evidence of altered BMP antagonist expression.

Conclusions: : Our data demonstrate that GDF6 mutations account for 1.6% of MAC cases with incomplete penetrance observed in two pedigrees. These data accord with Gdf6/gdf6a-deficient murine and zebrafish models, illustrating similar phenotypic variability. Preliminary findings of altered expression of BMP antagonists provide one explanation for incomplete penetrance and phenotypic heterogeneity, and may in time offer opportunities to target these molecules therapeutically.

Keywords: retinal development • genetics • growth factors/growth factor receptors