SIX2 gene haploinsufficiency leads to a recognizable phenotype with ptosis, frontonasal dysplasia, and conductive hearing loss
Heterozygous microdeletions of chromosome 2p21 encompassing only the SIX2 gene have been described in two families to date. The clinical phenotype comprised autosomal-dominant inherited frontonasal dysplasia with ptosis in one family. In the second family, conductive hearing loss was the major clinical feature described; however, the affected persons also had ptosis. Here, we present a large family combining all three predescribed features of SIX2 gene deletion. The phenotype in four affected family members in three generations consisted of bilateral congenital ptosis, epicanthus inversus, frontonasal dysplasia with broad nasal bridge and hypertelorism, frontal bossing and large anterior fontanel in childhood, narrow ear canals, and mild conductive hearing loss with onset in childhood. Thus, the phenotypic spectrum of SIX2 haploinsufficiency is widened. Moreover, 2p21 microdeletions with SIX2 haploinsufficiency appear to lead to a recognizable phenotype with facial features resembling blepharophimosis–ptosis–epicanthus inversus syndrome.