Microphthalmia is a highly morbid birth defect that is heterogeneous in etiology . A genetic cause of microphthalmia, suspected in many cases, is confirmed in only a minority because of significant locus heterogeneity that has only been partially captured to date. SIX6 is a compelling candidate gene for microphthalmia but no conclusive point mutations have been reported previously. We describe in this communication a multiplex family in which microphthalmia associated with anterior and posterior segment dysgenesis (complex microphthalmia) is caused by a homozygous SIX6 truncating mutation.
This is a Syrian family in which parents are first cousins, young and healthy. They have two sons 5 years old and 16 months old, both born with microphthalmia but without evidence for coloboma. The younger child had buphthalmos, corneal scarring and retinal detachment in the right eye apparently from neglected infantile glaucoma (Fig. 1). A thorough medical evaluation did not identify any associated signs or symptoms. After signing an Institutional Review Board-approved written informed consent (KFSHRC RAC#2070008) blood was collected from both children and their parents for DNA extraction. This was followed by genotyping on Axiom platform and autozygome analysis using autosnpa essentially as described before. For autozygome-guided mutation analysis, we sequenced all human and mouse microphthalmia genes that reside within an autozygosity interval that is shared between the two children (Table 1). Only one of these genes was found to harbor a novel variant; a homozygous frameshift deletion c.532_536del in SIX6 (NM_007374.2) (Fig. 1). Both children were homozygous while parents were heterozygous. This mutation is absent in 200 Saudi (with Saudis being closely related ethnically to Syrians) exomes and in the 1000 genomes project.
|chr14:56930247-78430247||OTX2, SIX6, SMOC1, VSX2|
SIX6 is a transcription factor with an established and remarkably conserved role in early eye development. It binds specific DNA motifs to downregulate, in conjunction with co-repressors, the expression of genes. Evidence suggests a model wherein SIX6 suppresses inhibitors of proliferation such as p27 in order to promote growth of early eye structures . It is also suggested that it is through the suppression of non-retinal selector genes that SIX6 permits cells of early eye structures to assume their proper identity . Genetic manipulation of SIX6 such as in its knockout mouse model resulted in small eyes and a small pituitary gland (areas where Six6 is highly expressed) . Consistent with the normal phenotype we observe in the heterozygous parents, heterozygous Six6 knockout mice lacked the eye and the pituitary findings. Inversely, overexpression of SIX6 results in the formation of large eyes in Xenopus . Remarkably, ectopic expression of SIX6 has also been shown to induce the formation of ectopic eye structures [3, 5].
These lines of evidence strongly support the candidacy of SIX6 as a disease gene for microphthalmia in humans. However, supporting evidence is circumstantial at best. For instance, several cases in which a genomic deletion encompassing SIX6 have been reported in association with microphthalmia/anophthalmia (reviewed by ). In none of these cases, however, was hemizygosity confined to SIX6 thus leaving open the possibility that other deleted sequences may have been the cause of the phenotype instead, e.g. BMP4 haploinsufficiency is known to cause microphthalmia and is only a few megabases away so it may have been disrupted by these large genomic deletions. Several large series have also failed to identify unambiguous deleterious point mutations in SIX6 among patients with microphthalmia . The strongest candidate variants reported to date are two missense changes that were also observed in clinically normal parents so their significance is unclear [7, 8]. In contrast, what we report here is a homozygous truncating mutation that segregates with the phenotype in two children with complex microphthalmia, adding to the list of autosomal recessive causes of microphthalmia in humans.