Human microphthalmia associated with mutations in the retinal homeobox gene CHX10

Abstract

Isolated human microphthalmia/anophthalmia, a cause of congenital blindness, is a clinically and genetically heterogeneous developmental disorder characterized by a small eye and other ocular abnormalities. Three microphthalmia/anophthalmia loci have been identified, and two others have been inferred by the co-segregation of translocations with the phenotype. We previously found that mice with ocular retardation (the or-J allele), a microphthalmia phenotype, have a null mutation in the retinal homeobox gene Chx10 (refs 7,8). We report here the mapping of a human microphthalmia locus on chromosome 14q24.3, the cloning of CHX10 at this locus and the identification of recessive CHX10 mutations in two families with non-syndromic microphthalmia (MIM 251600), cataracts and severe abnormalities of the iris. In affected individuals, a highly conserved arginine residue in the DNA-recognition helix of the homeodomain is replaced by glutamine or proline (R200Q and R200P, respectively). Identification of the CHX10 consensus DNA-binding sequence (TAATTAGC) allowed us to demonstrate that both mutations severely disrupt CHX10 function. Human CHX10 is expressed in progenitor cells of the developing neuroretina and in the inner nuclear layer of the mature retina. The strong conservation in vertebrates of the CHX10 sequence, pattern of expression and loss-of-function phenotypes demonstrates the evolutionary importance of the genetic network through which this gene regulates eye development.

Analysis of opo cis-regulatory landscape uncovers Vsx2 requirement in early eye morphogenesis

“…Initial analysis of Vsx2 function described that its mutation in mice (ocular retardation, or j ) resulted in microphthalmia, and revealed its essential role in neuroepithelial proliferation and bipolar cells differentiation 13 . Similar phenotypes were also observed in human Vsx2 mutations 14 and in vsx2-depleted embryos in zebrafish 15,16 . More recent studies on early stages of eye development have shown that Vsx2 represses Mitf and hence RPE identity, thus acting as a central component of the gene regulatory network involved in the specification of the neural retina domain [17][18][19][20] .…”

“…This TF was initially identified in mutant mice and human patients as an important regulator of retinal precursors' proliferation and lineage specification 13,14 . Later studies in mice found that Vsx2 played an essential role in the specification of the neural retina domain, mainly by repressing Mitf family members Mitf and Tfec, and hence preventing the activation of the RPE genetic programme 17,18,20 .…”

“…The fact that Vsx2 sites overlap with the binding sites of closely related homeodomain proteins opens up the possibility that several retinal determination genes cooperate with Vsx2 during optic cup morphogenesis, either through the opo H6_10137 enhancer or by regulating other unidentified morphogenetic genes. These differential regulatory properties may even vary within vertebrates, as optic cup malformations (that is, coloboma) have been reported in families affected with non-syndromic microphthalmia 14,62 . In fact, examples of differential input into gene regulatory networks have been already described during optic vesicle patterning in vertebrates.…”

Clinical utility gene card for: Non-Syndromic Microphthalmia Including Next-Generation Sequencing-Based Approaches

“…However, deletion of exon 3 has also been described. [8][9][10] Autosomal-recessive variants in RAX (MCOP3) [10][11][12] and ALDH1A3 (MCOP8) can be missense, nonsense or frameshift, with some splice donor variants. A RAX gene deletion has also been described in one patient with bilateral anophthalmia, with no other ocular or systemic abnormalities reported.…”

“…26 Homozygosity for missense and a splice variant in VSX2 have been described in MCOPCB3 cases. 27,28 MCOPCB4 is isolated microphthalmia associated with colobomatous cyst and is transmitted as an autosomal-recessive trait. The genetic cause remains unknown.…”