This is the second report of human anophthalmia-associated mutations of the RAX
homeobox gene (11
). While the parents were not carefully examined, they did not complain of any visual impairment at the time their child was evaluated. The proband was demonstrated to bear composite heterozygous mutations on both alleles of the RAX
gene. The parents are thus likely to each be healthy carriers of a heterozygous mutation, unless one of these mutations appeared de novo
. This would confirm the recessive inheritance of RAX
mutations in ocular dysgenesis.
The phenotype, reported here, consisting in bilateral and symmetric anophthalmia is more severe than the one previously described. This first patient had right anophthalmia and left microphthalmia and sclerocornea (11
). One of the causative mutations (p.Gln147X) induced, as predicted for the two mutations reported here, a truncation of the protein. The other was a missense p.Arg192Gln, with a milder effect on the protein, which conserved a low activity. This could suggest that the observed phenotypic variability be correlated with the mutation severity. However, definite conclusions cannot be drawn in view of the limited number of observations.
In animal models, all truncating mutations have been reported to have severe effects and lead to the absence of eye development (9
). In contrast, antisense or morpholino inhibition in Xenopus act in a dose-dependant manner, leading to graduated phenotypes ranging from eye reduction to anophthalmia (14
). In the present report, the location of the mutations in the last exon makes nonsense-mediated mRNA decay unlikely (15
). This is in accordance with the observation that, in the cellular model used by Voronina et al (11
), the more proximal p.Gln147X mutation allowed translation of a large amount of protein. These facts suggest that the two mutations we report here lead to truncated proteins, both lacking the C-terminal part containing the critical OAR functional domain (Otp,Aristaless,Rax
). Absence of RAX C-terminus is known to abolish it’s proliferative effect in Xenopus (14
). Furthermore, regulation of transcriptional activity of several other homeobox genes by the OAR domain has been suggested in other studies (7
). Thus p.Ser222ArgfsX62 and p.Tyr303X are thought to drastically impair RAX target genes expression. The precise delineation of the mechanistic effects of these mutations must therefore await binding studies and an important goal for future research will be the identification of the putative genes that can modulate RAX
activity through direct interaction.
To date, no cerebral malformation has been associated with RAX
mutations in man. This is surprising in the light of the observations in insect, batracian, fish and rodent models, where RAX
consistently participates in brain development and homozygous null alleles cause severe cerebral malformations (9
). A similar situation is seen, however, with respect to the Hesx1 homeobox-containing transcription factor, which in mice has a similar early role and an overlapping domain to that of Rax
but is downstream of Pax6
) and Rax
). While Hesx1
mouse mutants can demonstrate anophthalmia in addition to cerebral anomalies, human patients have either isolated pituitary malformations or septo-ocular dysplasia, with no further retinal involvement (22
). In a complementary fashion and unlike SOX2
mutations, no extraocular malformations have been observed in RAX
ocular dysgenesis patients. The patient reported previously by Voronina et al.
) was diagnosed as autistic. The patient reported here seems to have normal psychomotor development, although she is too young to exclude the possibility of developmental delay and/or autistic features. Thus, RAX
phenotypic spectrum is still unclear, and due to the limited number of cases reported so far, the existence of RAX
involvement in syndromic forms of anophthalmia cannot be excluded.