Like other genes implicated in several developmental processes, regulation of IRX gene expression is coordinated in a spatial and temporal manner. Such coordination can be accomplished by tissue-specific transcription factors binding of enhancers, sometimes at distances ranging over 1 Mb 5′ or 3′ to the coding sequences that control either the expression of individual homeobox genes or the entire cluster. Examples are the
SOX9 gene or the HOXD gene cluster, respectively.
55,56 The human IRXA cluster spans 1.8 Mb, and the IRXB cluster spans 1.3 Mb. No other genes are interspaced between the IRX genes in the clusters. In the IRXB cluster,
IRX3 is located at the centromeric end and has a 5′ telomere to 3′ centromere transcriptional orientation, which is opposite that of
IRX5 and
IRX6. In 2005, de la Calle-Mustienes et al.,
23 using transgenic
Xenopus and zebrafish embryos, demonstrated that 22 highly conserved noncoding regions (HCNCRs) that lie within the IrxB cluster (and one located just telomeric to the
Irx6 locus) direct the spatiotemporally specific expression of IrxB genes. In 2006, Pennacchio et al.
57 tested 167 HCNCRs, conserved between humans and the pufferfish
Takifugu rubripes or ultraconserved between humans and rodents, in a transgenic mouse assay. A meticulous analysis of these data showed that 21 HCNCRs lay within the IRXB cluster, 17 between
IRX3 and
IRX5 and 4 between
IRX5 and
IRX6 (
Fig. 4). Functional tests showed that only 4 HCNCRs among the 21 have an enhancer activity in the eye during the mouse's embryonic development. Enhancers 26 and 27 are located between
IRX5 and
IRX6 and are probably involved in
IRX5 and
IRX6 expression. Enhancer 157, located 330 kb centromeric to
IRX3 in intron 11of the
FTO gene induces in a murine transgenic assay an eye-specific expression resembling that of
IRX3. This gene is involved in controlling body fat. Its expression during embryonic development and its invalidation in mice did not show any activity in ocular embryogenesis.
58,59 Enhancer 59 is located in the first intron of the
RPGRIP1L gene (retinitis pigmentosa GTPase regulator interacting protein 1–like gene). This gene is expressed in several types of fetal and adult tissues but mainly in the brain, kidneys, ovaries, and testes and at the level of the retinal photoreceptors.
60,61 Knockout mice
Rpgrip1l −/− which are not viable, show exencephaly, polydactyly, abnormalities of lateralization, and microphthalmia.
60 In humans, mutations of
RPGRIP1L are found in patients presenting with the recessive syndromes oculo-cerebro-renal or type B Joubert and Meckel syndromes.
60,61 This gene is involved in eye embryogenesis. It is reasonable to assume that enhancer 59 is associated with the control of the expression of
RPGRIP1L in the eye. We therefore hypothesize that enhancer 157 plausibly plays a role in IRXB gene expression in the developing human eye. In addition, the chromosomal rearrangement delocalizes the IRXB cluster at the 2p22.3 region covered by the RP11-68N21. No known genes or HCNCRs were found. Accordingly, the translocation separates the IRXB cluster from enhancer 157, which has a specific eye expression thus deregulating the complex spatiotemporal expression and generating the ASOD observed in our patient.