Inherited retinal degenerations are a significant cause of morbidity in the Western world. Unraveling the underlying genes and their functions has led to much progress in assessing and treating these conditions and, in addition, has contributed significantly to our understanding of retinal degenerative mechanisms, including those underlying common conditions such as age-related macular degeneration (1
). One such genetic disease is choroideremia (CHM) (OMIM 303100) (4
). CHM has an incidence of 1 in 50,000 and exhibits X-linked inheritance. Affected males develop night blindness in their teens, which progresses to loss of peripheral vision and complete blindness 2–3 decades after onset. The majority of heterozygous females usually do not develop blindness, although examination of the fundus often reveals a patchy distribution of abnormal areas and there is a wide range of severity due to random X-inactivation.
Loss-of-function mutations in the CHM
gene underlie CHM (7
). The CHM gene was renamed Rab escort protein-1 (REP1) given its role as a regulator of Rab GTPases (9
). Rabs are monomeric GTPases, which function as regulators of intracellular vesicular transport and organelle dynamics (10
). To bind to intracellular membranes, Rabs have to undergo lipid modification with geranylgeranyl groups, a process generally known as prenylation (12
). REP binds a newly synthesized Rab, presents it to Rab geranylgeranyl transferase, which catalyzes geranylgeranyl additions to Rab, and then “escorts” and delivers it to its target cellular membrane (14
). More than 60 Rabs have been identified in the human genome, of which some are expressed ubiquitously and others in specialized cell types, depending on the specific vesicular transport pathway they regulate (11
In CHM, the loss of REP1 is apparently largely compensated for by REP2, and both REPs are ubiquitously expressed (16
). Intriguingly, however, the loss of REP1 is not fully compensated for in the eye, and this leads to the slow-onset retinal degeneration characteristic of CHM (17
). Our previous studies suggested that the molecular basis of CHM resides in the existence of some Rabs that are selectively underprenylated in the absence of REP1. One such candidate is Rab27a, which was found to be underprenylated in lymphoblasts of CHM patients (17
The pathogenesis of CHM remains unclear. The name reflects the fact that the choroid is severely affected, especially at the late stages of the disease. However, 2 other layers of the eye are also damaged: the retinal pigment epithelium (RPE) and the photoreceptors. It is not known at present whether the 3 affected layers, photoreceptors, RPE, and choroid, degenerate independently or in a cascade mechanism, and, if the latter is true, which layer degenerates first.
A previous attempt to generate a KO mouse model of CHM showed, surprisingly, that null mutations of the X-linked Chm
gene are embryonically lethal in males and also in heterozygous females when the mutant allele is inherited from a heterozygous female (19
). The lethality is due to defects in trophoblast development and vascularization (20
). In heterozygous female offspring, this is presumably due to the preferential inactivation of the paternal (wild-type) X chromosome in murine extraembryonic tissues, resulting in expression of the maternal (mutant) copy (19
). Recently, a chm
knockout in zebrafish was generated by random mutagenesis. The results were similar to those in mice: chm
loss of function caused lethality of the mutant larvae, which died at 6 days after fertilization (21
In this work, we report the generation of several mouse models of CHM. To avoid embryonic lethality and to circumvent the breeding problems caused by the inability to transmit the Chmnull allele from carrier females, we used a conditional approach in which Cre/loxP site-specific recombination allows spatial and temporal control of the actual knockout event. The biochemical, histological, and functional analysis of these mice suggests a cell-autonomous degeneration model for this disease and reveals novel insights into retinal degenerations in general.