Exploring the Variable Phenotypes of RPGR Carrier Females in Assessing their Potential for Retinal Gene Therapy

Inherited retinal degenerations are the leading cause of blindness in the working population. X-linked retinitis pigmentosa (XLRP), caused by mutations in the Retinitis pigmentosa GTPase regulator (RPGR) gene is one of the more severe forms, and female carriers of RPGR mutations have a variable presentation. A retrospective review of twenty-three female RPGR carriers aged between 8 and 76 years old was carried out using fundoscopy, autofluorescence imaging (AF), blue reflectance (BR) imaging and optical coherence tomography (OCT). Confirmation of the genetic mutation was obtained from male relatives or Sanger genetic sequencing. Fundus examination and AF demonstrate phenotypic variability in RPGR carriers. The genetic mutation appears indeterminate of the degree of change. We found four distinct classifications based on AF images to describe RPGR carriers; normal (N) representing normal or near-normal AF appearance (n = 1, 4%); radial (R) pattern reflex without pigmentary retinopathy (n = 14, 61%); focal (F) pigmentary retinopathy (n = 5, 22%) and; male (M) phenotype (n = 3, 13%). The phenotypes were precisely correlated in both eyes (rs = 1.0, p < 0.0001). Skewed X-inactivation can result in severely affected carrier females-in some cases indistinguishable from the male pattern and these patients should be considered for RPGR gene therapy. In the cases of the male (M) phenotype where the X-inactivation was skewed, the pattern was similar in both eyes, suggesting that the mechanism is not truly random but may have an underlying genetic basis.