Objective: To identify genes responsible for cone dystrophies and determine the functional consequences of their underlying mutations.
Design: Case-control study.
Participants: Two hundred forty unrelated patients diagnosed with cone dystrophy, cone-rod dystrophy, macular dystrophy, macular degeneration, or Stargardt disease, 95 control individuals, and 2 unrelated families with a distinctive type of cone dystrophy.
Methods: The DNAs of the 240 probands were screened for sequence variants in the PDE6H gene (that encodes the inhibitory gamma-subunit of cone cyclic guanosine monophosphate [cGMP]-phosphodiesterase [PDE]) by single-strand conformation polymorphism electrophoresis. The effect of a nucleotide substitution in the DNA of a patient on gene expression efficiency was analyzed by in vitro transcription/translation.
Main outcome measures: Cone-specific gene variants, fundus, visual field and electroretinogram (ERG) findings, and protein synthesis efficiency.
Results: We found a heterozygous G to C substitution in the 5' untranslated region (UTR) of the PDE6H gene in the DNA of a patient with a distinctive form of cone dystrophy, her sibling, and their father. This rare form of disease is very different in manifestation from other cone dystrophies and has been described as "cone dystrophy with nyctalopia and supernormal rod responses," "cone dystrophy with supernormal scotopic ERGs" and "supernormal and delayed rod ERG syndrome." Among the 240 patients that we studied, only 1 proband had the G to C variant. Furthermore, none of the 95 controls used in this study had this nucleotide change. We also determined that the PDE6H variant was not present in another family affected with this particular type of cone dystrophy. Because the 5' UTR of mRNAs plays a critical role in the regulation of protein synthesis, we determined the effect of the G to C change in this process. By use of in vitro transcription/translation experiments, we demonstrated that this substitution could lead to an increase in PDE6H gene expression.
Conclusions: Our results indicate that mutations in the PDE6H gene are not common, because only 1 of 240 patients with cone dystrophy showed a single nucleotide substitution in the 5' UTR of PDE6H mRNA that could be associated with the disease. If the effect of the G to C substitution we observed in vitro also occurs in vivo, it will lead to PDE6H overexpression in the photoreceptors. Excess of PDEgamma may affect normal cone cGMP-PDE function by inhibiting the catalytic PDEalpha,beta activity and lead to pathogenic elevation of cGMP and eventual degeneration of cone photoreceptors.