Purpose: Cone transducin plays an important role in interacting with the cone photoreceptor visual pigments and activating the cGMP-dependent phosphodiesterase. The human gene for the alpha-subunit of cone transducin (GNAT2) has been cloned and characterized. Recently achromatopsia has been associated with mutations in this gene. Cone and cone-rod dystrophies are a genetically heterogeneous group of photoreceptor diseases, in which mutations of a single gene may cause a variety of phenotypes. In this study we tested the hypothesis that mutations in GNAT2 cause cone-rod degeneration (CRD).
Methods: PCR-SSCP and heteroduplex analysis combined with automated sequencing was used for mutation detection in GNAT2 in 13 independent pedigrees with CRD. We used co-segregation analysis to establish or reject causation, when possible. Molecular computer modeling was utilized to examine the possible consequences of mutations onto GNAT2 protein structure.
Results: We found a novel 3 base-pair deletion, predicted to cause the loss of a highly conserved lysine at position 270 (K270del) in a French-Canadian CRD pedigree. We detected this deletion in a CRD proband, but also in his unaffected son, the proband's unaffected father and the proband's unaffected brother. However, we did not find this defect in 12 other CRD pedigrees, nor in 100 normal, culturally matched chromosomes. According to literature and our molecular computer modeling, the K270 plays an important role in securing the guanine ring in the nucleotide binding cleft of the molecule and in creating a salt bridge between the helical and GTPase domains of GNAT2. However, the K270del in GNAT2 does not appear to have extensive consequences to the structure and the function of the GNAT2. Apparently, there is a compensatory effect of lysine (K-271), which forms a hydrogen bond with the N1 ring nitrogen substituting for the loss of the lysine at position 270.
Conclusions: We detected a deletion of a highly conserved lysine at codon 270 in a critical functional area of the alpha-cone transducin molecule. The co-segregation analysis showed that the deletion is not co-inherited with the disease phenotype and therefore is not the disease causing mutation. Apparently the function of this molecule is not altered by this mutation, due to a compensatory effect of aminoacid 271. Taken together, the presence of this deletion in healthy individuals, and our protein modeling results, predict that alpha-cone transducin molecule is able to tolerate structurally and functionally the K270del.