In our screening for exonic mutations in
PDE6C in 456 unselected patients with several different forms of retinal
disease, we found a number of sequence variants. Five were common
polymorphisms present in equal numbers in patients and control
subjects, whereas seven others were rare heterozygous sequence variants
(Table 3) .
The variants were three rare amino acid changes: Asp157Glu, Lys
822Asn, and Glu834Gly. None of them could be associated with disease
because of the absence of a second variant in autosomal recessive
disease, and/or the absence of cosegregation of variant with disease in
the probands’ families. The Lys822Asn variant was interesting, because
this lysine residue is in the catalytic site of the α′-subunit and is
conserved in bovine and chick cone cGMP-PDE α′-subunits and in human,
dog, mouse, and bovine rod cGMP-PDE α-subunits. However, there was no
evidence for autosomal recessive disease, because no second variant was
found in the patient, and autosomal dominant disease was ruled out
because the mother carried the variant without disease. Moreover, the
amino acid change was conservative. In addition to not cosegregating
with disease in one of the proband’s families, the Glu834Gly variant
was also not significant, because this amino acid residue is in the
C-terminal portion of the protein between the highly conserved
catalytic site and the highly conserved isoprenylation site in an area
with no known functional significance.
There were four additional heterozygous rare sequence variants
(Table 3) that were not present in control subjects. None of these
could be associated with disease, because second variants could not be
found in autosomal recessive cases, and/or they did not cosegregate
with disease in the probands’ families, and/or they did not create new
or influence existing branch points, and/or they did not influence the
sequences around them to create new or influence existing splice sites.
It should be noted that PDE6C (or any gene for which
negative exon screening results have been obtained) cannot be
definitively ruled out as the site for mutations responsible for
disease. This is because the SSCP or DGGE techniques may not detect all
sequence variants (although only a very small percentage of mutations
may remain undetected), mutations may be present in 5′, 3′, or intronic
sequences not screened, mutations in this gene may be so rare that not
enough patients were screened to detect them, and mutations in this
gene may be present only in specific diseases not screened.
Nevertheless, in the patients analyzed, we could find no sequence
variants in the gene encoding the α′-subunit of cone cGMP-PDE that
could be associated with simplex or autosomal recessive retinal
degenerations that predominantly affect cone-mediated function and
rod-mediated function or that have a predilection for disease in the
macula.