To explore the possible influence of defined genetic backgrounds
on photoreceptor viability and function in mice carrying a targeted
disruption of the rhodopsin gene, the severities of retinopathies
in Rho-/- mice on C57BL/6J and 129Sv congenic backgrounds
were compared by light microscopy and electroretinography and
qualitatively by in situ end labeling of DNA in apoptotic
photoreceptor nuclei of retinal sections. Cone photoreceptor
viability and function were shown to deteriorate more slowly
on the C57BL/6J background in comparison to that of the 129Sv,
with significantly greater numbers of outer nuclear layer nuclei
in the retinas of C57BL/6J mice at 3 and 4 months of age. Both
amplitude and waveform features of the ERG were shown to be
remarkably different in the two strains, indicating an
approximately 6-fold difference in C57BL/6J Rho-/-
mice compared to 129Sv Rho-/- mice at 80 days. Thus,
in comparison with the 129Sv strain, genetic modifiers appear
to constitute a component of the C57BL/6J background, the
expression of which significantly protects cone photoreceptors
from apoptotic death in a mutation-induced murine retinopathy.
The differences in phenotype revealed in this study are sufficient
in principle to provide a basis for comparisons to be made between
QTLs in light-induced and mutation-induced systems.