Vertebrate photoreceptor cells are the basic sensory apparatus of the retin
a, capable of converting the energy of absorbed photons into neuronal signa
ls, The proximal portions of mammalian photoreceptor outer segments are syn
thesized daily by cell bodies, and outer segment tips are shed with a circa
dian rhythm, resulting in a complete turnover of outer segments about every
9 days, The shed outer segments are phagocytosed by adjacent retinal pigme
nt epithelial (RPE) cells, and metabolites are recycled to photoreceptors,
The Royal College of Surgeons (RCS) rat is a widely studied, classic model
of recessively inherited retinal degeneration in which the RPE fails to pha
gocytose shed outer segments, and photoreceptor cells subsequently die. We
have used a positional cloning approach to study the rdy (retinal dystrophy
) locus of the RCS rat, Within a 0.3 cM genetic inclusion interval, we have
discovered a small deletion of RCS DNA that disrupts the gene encoding the
receptor tyrosine kinase Mertk, The deletion includes the splice acceptor
site upstream of the second coding exon of Mertk and results in a shortened
transcript that lacks this exon, The aberrant transcript joins the first a
nd third coding exons, leading to a frameshift and a translation terminatio
n signal 20 codons after the AUG, The concordance of these and other data i
ndicate that Mertk is probably the gene for rdy, Our results provide geneti
c evidence for an essential role of a receptor tyrosine kinase in a special
ized form of phagocytosis and suggest a molecular model for ingestion of ou
ter segments by RPE cells.