In patients with autosomal-recessive retinitis pigmentosa (arRP), homozygosity mapping
was performed for detection of regions harboring genes that might be causative for
RP. In one affected sib pair, a shared homozygous region of 5.0 Mb was identified
on chromosome 6, within the RP25 locus. One of the genes residing in this interval
was the retina-expressed gene EGFL11. Several genes resembling EGFL11 were predicted
just centromeric of EGFL11. Extensive long-range RT-PCR, combined with 5'- and 3'-
RACE analysis, resulted in the identification of a 10-kb transcript, starting with
the annotated exons of EGFL11 and spanning 44 exons and 2 Mb of genomic DNA. The transcript
is predicted to encode a 3165-aa extracellular protein containing 28 EGF-like and
five laminin A G-like domains. Interestingly, the second part of the protein was found
to be the human ortholog of Drosophila eyes shut (eys), also known as spacemaker,
a protein essential for photoreceptor morphology. Mutation analysis in the sib pair
homozygous at RP25 revealed a nonsense mutation (p.Tyr3156X) segregating with RP.
The same mutation was identified homozygously in three arRP siblings of an unrelated
family. A frame-shift mutation (pPro2238ProfsX16) was found in an isolated RP patient.
In conclusion, we identified a gene, coined eyes shut homolog (EYS), consisting of
EGFL11 and the human ortholog of Drosophila eys, which is mutated in patients with
arRP. With a size of 2 Mb, it is one of the largest human genes, and it is by far
the largest retinal dystrophy gene. The discovery of EYS might shed light on a critical
component of photoreceptor morphogenesis.