LONDON _ Researchers in Scotland and Germany have identifiedone of the main genes responsible for the devastating eye diseaseknown as retinitis pigmentosa, which causes blindness. The genecodes for a previously unknown protein and researchers hope theirdiscovery will lead both to new screening tests for carriers of thedisease and to novel therapies.
Retinitis pigmentosa causes the retina of the eye to degenerate,leading to night blindness and tunnel vision. The term encompasses aheterogenous group of conditions, known to be caused by mutationsin many different genes. In the general population, between one andtwo in every 5,000 people suffer from some form of retinitispigmentosa.
The joint British/German discovery, reported in the May issue ofNature Genetics, relates to one of the most common forms of retinitispigmentosa, known as X-linked retinitis pigmentosa (xlRP). This isthe most severe form of retinitis pigmentosa and affects one in 25,000people. Sufferers begin to show night blindness during the firstdecade of life. Later, as more of the cells in the retina die, tunnelvision develops, so affected individuals are blind by their 30s or 40s.
Because the gene for this form of the disease is carried on the Xchromosome, it mainly affects males. Women who carry the genehave a 50 percent chance of having an affected son, and a 50 percentchance of having a daughter who also is a carrier. There is no reliabletest for carrier status, as these women do not normally show anysymptoms of the disease.
Kate Dry, postdoctoral scientist at the Medical Research Council'sHuman Genetics Unit, in Edinburgh, and one of the authors of thepaper, said: "This is a particularly devastating condition becausethere is no cure for it. However, we hope that this discovery willallow us to develop some sort of DNA screening test to identifycarriers and permit early diagnosis. Our ultimate objective will be totry to develop some sort of therapy. That is a very long way off butfinding the gene is an important step in the right direction."
Chris Inglehearn, senior research fellow at the Institute ofOphthalmology, in London, describes the discovery as "veryimportant." Other genes that have been implicated in retinitispigmentosa code for known proteins, such as rhodopsin, the retinalphotopigment which traps light falling on the retina. "What is soimportant about this finding is that the group has found a completelynew gene that causes retinitis pigmentosa when mutated. It will tell ussomething about the function of the normal eye and may provide bigclues to the causes of other retinal diseases," he said.
Dry explained that work on finding the genes responsible for retinitispigmentosa began in Edinburgh about 15 years ago when AlanWright set about collecting information from a large number offamilies affected by the disease. Much of the initial work focused onthe inheritance of DNA markers to see which ones were co-inheritedwith the disease.
Earlier Localization Work May Have Been Misleading
"One of the early important breakthroughs occurred when Dr. Wrightand his colleagues at that time determined that there were probablytwo genes on the X chromosome that cause this condition," Dry said."Now, 12 years later, we have identified one of them."
This early work localized the gene to between two known genes,CYBB which causes the condition chronic granulomatous diseaseand OTC which codes for the enzyme ornithine transcarbamylase, inthe region of the X chromosome called Xp21.1. Initial attempts toisolate the gene focused on a patient with part of his X chromosomemissing, including Xp21.1. This now appears to have given amisleading localization for the X-linked retinitis pigmentosa gene andthe cause of the retinitis pigmentosa in this particular patient remainsa mystery, the researchers said.
One year ago, both the team in Edinburgh and a team from Germany,including Thomas Meitinger and Alfons Meindl, from the departmentof Pediatric Genetics at the University of Munich, independentlyidentified a candidate gene that lay between CYBB and OTC.Although they later excluded this as the gene for X-linked retinitispigmentosa, this led them to collaborate in studying a new patientwith a very small deletion spanning only 75 kilobases of DNA. Thissmall region became the focus for a renewed search for the gene.
The researchers sequenced this section of DNA, before applyingcomputer programs which can identify pieces of DNA that have thefeatures of a gene. "We ended up with very small fragments ofDNA," Dry said. "We knew that if these fragments were parts of realgenes, they would be expressed in the correct tissue _ in other words,in the retina." Tests showed that the fragments were, indeed, identicalto fragments of copy DNA derived from the retina and other eyetissues.
"This was very encouraging," Dry said. The researchers then setabout screening all their patients with xlRP for mutations within thesesequences, using a technique called single-stranded conformationalanalysis. This involves obtaining single-stranded DNA from thesequences in question and running them on a non-denaturingpolyacrylamide gel. Single-stranded DNA which contains mutationstakes on a different conformation to single stranded normal DNA,and thus runs at a different speed on the gel. "We identified a numberof mutations in this way, which confirmed that this was the rightgene," Dry said.
The researchers have called the gene that they have identified theretinitis pigmentosa GTPase regulator (RPGR). Part of the proteinmade by RPGR is similar to another molecule, RCC1 (regulator ofchromosome condensation), which is a guanine nucleotide exchangefactor.
RCC1 regulates a protein that belongs to a family of small GTPaseproteins that are important for many cellular processes. The similarityof RCC1 with RPGR suggests that RPGR, too, may play a role in theexchange of guanine nucleotides.
However, Dry said, "We know very little about the function of theprotein at this stage. We can only speculate that because of thissimilarity, our protein may have a similar function to RCC1 and beinvolved in the transport or exchange of guanine nucleotides, whichare involved in cellular signaling."
No biotechnology companies have so far expressed an interest inbecoming involved in developing a DNA screening test for the gene.
Apart from a test for carriers, the researchers said they hope todevelop some form of gene therapy. "The eye is highly amenable togene therapy because it is so accessible," Dry said. n
-- Sharon Kingman Special To BioWorld Today
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