New mosaic retinitis pigmentosa mutation reported

Investigators in Israel have discovered a retinitis pigmentosa-causing mutation for which unaffected individuals can be mosaic, followed by autosomal dominant inheritance in successive generations.

The discovery arose out of the first ever analysis of the spectrum of rhodopsin (RHO) mutations in the Israeli and Palestinian populations, write Avigail Beryozkin of Hadassah-Hebrew University in Jerusalem, Israel and colleagues.

They published their findings in Investigative Ophthalmology & Visual Sciences.

A heterogeneous group of common retinal degenerations, retinitis pigmentosa affects about 1 person in 4500 in the United States and Europe, and about 1 in 2100 in the vicinity of Jerusalem, they report.

The syndrome typically begins with loss of night and peripheral vision, and eventually leads to total vision loss in most patients.

People inherit it in different patterns, including autosomal recessive, autosomal dominant and X-linked.

In search of the cause, researchers first identified mutations in the RHO gene. This includes a single mutation, p.Pro23His, in a large number of American patients with autosomal dominant retinitis pigmentosa, the most common retinitis pigmentosa mutation in North America.

Since then researchers have found more than 150 RHO mutations responsible for 16% to 35% of autosomal dominant retinitis pigmentosa and autosomal dominant congenital stationery night blindness.

Analyses of the Israeli and Palestinian populations in the past decade have turned up a lot of founder mutations as the cause of autosomal recessive retinitis pigmentosa, but not mutations associated with the autosomal dominant form of the condition.

So Beryozkin and colleagues undertook a survey of RHO mutations in a cohort of 38 families with the autosomal dominant condition, 3 families of Arab Muslims and 29 families of Israeli Jews.

The investigation included clinical evaluations of the patients’ family histories, full ophthalmologic examinations, electroculography, full-field electroretinography, colour vision testing using the Ishihara 38-plate test and Farmsworth D-15 panel, optical coherence tomography, colour and infrared fundus photography, autofluorescence imaging and fluorescein angiography.

The researchers extracted genomic DNA from peripheral blood of the participants using FlexiGene DNA kits (Qiagen, Hilden German). They performed whole exome sequencing analysis on 9 samples.

They screened the DNA samples for RHO exons and flanking intronic sequences with protease chain reaction (PCR) screening followed by Sanger sequencing.

They identified the cause of the disease in 2 families: a PRPF3 mutation in one family and a previously reported RHO mutation in another.

Sanger sequencing analysis of the 5 RHO exons in the remaining 30 index cases did not reveal any mutations.

But a combination of whole exome sequencing and Sanger sequencing revealed 8 disease-causing mutations in 9 families of which 1 mutation was novel.

The 7 previously known mutations are missense. Five of them alter amino acid residues that have been shown to be affected by other mutations. The newly discovered mutation is a tandem duplication of 91 nucleotides.

Twelve patients with heterozygous RHO mutations all had clinical symptoms and signs of retinitis pigmentosa. The duration of the disease ranged from 6 to 43 years and the visual acuity from 0.3 to 1.0. The fundus appearance was compatible with typical retinitis pigmentosa, except for one patient, who was diagnosed with sector retinitis pigmentosa.

The investigators concluded that RHO mutations are most likely the major cause, of autosomal dominant retinitis pigmentosa in Israel and Palestine, since these mutations caused about 28% of the disease in this cohort. That proportion is similar to findings other countries, including Italy, the United Kingdom, the United States and France.

“The fact that most mutations identified here were already reported in other populations is surprising since marked differences in mutation spectrum have been reported in autosomal recessive [retinitis pigmentosa]-causing genes among the Israeli/Palestinian populations versus the European/US populations,” the researchers write.

Only one of the mutations, c.4035C>T, p.Arg135%rp, was shared by patients from unrelated families. Since one of these families is from Turkmenistan Jewish origins while the other is from North African Jewish origins, the researchers speculate that this mutation occurred in a mutational hotspot.

The novel mutation uncovered in this investigation, c.548-638dup91bp, might shed light on the amount of normal rhodopsin protein required to prevent retinal degeneration, the researchers wrote.

The mutation affected two generations who were diagnosed with retinitis pigmentosa at an early age. A detailed ocular evaluation, including electroretinography, did not show any visual abnormalities among the grandparents. But a detailed genetic analysis of a grandfather showed that a small proportion of his blood cells carried the mutation.

Two of his five children have retinitis pigmentosa, and one of these, who was available for the study, carried the duplication heterozygously.

The researchers therefore made the assumption that the grandfather carries a de novo retinitis pigmentosa-causing mutation appearing in both in somatic cells and the germ line. This would make him the first individual reported as mosaic for a retinitis pigmentosa mutation, they write.

By using a sensitive PCR assay, they determined that 6.5% of the alleles in his blood cells were mutated, resulting in 13% of cells that were heterozygous for the mutation.

If this fraction reflects other organs, they reason, then it is not sufficient to cause retinal degeneration.

The finding leads the researchers to suggest that screening for retinitis pigmentosa mutations should take into account the possibility that isolated retinitis pigmentosa cases and non-conclusive inheritance patterns can result from such de novo mutations, including mosaic ones.