Retinitis Pigmentosa 3, X-Linked

Clinical Characteristics
Ocular Features: 

Retinitis pigmentosa is a large group of disorders with great clinical and genetic heterogeneity.  The ocular disease is characterized by night blindness, field constriction, and pigmentary changes in the retina.  The later may have a ‘bone corpuscle’ appearance with a perivascular distribution.  A ring scotoma is sometimes evident.  Age of onset and rate of progression is highly variable, even within families.  In this, an X-linked form of the disease, the first symptoms often appear early in the second decade of life.  The rods are impacted early but cone deterioration with loss of central vision usually follows.  Some patients complain of dyschromatopsia and photophobia.  The ERG generally documents this progression but the mfERG shows wide variations in central cone functioning.  Legal blindness is common by the 4thor 5thdecades of life.  The course of clinical and ERG changes is more aggressive in the X-linked form than in autosomal dominant retinitis pigmentosa disease resulting from RHO mutations.  The final common denominator for all types is first rod and then cone photoreceptor loss through apoptosis.

Up to 50% of adults develop cataracts beginning in the posterior subcapsular area.  The vitreous often contains cells and some patients have cystoid macular edema.  A waxy pallor of the optic nerve is sometimes present especially in the later stages of the disease.

Female carriers generally are asymptomatic but may also have severe RP.  Occasionally they have an unusual tapetal reflex consisting of a ‘beaten metal’ appearance or sometimes scintillating, golden patches. 

Systemic Features: 

There is no systemic disease in ‘simple’ or non-syndromic retinitis pigmentosa but pigmentary retinopathy is associated with a number of syndromes (syndromal RP) e.g.,  Usher syndromes, Waardenburg syndrome, Alport syndrome, Refsum disease, Kerns-Sayre syndrome, abetalipoproteinemia, neuronal ceroid lipofuscinosis, mucopolysaccharidoses types I, II, III, and Bardet-Biedl syndromes. 

The RPGR gene is important to the normal function of cilia throughout the body.  For this reason disorders resulting from RPGR mutations such as CORDX1 (304020) and this one are sometimes classified as primary ciliary dyskinesias or ciliopathies.  The gene products of the RPGR gene, for example, are localized to connecting cilia of the outer segments of rods and cones and in motile cilia in the airway epithelia.  A subset of families with RP3 have chronic and recurrent upper respiratory infections including sinusitis, bronchitis, pulmonary atelectasis, and otitis media (300455) similar to that seen in the immotile cilia syndrome (244400).  Female carriers in these families have few retinal changes but may suffer recurrent and severe upper respiratory infections similar to hemizygous males.  Severe hearing loss also occurs in both sexes with the RPGR mutations and there is some evidence that this may be a primary sensorineural problem, perhaps in addition to conductive loss from recurrent otitis media.

Genetics

Mutations in more than 100 genes may be responsible for retinitis pigmentosa but sporadic disease occurs as well.  Between 5 and 10% of individuals have X-linked disease.  Perhaps 70% of X-linked RP is caused by mutations in RPGR (Xp11.4) as in this condition.  The same gene is mutant in one form of X-linked cone-rod dystrophy (CORDX1; 304020). These  disorders are sometimes considered examples of X-linked ocular disease resulting from a primary ciliary dyskinesia (244400).

Another form of X-linked RP (RP2) with more choroidal involvement is caused by mutations in the RP2 gene (312600 ; Xp11.23). 

Many genes associated with retinitis pigmentosa have also been implicated in other pigmentary retinopathies.  In addition numerous phenocopies occur, caused by a variety of drugs, trauma, infections and numerous neurological disorders.  To make diagnosis even more difficult, the fundus findings and ERG responses in nonsyndromic RP in most patients are too nonspecific to be useful for classification. Extensive systemic and ocular evaluations are important and should be combined with genotyping in both familial and nonfamilial cases to determine the diagnosis and prognosis. 

Treatment
Treatment Options: 

Photoreceptor transplantation has been tried in 8 patients without improvement in central vision or interruption in the rate of vision loss.  Longer term results are needed.  Resensitizing photoreceptors with halorhodopsin using archaebacterial vectors shows promise in mice.  High doses of vitamin A palmitate slow the rate of vision loss but plasma levels and liver function need to be checked at least annually.  Oral acetazolamide can be helpful in reducing macular edema.

Low vision aids and mobility training can be facilitating for many patients.  Cataract surgery may restore several lines of vision at least temporarily.

Several pharmaceuticals should be avoided, including isotretinoin, sildenafil, and vitamin E. 

References
Article Title: 

References

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Sandberg MA, Rosner B, Weigel-DiFranco C, Dryja TP, Berson EL. Disease course of patients with X-linked retinitis pigmentosa due to RPGR gene mutations. Invest Ophthalmol Vis Sci. 2007 Mar;48(3):1298-304.

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Janaky M, Palffy A, Deak A, Szilagyi M, Benedek G. Multifocal ERG reveals several patterns of cone degeneration in retinitis pigmentosa with concentric narrowing of the visual field. Invest Ophthalmol Vis Sci. 2007 Jan;48(1):383-9.

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Berger AS, Tezel TH, Del Priore LV, Kaplan HJ. Photoreceptor transplantation in retinitis pigmentosa: short-term follow-up. Ophthalmology. 2003 Feb;110(2):383-91.

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Zito I, Downes SM, Patel RJ, Cheetham ME, Ebenezer ND, Jenkins SA, Bhattacharya SS, Webster AR, Holder GE, Bird AC, Bamiou DE, Hardcastle AJ. RPGR mutation associated with retinitis pigmentosa, impaired hearing, and sinorespiratory infections. J Med Genet. 2003 Aug;40(8):609-15.

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