Main

Sir,

The reported prevalence of retinitis pigmentosa (RP) in China is 25 per 100 000,1 which is similar to the rate of 19–27 per 100 000 observed in Western countries.2 Genes causing RP have been identified by a combination of linkage mapping, cloning, and candidate testing. At present, close to 100 rhodopsin mutations have been identified in adRP patients and the existence of other adRP loci has been established.3 Genetic heterogeneity, allelic heterogeneity, and clinical heterogeneity have been demonstrated among patients with adRP, arRP, and X-linked patterns of inheritance.4,5,6 Digenic RP caused by the simultaneous presence of a mutation in the peripherin/RDS gene and a mutation in the Rom1 gene, and one family with RP involving mitochondrial inheritance, have also been described.7,8

Case report

RP has been clinically diagnosed in 32 members of a five-generation family of 111 persons, aged 3–82 years, from Hubei Province, People's Republic of China (Figure 1). The proband is a 58-year-old male (III17) diagnosed with night-blindness in early childhood, and showing gradual deterioration of peripheral sight and almost total loss of vision by 52 years of age. Symptoms in the various affected family members include night-blindness, impaired vision, and visual field loss. Their visual deterioration commenced between 20 and 30 years of age, with symptoms usually starting in one eye and progressing bilaterally. Progressive myopia and astigmatism, and retinochoroidal dystrophy are found in all patients. The family members underwent clinical examination, testing for visual acuity (VA) and field loss, fundus examination and fundus photography in Tongji Medical College, Huazhong University of Science and Technology, China. The earliest ocular changes in members of the family were a pale fundus with pigment granularity, followed by narrowed vessels and bone spicule-like pigment accumulation by 10–20 years of age. The pigment deposits increased and approached the posterior pole by 30–50 years of age (Figure 2). Eight patients over 30 years of age have severe retinochoroidal atrophy and a bull's-eye macula is seen in most patients. Teenage patients in the family have peripheral lens opacities, and young adult patients have early nuclear sclerosis confirmed by lens opacitometry. A 23-year-old patient (IV61) with VA of 0.02 also has posterior subcapsular cataracts and retinal detachment in his left eye. Several persons over the age of 30 years old are completely blind (III22, III23, III24, and IV39), and other individuals over the age of 50 years (II8, II9, and III12) are severely visually handicapped (VA=0.00). Besides visual impairment, other abnormalities including dementia (III30), digital abnormalities (II4, III16, and III18), deaf-mutism (IV45), and mental retardation (IV78 and V109) have also been observed in this pedigree.

Figure 1
figure 1

Pedigree of the family with RP patients. Persons who married into the family were not numbered. □, normal male; ○, normal female; ▪, affected male; •, affected female; /, deceased; ↗, proband.

Full size image
Figure 2
figure 2

Fundus examination of the proband: pale fundus with pigment granularity, followed by narrowed vessels and bone spicule-like pigment accumulation.

Full size image

Primary candidate genes (ie RP1, RP7, RP9, RP11, RP13, Rom1, Rom3, rhodopsin, and peripherin/rds) have been screened by a combination of SSCP and RFLPs techniques in eight affected (III17, III18, III23, III26, IV61, IV78, IV84, and V109) and 10 unaffected family numbers (III19, III20, III21, III28, IV58, IV59, IV62, IV71, IV76, and IV83) available for blood sampling from the pedigree. A previously undescribed missense mutation was identified in the rhodopsin gene at exon 1, codon 52 (TTC → TAC) in all eight affected individuals, resulting in a substitution of phenylalanine (Phe) to tyrosine (Tyr). The mutation was confirmed by direct DNA sequencing.

Comment

Although both Tyr and Phe have similar hydroxyl groups, Tyr is an uncharged polar residue, whereas Phe is nonpolar. The mutation could therefore potentially affect a crucial ligand-binding site or a particular fold. The programme PredictProtein (http://maple.bioc.columbia.edu/pp/) indicated that the mutation did not significantly alter the secondary protein structure. However, it is possible that Tyr interferes with the helical structure encoded by the upstream motif WQFSMLAAYMFLLI.

MaxHom analysis of rhodopsin homologues showed that Phe is conserved across mammalian species,9 which suggests that it may be critical in ensuring normal visual function. This adds support to the proposition that the Phe → Tyr substitution is the disease mutation. Owing to no restriction enzyme cutting site created or ablated by the sequence change, the initial SSCP method was employed to screen the mutation in 50 unrelated Chinese individuals (25 male and 25 female) from the same Han Chinese ethnic group in Wuhan. The sequence change in the rhodopsin gene was not found in these 100 Chinese control chromosomes, indicating that the mutation is not a polymorphism. Given the level of clinical heterogeneity observed in the family, cytogenetic analysis and mtDNA sequencing will also be undertaken to identify other potential causes associated with additional or alternative candidate regions.