Codon 269 3bp insertion.

The 48 year old proband from this family and her 56-year-old sister were both

completely asymptomatic, with visual acuities of 6 /6 right and left. Each was referred

following a routine optometric examination. Fundal examination revealed an area of bone spicule pigmentation interiorly in both retinas that was smaller and more discrete than that typically seen in age matched individuals from the families with other

rhodopsin mutations.

Perimetry in the proband demonstrated a corresponding, relatively localised superior visual field defect with unrecordable rod and cone thresholds at 10-30° off axis superiorly and normal thresholds in the inferior fields (Figure 5.7).

Electrophysiology also demonstrated relatively mild abnormalities of rod and cone function. The light rise of the EOG was normal in both subjects. In the 48 year old proband both rod ERG amplitudes and the a-wave of the maximal ERG response were mildly subnormal. The implicit times of her photopic ERG were mildly increased. In her 56 year old sister the scotopic b-wave and the P50 component of the PERG were of mildly subnormal amplitude in the right eye, but normal in the left. Cone derived ERG’s showed no abnormality of amplitude or implicit time.

cSLO imaging did not reveal a ring of hyperfluorescence around the fovea similar to that observed in patients with the Thr58Arg mutation.

Rhodopsin Mutations in Sectorial RP

Figure 5.7 Scotopic (blue stimulus) 60° visual field of a subject with the codon

269 3bp insertion.

Visual field of the right eye of patient 11:2, aged 48 years, from family 631811. The area of superior field loss is markedly smaller than that seen in the other families.

RIGHT QUAD TOTAL 642 80® . :: FOVEA: 7 OB QUAD TOTAL 557 QUAD TOTAL 1040 QUAD TOTAL 1068 GRAYTONE SYMBOLS SYM ASB OB iiiiiiii

M

M

Ri

5.4

Discussion.

In this study rhodopsin mutations were found in 11 out o f 16 apparently

unrelated families with sectorial RP (69%). Only five different mutations were

identified, indicating that a limited number of rhodopsin mutations may account for a

large proportion of cases of sectorial RP in the U.K. We did not find any individuals

with the Pro23His mutation, which is the commonest rhodopsin mutation in North

America, and is associated with a sectorial or regional phenotype. The presence of the Thr58Arg mutation in several families, one of which is of Indian origin, suggests that

this may be a mutational hotspot, similar to the rhodopsin codon 347 mutation (Gal et

al, 1997).

Prior to the development of molecular classifications, it was shown that adRP

could be classified according to the pattern o f photoreceptor degeneration (Lyness et

al, 1985; Massof and Finkelstein,1987; Kemp et al, 1988)”. Type-1 or diffuse adRP

is characterised by diffuse loss of rod function with relative preservation of cone function in the early stages of the disease. Patients frequently present early, with night blindness evident before the age of 10 years. Further visual symptoms and retinal

pigmentary changes may, however, be delayed for 1 0 -2 0 years after the onset of

disease. There is good evidence that loss of visual sensitivity is due to cell dysfunction rather than cell death in the early stage of disease. In contrast, Type-2 or regional adRP, which is more common, displays regional or patchy loss o f rod function accompanied by concomitant loss of cone function in the affected areas. Type-2 adRP has a variable age of onset, and may rapidly progress to develop symptomatic visual field loss. In this form o f RP loss of function can be explained by cell death or loss of outer segments.

Rhodopsin Mutations in Sectorial RP

The sectorial RP phenotypes observed in our subjects consistently exhibited simultaneous loss of rod and cone function confined, largely or exclusively, to the inferior retina. This was typically associated with a relatively late onset of nyctalopia, with progression to symptomatic visual field loss within five to ten years, and with the presence of inferior intraretinal pigmentation even in asymptomatic individuals. It is reasonable, therefore, to consider sectorial RP as a comparatively benign variant of regional or Type 2 adRP.

Electrodiagnostic and psychophysical assessment of sectorial RP patients of differing ages suggests a progressive concomitant loss of rod and cone function with

increasing age. This is consistent with the findings of earlier studies (Fishman et al,

1991; Fishman et al, 1992a; Fishman et al, 1992b; Fishman et al, 1992c), and

indicates that lack o f disease progression should probably not be used as a diagnostic criterion for sectorial RP.

Whilst the findings on clinical examination in many of these families are similar, more detailed evaluation of the ocular phenotypes reveals significant differences between them. For example, whilst electrophysiological assessment of a 38 year old subject with the Thr58Arg mutation revealed no unequivocal abnormality, a 34 year old patient with a Aspl90Asn mutation was found to have a bilaterally subnormal EOG light rise, subnormal rod and cone derived ERG amplitudes, an increased 30Hz flicker implicit time, and a subnormal PERG.

The ‘negative’ ERG observed in the subject with a Glyl06Arg rhodopsin

mutation appears to be unique. It is difficult to explain why a mutation in a protein exclusively expressed in rod photoreceptors should predominantly affect the b-wave of the ERG, which is derived from post-receptoral neural elements, rather than the a- wave, which is generated in relation to phototransduction. A ‘negative’ ERG is more

usually seen in conditions that predominantly affect the inner retina, such as central retinal artery occlusion. X-linked juvenile retinoschisis, resulting from mutations in

the gene XLRSl, is one of the few inherited dystrophies that give rise to an

electronegative ERG (Sauer et al, 1997). The XLRSl protein, retinoschisin, is

expressed exclusively in photoreceptor cells in murine retina, but it is also secreted into the inner retina, and may have a role in photoreceptor-Müller cell interaction

(Grayson et al, 2000).

Histological studies of human retinas affected by retinitis pigmentosa have demonstrated that rods, amacrine cells and horizontal cells may all undergo neurite sprouting in regions with significant photoreceptor loss’. When neurite sprouting does occur, the majority of rod neurites bypass the dendrites of horizontal and bipolar cells, the normal targets of rod axons in the outer plexiform layer, and directly contact the hypertrophied processes of Müller cells, which have undergone reactive gliosis in response to photoreceptor cell death, and the somata of amacrine cells. These changes in retinal neuronal circuitry may contribute to the electroretinographic abnormalities observed in RP, and in some instances to the development of an electronegative ERG.

An increased 30Hz flicker implicit time was noted in several patients in this study, and a review of the literature suggests that the sectorial RP phenotype

associated with virtually all rhodopsin mutations involves a delay in the cone-

mediated ERG in older subjects, even when a normal implicit time is observed in

younger individuals (Sullivan et al, 1993; Hayakawa et al, 1993; Stone et al, 1991;

Fishman et al, 1991; Fishman et al, 1992a; Fishman et a l, 1992b; Fishman et al,

1992c). A normal 30Hz flicker implicit time has previously been suggested as a diagnostic criterion for sectorial RP (Berson and Horward, 1971), but in view of these findings it seems possible that this may be an age-dependent phenomenon.

Rhodopsin Mutations in Sectorial RP

All patients in the present study had relative sparing o f the central 5° of the superior visual field, and frequently a selective loss of the inferior visual field immediately surrounding the central 5°. In some cases a ring of increased autofluorescence was observed around the fovea. These psychophysical and cSLO data suggest a possible mechanism for the concomitant loss of rod and cone function in the affected retina. Abnormal rod opsin molecules may interfere with outer segment disc assembly, structure, or stability. This may result in increased turnover of rod outer segments and a greater demand for phagocytosis of shed outer segments by the RPE, leading to overloading of the RPE. The affected RPE might then be unable to support cone function. The ring of increased autofluorescence surrounding the fovea,

and the selective loss of visual field at 1 0-2 0° eccentricity, may result from the

increased density of rods known to exist at this location, since increased rod density would be predicted to result in greater RPE overload. In the cone-dominated foveal area, however, the RPE would remain healthy and cone function would be preserved.

Given the enormous range of genotypes associated with RP, and retinal dystrophies in general, sectorial RP represents a rare example o f an RP phenotype in which there is a high probability of detecting mutations in a specific gene. Mutation

screening of the rhodopsin gene is a relatively simple, cost effective, and well-

established molecular genetic technique. It is practical, therefore, to screen all

individuals with the sectorial RP phenotype for rhodopsin mutations, and to utilise the

information gained in prognostic and genetic counselling.

Sectorial RP is also a fascinating phenotype from the perspective o f disease

pathogenesis, since the causative rhodopsin mutations are obviously present in every

rod photoreceptor throughout the retina, and yet the resulting retinal degeneration has an extremely strong predilection for the lower half o f the retina. A histological study

of the retinas of a subject with the rhodopsin ThrlTMet mutation demonstrated that the inferior retina contained no rods and rare cone somata, correlating with an absolute scotoma observed in the superior field, whilst the superior retinas had near-normal- appearing rods and cones in the far periphery and a gradient from the midperipheral to central retina of progressively shortened outer segments and loss of photoreceptors.

Evidently the rhodopsin mutation alone is necessary but not sufficient to bring

about rod cell death. Heckenlively and co-workers (1991) studied four affected individuals from two families with sectorial retinitis pigmentosa caused by the Pro23His mutation and suggested, based on their patients’ history of light exposure and the location of degeneration in the retina, that light phototoxicity might play a role in the pathogenesis of sectorial RP. This hypothesis remains unproven, and further studies will be required to elucidate the nature of the factors that interact with specific

rhodopsin mutations to produce the altitudinal distribution of retinal degeneration that

NRL SerSOThr Mutation