Extreme hyperopia is the result of null mutations in MFRP, which encodes a Frizzled-related protein

Abstract

Nanophthalmos is a rare disorder of eye development characterized by extreme hyperopia (farsightedness), with refractive error in the range of +8.00 to +25.00 diopters. Because the cornea and lens are normal in size and shape, hyperopia occurs because insufficient growth along the visual axis places these lensing components too close to the retina. Nanophthalmic eyes show considerable thickening of both the choroidal vascular bed and scleral coat, which provide nutritive and structural support for the retina. Thickening of these tissues is a general feature of axial hyperopia, whereas the opposite occurs in myopia. We have mapped recessive nanophthalmos to a unique locus at 11q23.3 and identified four independent mutations in MFRP, a gene that is selectively expressed in the eye and encodes a protein with homology to Tolloid proteases and the Wnt-binding domain of the Frizzled transmembrane receptors. This gene is not critical for retinal function, as patients entirely lacking MFRP can still have good refraction-corrected vision, produce clinically normal electro-retinograms, and show only modest anomalies in the dark adaptation of photoreceptors. MFRP appears primarily devoted to regulating axial length of the eye. It remains to be determined whether natural variation in its activity plays a role in common refractive errors.

Fig. 1.

Morphological features of nanophthalmos. A diagram of the eye, sagittal section, and light path is shown. (A) Normal, closer view. (B) Nanophthalmic. Adapted from ref. to reflect eye morphology of individual 7 in kindred A.

Fig. 2.

Inheritance of nanophthalmos. Kindred A, showing individuals relevant to the current study, and participants in an earlier study (patients 17–19) (10) are shown. All offspring are shown only in VI.

Fig. 3.

Clinical features of nanophthalmos in kindred A. (A) Right eye (RE) of patient 5, with vertical slit lamp illumination. (B) Central retinal fundus, RE of patient 7. (C) Lateral retina, RE of patient 7. (D) Ocular features of individuals 1–16. Refraction indicates spherical +1/2 cylindrical. LE, left eye; nd, not determined. (E) A-scan immersion ultrasound, RE of patient 7. Shown are the return times of echo peaks along the optic axis. An interpretation, in mm, is based on 10 independent scans. (F Left) ERG tracings, RE of patient 7. Time of a brief whole-field flash administered in the dark to a dark-adapted eye (scotopic) is indicated by vertical dashed lines. Electrical response of the retina, as measured between body and cornea, is indicated in μV. A bright flash over background illumination (photopic) or a repeating train of flashes was used to isolate cone responses. (F Right) Results from a second clinical examination of patient 7, indicating peak amplitudes in μV for A-waves (photoreceptor hyperpolarization) and B-waves (postsynaptic depolarization of retina). (G) (Upper) Dark-adaptation kinetics. Visual pigment of the whole eye is bleached by 5 min of bright white light (1,000 candelas per m²). Time in darkness immediately after the bleaching is indicated on the x axis. The y axis indicates minimum light intensity in microapostilbs that can be seen by the subject at a given time. The testing stimulus for rods and cones is a flashing disk 7° in diameter, placed 15° below a dim red focal target. (Lower) Dark adaptation, RE of patient 5, using LKC Technologies Ganzfeld.

Fig. 4.

Genetic linkage to the NNO2 disease locus. (A) Chromosomal position marked in Mb and cM. Two-point logarithm of odds (LOD) scores are shown. For Z,at θ = 0 (mutation coincident with marker), and for Z_max, maximum LOD score, at θ_max (recombination frequency between mutation and marker). (B) Map of 2.14-Mb disease interval and known genes, homozygous for all affected in generation VI. Black bar indicates 383-kb interval homozygous in patient 14.

Fig. 5.

Haplotypes. Chromosome 11 marker allele haplotypes for the linkage pedigree are shown below genotyped individuals. Boxes contain haplotypes of the disease chromosome.

Fig. 6.

Mutations in MFRP. (A) Automated sequencing trace of a homozygous exon 10 mutation in the Amish-Mennonite kindred. (B) A homozygous exon 5 amber stop codon mutation in the proband from kindred B. (C) Compound heterozygosity in exon 5 shows a frameshift deletion and a point substitution in two affected sisters of kindred C. (D) Diagram of normal MFRP (top row). Below are encoded mutant proteins in kindreds A–C and mouse rd6. N, amino terminus; C, carboxyl terminus; P, proline-rich interval; Mem, membrane spanning helix; STP, serine-threonine-proline-rich interval; CUB, cubilin-related; L, low-density lipoprotein receptor-related. CRD, Frizzled-related CRD.