Michael J. Denton

Mutations in a novel retina-specific gene cause autosomal dominant retinitis pigmentosa.

Inherited retinal diseases are a common cause of visual impairment in children and young adults, often resulting in severe loss of vision in later life. The most frequent form of inherited retinopathy is retinitis pigmentosa (RP), with an approximate incidence of 1 in 3,500 individuals worldwide. RP is characterized by night blindness and progressive degeneration of the midperipheral retina, accompanied by bone spicule-like pigmentary deposits and a reduced or absent electroretinogram (ERG). The disease process culminates in severe reduction of visual fields or blindness. RP is genetically heterogeneous, with autosomal dominant, autosomal recessive and X-linked forms. Here we have identified two mutations in a novel retina-specific gene from chromosome 8q that cause the RP1 form of autosomal dominant RP in three unrelated families. The protein encoded by this gene is 2,156 amino acids and its function is currently unknown, although the amino terminus has similarity to that of the doublecortin protein, whose gene (DCX) has been implicated in lissencephaly in humans. Two families have a nonsense mutation in codon 677 of this gene (Arg677stop), whereas the third family has a nonsense mutation in codon 679 (Gln679stop). In one family, two individuals homozygous for the mutant gene have more severe retinal disease compared with heterozygotes.

Laws of form revisited

efore Darwin, most biologists adheredto a platonic model of nature. Thisimplied that the biological realm con-sisted of a finite set of essentially immutablenatural forms that, like inorganic formssuch as atoms or crystals, are an intrinsicpart of the eternal order of the world. Justas, today, we account for the form of atomsand crystals by a set of physical laws or‘constructional rules’, so pre-darwinianbiologists sought to account for the originof biological forms in terms of a set ofgenerative physical laws often referred to asthe ‘laws of form’.For many biologists today, platonicbiology is an anachronism irretrievably laidto rest, and the idea that biological formsmight be intrinsic features of nature generat-ed by physical laws is treated with increduli-ty. However, recent advances in proteinchemistry suggest that at least one set ofbiological forms — the basic protein folds —is determined by physical laws similar tothose giving rise to crystals and atoms. Theygive every appearance of being invariantplatonic forms of precisely the type that thepre-darwinian biologists were seeking. Protein folds, the basic constructionalunits of proteins, each consist of a foldedchain of between 80 and 200 amino acids.Some proteins consist of a single fold, butmost are a combination of two or more.During the 1970s, as the three-dimensionalstructure of an increasing number of foldswas determined, it became apparent thatthe folds could be classified into a finitenumber of distinct structural families con-taining a number of closely related forms.The fact that protein folds could be classifiedin this manner provided the first line ofevidence that the folds might be naturalforms.Further evidence that the folds doindeed represent a finite set of naturalforms is provided by detailed structuralstudies carried out over the past two decadeswhich have revealed that the structure ofthe folds can be accounted for by whatamounts to a set of ‘constructional rules’governing the way that the various sec-ondary structural motifs, such as a-helicesand b-sheets, can be combined and packedinto compact three-dimensional structures.One is inevitably reminded of the atom-building rules governing the assembly ofsubatomic particles into the 92 atoms of theperiodic table. Consideration of these ‘constructionallaws’ suggests that the total number ofpermissible folds is bound to be restrictedto a very small number — about 4,000,according to one estimate. Confirmationthat this is probably so is provided by a differ-ent type of estimate, based on the discoveryrate of new folds. Using this method,Cyrus Chothia of Britain’s Medical ResearchCouncil estimated that the total number offolds utilized by living organisms may not bemore than 1,000. Subsequent estimates havegiven figures of between 500 and 1,000.Whatever the final figure, the fact thatthe total number of folds represents a tinystable fraction of all possible polypeptideconformations, determined by the laws ofphysics, reinforces the notion that the folds,like atoms, represent a finite set of built-innatural forms.The robustness of the folds offers anoth-er clue. The fact that the folds can retaintheir native conformations in the face ofmultiple different sorts of short-term defor-mations caused by the molecular turbulenceof the cell, and in the face of extensive, long-term evolutionary changes in their amino-acid sequences, is precisely what would beexpected if they are natural forms, specifiedby physical law. Again, the fact that thesame fold can be specified by many differ-ent, apparently unrelated amino-acidsequences, suggesting multiple separate dis-coveries during the course of evolution, isfurther evidence that the folds are intrinsicfeatures of the order of nature. Finally, thefact that in many cases the same fold isadapted to very different biochemical func-tions is precisely what would be expected ifprotein functions are secondary adaptationsof a set of primary, immutable, naturalforms. If forms as complex as the protein foldsare intrinsic features of nature, might someof the higher architecture of life also be deter-mined by physical law? The robustness ofcertain cytoplasmic forms, for example thespindle apparatus and the cell form of ciliateprotozoans such as

Analysis of linkage relationships of X-linked retinitis pigmentosa with the following Xp loci: L1.28, OTC, 754, XJ-1.1, pERT87, and C7

SummaryA number of variants of X-linked retinitis pigmentosa (XLRP) have been described. In one variant, listed in the McKusick (McK) catalogue (McKusick 1983) as entry no. 30320, the heterozygotes exhibit a golden metallic or tapetal reflex. Three large pedigrees segregating for XLRP with the characteristic tapetal reflex in the heterozygotes were examined, and the linkage between the XLRP locus and Xp loci, L1.28, OTC, 754, XJ-1.1, pERT87 and C7 was measured. The strongest linkage was found to be between the XLRP locus and OTC. In addition, recombinational evidence drawn from the three pedigrees suggests that the XLRP locus is distal to L1.28 and proximal to 754. This putative location of the XLRP gene between L1.28 and 754 taken together with the tight linkage to OTC, a locus already located between L1.28 and 754, leads us to propose a gene order of centromere-L1.28-OTC/XLRP-754-telomere.

Preliminary exclusion of an X-linked gene in Leber optic atrophy by linkage analysis

SummaryThe maternal inheritance in Leber optic atrophy suggests that it may be caused by a cytoplasmic or mitochondrial defect. However, the strong male bias and the strict tissue specificity can not be readily explained by a single mitochondrial gene defect alone. Wallace suggested a hypothesis that the disease could be the result of an interaction between an X-linked gene and a mitochondrial DNA defect. Linkage relationships between Leber optic atrophy and 15 X-chromosome markers were analyzed in three large Tasmanian families. The results of two-point linkage analysis showed no close linkage between Leber optic atrophy and any of the 15 markers. The results of multipoint linkage analysis suggested the exclusion of the assumed X-linked gene from almost the whole X chromosome in these families.

Two different genes for X-linked retinitis pigmentosa

Linkage analysis was carried out in three large multigenerational kindreds with X-linked retinitis pigmentosa using DNA markers on Xp. About 10% recombination has been found between the retinitis pigmentosa locus (RP2) and the marker locus DXS7, assigned to band Xp11.3, which was reported earlier to be closely linked to RP2 in several independent families. In the kindreds described in this paper, however, RP2 shows close linkage and no recombination with the marker loci OTC and DXS148, both assigned to Xp21, indicating that, contrary to previous linkage studies, there is evidence of an RP locus distal to DXS7. This suggests that X-linked retinitis pigmentosa is genetically heterogeneous, i.e., caused by mutations at different loci.

Autosomal recessive retinitis pigmentosa locus RP28 maps between D2S1337 and D2S286 on chromosome 2p11-p15 in an Indian family

Retinitis pigmentosa (RP) is a group of clinically and genetically heterogeneous disorders characterised by night blindness, constriction of visual field, and dystrophic changes of the retina. Previous genetic studies have shown extensive allelic and non-allelic genetic heterogeneity of RP. Here we describe an Indian family with multiple consanguineous marriages and a total of four patients with autosomal recessive (AR) RP. The homozygosity mapping strategy was successfully used and indicated close linkage between the disease locus and D2S380, D2S441, D2S291, and D2S1394 with maximum lod scores between 1.51-3.07 at θ=0.00. The analysis of multiply informative meioses maps the locus (RP28) for ARRP in this family between D1S1337 and D2S286 on 2p11-p15. The involvement of visinin (VSNL1), a promising candidate gene assigned to chromosome 2p by previous studies, has been excluded by the absence of linkage.

X-linked megalocornea: close linkage to DXS87 and DXS94

SummaryIn a family in which X-linked megalocornea is segregating, the disease locus was found to be closely linked to DXS87 (zmax=3.91, θmax=0.00) and DXS94 (zmax=3.34, θmax=0.00) in Xq21.3-q22.

Autosomal recessive retinitis pigmentosa locus maps on chromosome 1q in a large consanguineous family from Pakistan

A large Pakistani family with several consanguineous marriages is described, in which autosomal recessive retinitis pigmentosa is segregating. Linkage studies revealed close linkage between the disease locus and six loci on chromosome 1q (D1S158, F13B, D1S422, D1S412, D1S413, and D1S53) with maximum lod scores ranging from 0.988‐4.657 at Θ=0.065‐0.235. However, the analysis of individual nuclear families showed very close linkage without recombination in three branches and several recombinants and negative lod scores throughout in the fourth branch. These results strongly suggest that mutations of two different genes are responsible for the disease in the ‘linked’ and ‘unlinked’ branches. Parallel to the linkage heterogeneity, clear phenotypic differences have been observed among the ‘linked’ and ‘unlinked’ parts. Our findings demonstrate that in case of recessive disorders the possibility of non‐allelic genetic heterogeneity should always be considered, even within the same kindred and in genetic isolates if a largely extended pedigree is analysed.

Oguchi disease: suggestion of linkage to markers on chromosome 2q.

Oguchi disease is a rare autosomal recessive form of congenital stationary night blindness. The condition is associated with fundus discolouration and abnormally slow dark adaptation. Earlier studies suggested that the 48 kD protein S antigen may be involved in the recovery phase of light transduction. Previous cytogenetic and linkage studies have localised the S antigen gene (SAG) to chromosome 2q37.1. In the present study markers which map to distal chromosome 2q were typed in an inbred Oguchi pedigree. The segregation data obtained suggested that the affected subjects are homozygous by descent for a region between D2S172 and D2S345. An intragenic SAG polymorphism was homozygous in all affected people and a recombination event suggested that SAG maps proximal to D2S345. Collectively, these findings support the hypothesis that a defect in S antigen may be responsible for Oguchi disease.

Genetic mapping of RP1 on 8q11-q21 in an Australian family with autosomal dominant retinitis pigmentosa reduces the critical region to 4 cM between D8S601 and D8S285

Abstract The locus (RP1) for one form of autosomal dominant retinitis pigmentosa (adRP) was mapped on chromosome 8q11-q22 between D8S589 and D8S285, which are about 8 cM apart, by linkage analysis in an extended family ascertained in the USA. We have studied a multigeneration Australian family with adRP and found close linkage without recombination between the disease locus and D8S591, D8S566, and D8S166 (Zmax = 1.137– 4.650 at θ = 0.00), all mapped in the region known to harbor RP1. Assuming that the mutation of the same gene is responsible for the disease in both families, the analysis of multiply informative meioses in the American and Australian families places the adRP locus between D8S601 and D8S285, which reduces the critical region to about 4 cM, corresponding to approximately 4 Mb, which is completely covered by a yeast artificial chromosome contig assembled recently.