JtMed Genet 1998;35:641-645 641 Assessment of the interphotoreceptor matrix proteoglycan- 1 (IMPG1) gene localised to J Med Genet: first published as 10.1136/jmg.35.8.641 on 1 August 1998. Downloaded from 6ql3-ql5 in autosomal dominant Stargardt-like disease (ADSTGD), progressive bifocal chorioretinal atrophy (PBCRA), and North Carolina macular dystrophy (MCDR1) Andrea Gehrig, Ute Felbor, Rosemary E Kelsell, David M Hunt, Irene H Maumenee, Bernhard H F Weber Abstract assessment of the functional properties of the We have recently characterised the geno- gene product with regard to the disease pathol- mic organisation of a novel interphotore- ogy. Several criteria, such as tissue specificity of ceptor matrix proteoglycan, IMPG1, and gene expression, cellular localisation of the have mapped the gene locus to chromo- transcript, immunohistochemical data, possibly some 6ql3-ql5 by fluorescence in situ in combination with interspecies nucleotide/ hybridisation. As the interphotoreceptor protein sequence comparison or protein pattern matrix (IPM) is thought to play a critical database searches, may provide helpful infor- role in retinal adhesion and the mainte- mation on the probable function of a candidate nance of photoreceptor cells, it is conceiv- gene. With an increasing number of mapped able that a defect in one of the IPM genes and expressed sequence tags (ESTs) components may cause degenerative le- available through the international efforts ofthe sions in retinal structures and thus may be Human Genome Project, the positional candi- associated with human retinopathies. By date strategy has become the method of choice genetic linkage analysis, several retinal for the identification of disease genes.' dystrophies including one form of auto- Recently, a novel gene, IMPG1, encoding a somal dominant Stargardt-like macular major proteoglycan of the interphotoreceptor dystrophy (STGD3), progressive bifocal matrix (IPM), was identified and shown to be chorioretinal atrophy (PBCRA), and preferentially expressed in retina by both rod http://jmg.bmj.com/ Institut fuir North Carolina macular dystrophy and cone photoreceptor cells.2 'The IPM occu- Humangenetik, (MCDR1) have previously been localised pies the space between the neurosensory retina Biozentrum, to a on Universitat Wurzburg, region proximal 6q that overlaps and the retinal pigment epithelium (RPE) and Am Hubland, D-97074, the IMPGl locus. We have therefore as- is largely composed of insoluble glycoconju- Germany sessed the entire coding region of IMPG1 gates that appear to form specific structures A Gehrig by exon amplification and subsequent sin- around the rod and cone photoreceptor cells, U Felbor gle stranded conformational analysis in commonly referred to as cone matrix sheaths on September 30, 2021 by guest. Protected copyright. B H F Weber patients from 6q linked multigeneration and rod matrix domains.4 The IPM glycocon- Augenklinik, families diagnosed with PBCRA and jugates, which are primarily chondroitin sul- Universitat Wurzburg, MCDR1, as well as a single patient from an phate proteoglycans, have been proposed to D-97074 Wuirzburg, autosomal dominant STGD pedigree un- mediate interactions among various cell types, Germany linked to either of the two known STGD2 including the RPE, the photoreceptors, the glial U Felbor and STGD3 loci on chromosomes 13q and Muller cells, and the choriocapillaris8 and may 6q, respectively. No disease associated Institute of be vital for retinal adhesion and maintenance of Ophthalmology, mutations were identified. In addition, photoreceptor cells.'0II University College using an intragenic polymorphism, More recently, we have determined the London, London IMPG1 was excluded by genetic recombi- genomic organisation of IMPG1 and have EC1V 9EL, UK nation from both the PBCRA and the mapped the gene locus to chromosomal region R E Kelsell MCDR1 loci. However, as the autosomal D M 6ql 3-ql 512 (fig 1). Interestingly, several retinal Hunt dominant Stargardt-like macular dystro- dystrophies including autosomal dominant The Wilmer Eye phies are genetically heterogeneous, other Stargardt-like macular dystrophy (STGD3)," Institute, The Johns forms of this disorder, in particular progressive bifocal chorioretinal atrophy Hopkins Center for STGD3 previously linked to 6q, may be (PBCRA)," and North Carolina macular dys- Hereditary Eye caused by mutations in IMPG1. trophy (MCDR1)'5 have been localised to an Diseases, Baltimore, (7 Med Genet 1998;35:641-645) MD 21287-9237, USA overlapping region on the proximal long arm of I H Maumenee Keywords: Stargardt-like macular dystrophy; MCDR1; chromosome 6 by genetic linkage analysis (fig PBCRA; interphotoreceptor matrix proteoglycan-1 1). In order to assess whether mutations in Correspondence to: IMPG1 may be associated with one of these Dr Weber. macular dystrophies, we have now analysed the The Received 14 October 1997 positional candidate approach relies on the 17 coding exons of the gene for disease causing Revised version accepted for mapping ofa disease locus and a candidate gene mutations in a family with autosomal dominant publication 12 January 1997 to a common chromosomal region and the Stargardt-like macular dystrophy (ADSTGD), 642 Gehrig, Felbor, Kelsell, et al Ns ofthe IMPGl gene in STDG3 previously linked e,6 to proximal 6q.'3 The availability of the exon/ 26.2 cM 8.0 cM 11.0 cM 4.4 cM 5.0 cM intron structure will allow the further assess- J Med Genet: first published as 10.1136/jmg.35.8.641 on 1 August 1998. Downloaded from I %& i i 1' ment of the potential role of IMPG1 in various retinopathies of unknown aetiology. STGD3 PBCRA Materials and methods MCDR1 PATIENTS AND FAMILIES The clinical, histopathological, and ultrastruc- tural phenotype of a three generation pedigree Chr 6 with autosomal dominant Stargardt-like macu- lar dystrophy (also known as fundus flavimaculatus-like macular dystrophy) and late onset atrophic macular degeneration has been 6q114.2-q15 described in detail elsewhere.'6 In brief, mild blurring of central vision was first noted by the index patient at the age of 38. A paracentric 17 scotoma was present in both eyes. Ophthalmos- 1 2 3 4;56 7 8 9 10 1112 13 14 15161 copy showed a reddish foveal appearance TGA ATG 1MPG 1 surrounded by small yellowish-white, atrophic subretinal flecks and a reticular pattern of sub- Figure 1 Idiogram ofchromosome 6 and genetic map ofproximal 6q illustrati in the midperiphery of relative locations andflanking DNA markers ofgene locifor autosomal dominzangthe retinal pigmentation Stargardt-like macular dystrophy (STGD3),'3 North Carolina macular dystro)phy both eyes. Multiple focal and confluent areas of (MCDRI),"' andprogressive bifocal chorioretinal atrophy (PBCRA).'4 Below,,the genomic choroidal hyperfluorescence were detected by organisation of the gene encoding the interphotoreceptor matrix proteoglycan-1 (IMPGI) fluorescein angiography. Bilateral retinal pig- and its chromosomal localisation is shown. " ment epithelium (RPE) window defects were so far unlinked to STGD2 or STGD35,16 in three present in the fovea. There were areas of silent MCDR1 pedigrees of German desceint recently or dark choroid. The photopic electroretino- linked to proximal 6q'7 " and in a five Igeneration gram (ERG) was subnormal in both eyes British PBCRA pedigree previously rmapped to whereas scotopic ERG and electro-oculogram proximal 6q.14 Our results definitively exclude (EOG) were within the normal range. Inspec- IMPG1 as a candidate gene for MC-DRl and tion of the pedigree indicates autosomal domi- PBCRA on the basis ofSSCA and the analysis of nant inheritance (fig 2). For mutational analy- rmore, di- sis, a DNA sample from a single affected subject recombinant chromosomes. Further in shown any di,sease asso- was available (arrowed fig 2). rect sequencing has not In a recent study, the North Carolina macu- ciated mutations in our patient with S dDSTGD. lar dystrophy (MCDR1) phenotype was linked However, as autosomal dominant Sta in rgardt-clke three multigeneration families of German http://jmg.bmj.com/ macular dystrophies are known to be,genetIcally descent to DNA markers derived from chro- heterogeneous,'3 16 19 an involvement (of IMPGl mosome 6ql4-ql6.2,'7 18 a region known to in one form ofADSTGD cannot be ruled out at harbour the MCDRI gene.'5 The family mem- present and must await the mutational analyses bers underwent comprehensive ophthalmo- logical examination. The affected subjects in ADSTGD families L, A, and W presented with fundus abnormalities consistent with MCDR1 and are described in detail elsewhere.'7 18 The DNA on September 30, 2021 by guest. Protected copyright. ET0 from one affected subject per family was used for the mutational analyses (arrowed in fig 2). Genetic linkage analysis on a five generation MCDR1 -L MCDR1-A ACDR 1 -W British pedigree expressing the progressive E p bifocal chorioretinal atrophy (PBCRA) pheno- type has mapped the gene locus by two point linkage to an interval between D6S249 and D6S301 on proximal chromosome 6q.'4 De- tailed descriptions of ophthalmological findings in individual members ofthe PBCRA family are given in Douglas et ao20 and Godley et al.21 Briefly, PBCRA is a progressive disease charac- PBCRA terised by two distinct foci of atrophy, a tempo- ral focus which is present at birth and a nasal b * focus which appears in the second decade of life. In addition, it is associated with nystagmus, myopia, and a significant reduction in visual acuity and colour vision. Photopsia and retinal detachment