Stargardt Disease: towards developing a model to predict phenotype

European Journal of Human Genetics (2013) 21, 1173–1176 & 2013 Macmillan Publishers Limited All rights reserved 1018-4813/13 www.nature.com/ejhg SHORT REPORT Stargardt Disease: towards developing a model to predict phenotype This article has been corrected since online publication and a corrigendum is also printed in this issue Laura Heathfield1, Miguel Lacerda2, Christel Nossek1, Lisa Roberts1 and Rajkumar S Ramesar*,1 Stargardt disease is an ABCA4-associated retinopathy, which generally follows an autosomal recessive inheritance pattern and is a frequent cause of macular degeneration in childhood. ABCA4 displays significant allelic heterogeneity whereby different mutations can cause retinal diseases with varying severity and age of onset. A genotype–phenotype model has been proposed linking ABCA4 mutations, purported ABCA4 functional protein activity and severity of disease, as measured by degree of visual loss and the age of onset. It has, however, been difficult to verify this model statistically in observational studies, as the number of individuals sharing any particular mutation combination is typically low. Seven founder mutations have been identified in a large number of Caucasian Afrikaner patients in South Africa, making it possible to test the genotype–phenotype model. A generalised linear model was developed to predict and assess the relative pathogenic contribution of the seven mutations to the age of onset of Stargardt disease. It is shown that the pathogenicity of an individual mutation can differ significantly depending on the genetic context in which it occurs. The results reported here may be used to identify suitable candidates for inclusion in clinical trials, as well as guide the genetic counselling of affected individuals and families. European Journal of Human Genetics (2013) 21, 1173–1176; doi:10.1038/ejhg.2013.92; published online 22 May 2013 Keywords: ABCA4; age of onset; generalised linear model; genotype–phenotype; Stargardt disease INTRODUCTION Stargardt disease (STGD) (OMIM: 248200) is a juvenile onset form of macular dystrophy, characterised by central vision loss due to the degeneration of cone photoreceptor cells in the macula.1,2 The disease can follow both autosomal recessive (ar) and autosomal dominant (ad) inheritance patterns with symptoms usually presenting within the first two decades of life.3–6 The incidence of STGD is estimated to be 1 in 10 000 individuals, and it accounts for B 7% of all retinal disease.7 Mutations in the ATP-binding cassette subfamily A group 4 (ABCA4) gene give rise to the recessive form of STGD, but are also associated with other retinal degenerative disorders such as cone-rod dystrophy (CRD) and autosomal recessive retinitis pigmentosa (arRP),2,8,9 thereby characterising these dystrophies as ABCA4-associated retinopathies (AARs).3,10–12 It is proposed that malfunctioning ABCA4 protein prevents complete removal of retinoid products from the outer segment disc membrane of photoreceptors, resulting in the accumulation of downstream derivatives such as di-retinoid-ethanolamine in the retinal pigment epithelium (RPE). Such products can trigger RPE dysfunction through various mechanisms, consequently leading to degeneration of photoreceptors and ultimately vision loss.3,13 A high level of allelic heterogeneity is displayed in arSTGD, as over 600 mutations in ABCA4 have been identified.14 The vast number of identified mutations in ABCA4, their respective effects on the protein and the combination of mutations within a particular individual are largely accountable for the phenotypic heterogeneity in arSTGD patients.15 The considerable amount of both allelic and phenotypic heterogeneity associated with ABCA4 mutations has led to the proposal of a genotype–phenotype model, which attempts to define the relationship between different ABCA4 mutations and severity of the AAR phenotype.16,17 The model suggests that phenotype may be predicted by ABCA4 mutations depending on the functionality of the remaining ABCA4 protein. Thus, the lower amount of functional ABCA4 activity, the more severe the AAR. In 2006, Valverde et al18 hypothesised that both the nature and the combination of the ABCA4 mutations have a role in the age of onset (AOO) of the AAR, and the earlier the AOO, the more severe the disease. Within the context of a single AAR, a similar rationale could be applied whereby different mutations in ABCA4 could give rise to varying phenotypes in patients with STGD. The relative pathogenic contribution of individual alleles has been difficult to assess, since STGD is a highly heterogeneous disease and therefore, few individuals have been reported to share the same biallelic combination of mutations. This study considers a South African cohort of 118 individuals who express biallelic combinations of seven founder mutations in ABCA4 (c.454C4T (p.Arg152*), c.768G4T (p.Val256Val), c.1804C4T (p.Arg602Trp), c.2588G4C (p.Gly863Ala), c.4469G4A (p.Cys1490Tyr), c.5461–10T4C, c.6079C4T (p.Leu2027Phe)), which collectively account for 36% of STGD cases studied to date in South Africa.19,20 These data, together with clinical information for each patient, were used to test the genotype–phenotype model. A generalised linear model (GLM) was 1UCT/MRC Human Genetics Research Unit, Division of Human Genetics, Institute of Infectious Diseases and Molecular Medicine, Faculty of Health Science, University of Cape Town, Cape Town, South Africa; 2Department of Statistical Sciences, Faculty of Science, University of Cape Town, Cape Town, South Africa *Correspondence: Professor RS Ramesar, UCT/MRC Human Genetics Research Unit, Division of Human Genetics, Institute of Infectious Diseases and Molecular Medicine, Faculty of Health Science, University of Cape Town, Anzio Road Observatory, Cape Town, Western Cape 7925, South Africa. Tel: þ 27 21 406 6297; Fax: þ 27 21 406 6826; E-mail: Received 7 January 2013; revised 10 April 2013; accepted 16 April 2013; published online 22 May 2013 Stargardt disease model L Heathfield et al 1174 developed to predict the AOO of patients with a biallelic combination of the seven common mutations and to assess their relative pathogenic contribution. MATERIALS AND METHODS The cohort comprised 118 individuals from 106 Caucasian Afrikaner families, each carrying two of the common seven ABCA4 (RefSeq NG_009073.1) mutations and for whom a self-reported AOO was available. The seven mutations screened in this study were not novel, and were all listed in the Human Gene Mutation Database (http://www.hgmd.cf.ac.uk/ac/index.php) at the Institute of Medical Genetics in Cardiff (except for c.768G4T (p.Val256Val)) and the Retina International ABCA4 Mutation Database (http:// www.retina-international.org/files/sci-news/abcrmut.htm). AOO was used, as this was the only common phenotypic information available for all patients within this large cohort. Informed consent was obtained from all individuals according to the te (...truncated)