The genetic landscape of inherited eye disorders in 74 consecutive families from the United Arab Emirates

Abstract Genetic eye diseases are phenotypically and genetically heterogeneous, affecting 1 in 1,000 people worldwide. This prevalence can increase in populations where endogamy is a social preference, such as in Arab populations. A retrospective consecutive cohort of 91 patients from 74 unrelated families affected with non‐syndromic and syndromic inherited eye disease presenting to the ocular genetics service at Moorfields Eye Hospitals United Arab Emirates (UAE) between 2017 and 2019, underwent clinically accredited genetic testing using targeted gene panels. The mean ± SD age of probands was 27.4 ± 16.2 years, and 45% were female (41/91). The UAE has a diverse and dynamic population, and the main ethnicity of families in this cohort was 74% Arab (n = 55), 8% Indian (n = 6) and 7% Pakistani (n = 5). Fifty‐six families (90.3%) were genetically solved, with 69 disease‐causing variants in 40 genes. Fourteen novel variants were detected with large deletions in CDHR1 and TTLL5, a multiexon (1–8) duplication in TEAD1 and 11 single nucleotides variants in 9 further genes. ABCA4‐retinopathy was the most frequent cause accounting for 21% of cases, with the confirmed UAE founder mutation c.5882G>A p.(Gly1961Glu)/c.2570T>C p.(Leu857Pro) in 25%. High diagnostic yield for UAE patients can guide prognosis, family decision‐making, access to clinical trials and approved treatments.


| INTRODUCTION
Inherited eye disease is phenotypically and genetically heterogeneous with over 430 known disease-causing genes (Patel et al., 2019).
Approximately 1 in 1,000 people worldwide are affected with either progressive, non-progressive, syndromic, or non-syndromic genetic pathologies falling into the spectrum of development eye disorders, corneal and retinal dystrophies, and/or hereditary optic neuropathies (Stone, 2007). Retinal dystrophies encompass rod-dominant diseases [such as retinitis pigmentosa (RP) or rod-cone dystrophy (RCD), early onset retinal dystrophy (EORD), and Leber congenital amaurosis (LCA)] and cone-dominant diseases (including cone/cone-rod dystrophy, Stargardt disease, and macular dystrophies), with or without extraocular features. For those originating from the Arabian Gulf, approximately 5% of the population are affected with genetic disease involving the eye and adnexa (Tadmouri, Al-Haj Ali, Nair, & Fareed, 2006), in comparison to just 0.0132% of children in the United Kingdom (Rahi & Cable, 2003). The most common mode of inheritance is autosomal recessive in 60%, autosomal dominant in 25%, X-linked in 5%, and less than 1% are mitochondrial (Tadmouri et al., 2006). Endogamy and large families are a social preference in several Arab populations, and the consanguinity rate in the United Arab Emirates (UAE) is between 39 and 54.2% (Al-Gazali & Hamamy, 2014). Together this contributes to the high number of cases with autosomal recessive diseases due to homozygous disease-causing variants (Al-Gazali & Hamamy, 2014;Tadmouri et al., 2006). Clinically accredited genetic testing permits the identification of disease-causing gene variants and supports informed genetic counseling for family planning, potential therapies, and clinical trials (Prado, Acosta-Acero, & Maldonado, 2020). In 2019 the UAE approved the use of Luxturna (or voretigene neparvovec), the first retinal gene therapy for patients with autosomal recessive RPE65-retinopathy. We report herein the genetic outcomes for 74 unrelated families, with at least one member (proband) affected with nonsyndromic or syndromic inherited eye disease presenting consecutively to the ocular genetics service at Moorfields Eye Hospitals UAE over a 17-month period from December 2017 to September 2019.

| MATERIAL AND METHODS
A retrospective case note review of all consecutive patients presenting to the ocular genetics service at Moorfields Eye Hospitals UAE, Dubai and Abu Dhabi sites, from December 2017 to September 2019 was conducted. If the patient did not have a previously established genetic result, they were offered molecular testing using comparable targeted gene panel testing through the Rare & Inherited Disease Genomic Laboratory at Great Ormond Street Hospital (London, UK) or Blueprint Genetics (Helsinki, Finland). Coding exons and flanking intronic regions of genes associated with genetic eye diseases and selected deep intronic variants were screened and analyzed as previously reported (Patel et al., 2019).
One proband (46-1 from family 46) with aniridia initially had a microarray-based comparative genomic hybridization for deletion screening of WT1 and PAX6 for Wilms tumor, aniridia, genitourinary anomalies, and mental retardation (WAGR) syndrome (OMIM #194072), this was negative, so then underwent PAX6 (OMIM *607108) gene screening with Sanger sequencing through the Wessex Regional Genetics Laboratory (Salisbury, UK). Variant classification followed American College of Medical Genetics and Genomics (ACMG) guidelines (Richards et al., 2015).

| RESULTS
Ninety-one patients from 74 unrelated families (with 74 probands), aged between 2 and 80 years old (mean ± SD was 27.4 ± 16.2 years), with 45% being female (41/91), presented to the ocular genetics service at Moorfields Eye Hospitals UAE. The ethnicity of families were Arab (74%, n = 55), Indian (8%, n = 6), Pakistani (7%, n = 5), Caucasian (5%, n = 4; Italian, British, South African, and Russian), Egyptian (3%; n = 2), Sudanese (1%, n = 1) and Japanese (1%, n = 1). Seventy-eight percent of families (n = 58) reported consanguinity on direct questioning. Seven families presented with a predetermined genetic result and five families did not proceed with genetic testing due to the cost involved (Table S1); these 12 families were excluded from further analysis. Sixty-two families proceeded with genetic testing to identify the gene variant(s) associated with their inherited eye disease using targeted gene panels, except one (family 46 with aniridia, as per the methods). All patient demographics including clinical and genetic details are listed in Table S1.
Of the 62 families who opted for molecular testing, they were divided into 27 rod-cone dystrophies (43.5%, including RP, EORD, LCA), 16 cone/cone-rod dystrophies (25.8%, including Stargardt disease and macular dystrophies), 10 syndromic retinal dystrophies (16.1%), 3 achromatopsia (4.8%), 2 retinoschisis (3.2%), 2 albinism (3.2%) and 2 "others" (3.2%) including 1 aniridia and 1 pathological myopia ( Figure 1a). The majority of conditions were inherited autosomal recessively with 46 affected families (74.2%), autosomal dominant in 5 families (8.1%), X-linked recessive in 2 families (3.2%) and 3 with an unclear pattern of inheritance (4.8%) where the diagnosis remained unconfirmed (families 54, 55, and 56) (Figure 1b). In total, 56 families (90.3%) received a genetic diagnosis (Table 1; Figure 1c). Sixty-nine variants were identified in 40 genes associated with inherited eye diseases (Table S2). The most prevalent gene was ABCA4 found in Family 19 were found to have a novel heterozygous multi-exon (exons 1-8) duplication in TEAD1 displaying a clinical phenotype consistent with Sveinsson chorioretinal atrophy. The proband (19-1), now 63 years old, noticed difficulties with his night vision and peripheral visual field when he was age 30 years, he was diagnosed with RP and primary acute angle closure for which he had a left yag laser peripheral iridectomy (PI). He had a right phaco and IOL for cataract extraction age 54 and has a left cataract in situ. His color vision is normal but over the past 5 years the nyctalopia has worsened. No systemic features, a past medical history of prostate cancer, but nil else of note. The family is nonconsanguineous, but shows a dominant inheritance with an affected daughter (19-3) who is 33 years old, an affected older sister (192) who is 67 years old, his late father and paternal aunt were also affected (see Figure 4a). On examination, BCVA of 0.82 in the right eye and 0.90 in the left eye, intraocular pressure was normal. Anterior segment showed F I G U R E 1 Disease subgroups and inheritance patterns of genetically solved and unsolved families. (a) Seventy-four families from the UAE were grouped as follows: 27 rod-cone dystrophies (RCD), 16 cone-rod dystrophies (CRD), 10 syndromic retinal dystrophies (syndromic IRD), 3 achromatopsia, 2 retinoschisis, 2 albinism and 2 "others" (which include an aniridia and pathological myopia family). (b) Distribution of mode of inheritance amongst the 74 families. Three families remained "unconfirmed" with variants of unknown pathogenic significance or multiple mutations leading to uncertainty of the clinical diagnosis. NPF, no primary findings. (c) Number of families solved or with no primary finding (NPF) in each group T A B L E 1 Variant details and confirmed phenotype for the 56 solved families presenting to the ocular genetics service.  (Billingsley, Vincent, Deveault, & Héon, 2012).
Genetic modifiers may explain the variability of onset and severity in these patients (Meyer & Anderson, 2017).
Despite the high rates of molecular diagnosis, 9.7% of families remained without any primary findings; the largest group were the rod-cone dystrophies with five unsolved families. In the "other" group, Whole genome sequencing (WGS) can cover deep intronic, 3 0 -and 5 0 -untranslated regions and noncoding regulatory elements, whilst also covering novel genes, and is beneficial for unsolved cases. In family 56, multiple disease-causing variants were identified leaving the molecular diagnosis inconclusive until further clinical investigations such as electroretinography (ERG) or WGS is undertaken.
Proband 56-1 was a 9-year-old male presenting with nystagmus from birth and reduced vision from age 1 with photophobia and a hyperme-  Table 1 for details). (a) Family tree highlighting the proband (56-1, arrowhead), no reported consanguinity.
(b) The clinical phenotype in proband 56-1 aged 9 years shows a normal fundus appearance on the UWF color fundus images. UWF FAF revealed foveal hyperautofluorescence. SD-OCT was not available for this patient due to their nystagmus. The clinical features appear consistent with CNGA3-related achromatopsia this condition previously (Fossdal et al., 2004). In family 6, two affected patients 6-1 (8 years old) and 6-2 (4 years old) with a severe ABCA4 variants are amongst the most common in the UAE cohort (Burke et al., 2012;Guymer et al., 2001;Khan, 2019aKhan, , 2019b. Family suggested to be the UAE founder mutation (Khan, 2019b). This was seen in three families (23, 27, and 31) in association with a further heterozygous change, but in family 31 both variants were found in a homozygous arrangement (Table 1). The variant frequency of c.5882G>A p.(Gly1961Glu) was 5.9% (5/85 individuals) in this cohort compared with 0.46% in the worldwide population (gnomAD). A further ABCA4 splice variant c.5714+5G>A was observed in a compound heterozygous state in three unrelated families (22, 26, and 28), this is considered a "mild" variant also found in those of European descent and whose frequency has increased in Newfoundland, Canada due to a founder effect (Green et al., 2020). A larger UAE molecularly confirmed cohort will further support the evidence for founder mutations, which may be abundant in this population due to the high consanguinity rate. In this study we report the molecular diagnosis of 56 unrelated families originating or residing in the UAE. The majority of patients presenting to the clinic were affected with inherited retinal disease, highlighting a dearth of other nonretinal genetic eye conditions such as primary congenital glaucoma, congenital cataract, and ocular malformations. This may reflect referring practice and a lack of awareness relating to genetic testing for these conditions as extensive nonretinal targeted gene panels covering these conditions exists that can be offered to families. Patients who presented for genetic testing had a high diagnostic yield, but in some cases the number of pathogenic variants due to consanguinity confound the overall retinal pathology and make therapeutic choices difficult. The visual prognosis in these patients is variable. Nonetheless, for the majority it will guide eligibility into clinical trials and future approved therapies such as voretigene neparvovec, which is now available in the region. Further large-scale studies in the UAE population will reveal founder mutations associated with inherited eye diseases due to the engrained endogamy and consanguinity. Those who remain with no primary findings or inconclusive results will benefit from whole genome sequencing, which will become the gold-standard genetic test for all patients in the future.