Genetic profile of syndromic retinitis pigmentosa in Portugal

Retinitis pigmentosa (RP) comprises a genetically and clinically diverse group of inherited retinal degenerations (IRDs), primarily characterized by rod-cone degeneration. With an estimated prevalence of 1:4000 individuals, it is the most frequent form of IRD [1]. While most cases of RP are not associated with systemic abnormalities, 20–30% of patients exhibit extra-ocular disease and are referred to as syndromic RP [1‐3]. Usher syndrome features sensorineural hearing loss (and in some forms vestibular impairment) in association with RP and is overall the most frequent form of syndromic RP [2‐4], followed by Bardet-Biedl syndrome. In the latter, polydactyly, intellectual disability, and truncal obesity are among the most prevalent extra-ocular manifestations [2‐4].

Genetic profiling of IRDs takes on an ever-growing significance for the affected individual, not only with regard to disease prognosis and genetic counseling but also for treatment prospects [5], which recently became a reality with the introduction of gene therapy for RPE65-associated retinal degeneration [6]. Even though therapies targeting the retinal phenotype of syndromic RP are not currently available, the genetic landscape of syndromic RP has been receiving increased interest worldwide, including a few European studies [7‐10]. Although there are some similarities in genetic profiles, there is significant variation among regions and ethnic groups. This genetic diversity between populations may be partly explained by founder mutations [8, 11, 12], thus highlighting the importance of obtaining reference population-based data.

In Portugal, data on the genetic architecture of syndromic RP is currently scarce. By conducting a national, multicenter study, we aimed at characterizing the genetic landscape of syndromic RP in a large Portuguese cohort.

Genetic profiling of IRDs is of major importance for patients and, through genetic counseling, for family members as well. Nevertheless, constraints in access to genetic testing may hinder the goal of obtaining a molecular diagnosis for every affected patient [17]. A paradigm shift is in progress, with a recent increase in the number of publications contributing to improve knowledge of the genetic landscape of IRDs in Portugal [13, 18‐24]. One of such publications included a cohort of 230 Portuguese families with IRDs, but only 23 probands had syndromic RP [13]. In this nationwide, multicenter study including 122 patients from 100 families, we describe the genetic landscape of syndromic RP in Portugal.

Overall, disease-causing variants were identified in 86/100 families for a diagnostic yield of 86%. Even though this figure is much higher than what is usually obtained for non-syndromic forms of the disease [7, 25], it is in line with a previous study by Karali et al. [10], reporting genetic testing sensitivity upwards of 80% for syndromic IRDs.

Given the geographic proximity between Portugal and Spain, as well as the genetic similarities observed between its inhabitants [26], studies on the genetic landscape of syndromic RP in Spanish cohorts are a natural reference for comparison purposes, and thus, one could anticipate somewhat similar genetic findings for a Portuguese cohort. As expected, Usher (n = 62 families) and Bardet-Biedl (n = 19 families) syndromes were found to be the most frequent causes of syndromic RP in our cohort. USH2A and MYO7A variants were the major causes of Usher syndrome type II and type I, respectively. Similar findings were reported by Perea-Romero et al. [7] in their large Spanish cohort (n = 577 syndromic IRD families) and are observed as well in most studies from different populations [27‐29]. Additionally, the BBS1 variant c.1169 T > G p.(Met380Arg) was the most frequently identified causative variant for Bardet-Biedl cases. This is in line with other Caucasian cohorts, where it was shown that ~ 80% of patients with BBS1-related disease carry this pathogenic variant [30, 31].

Even so, significant differences were found in the genetic architecture of Usher syndrome for the present cohort, as illustrated by the comparatively high prevalence of ADGRV1 variants, present in 14.5% of families, but found to be less common in Spanish [8] or North American [25] cohorts. Conversely, PCDH15 mutations were a prevalent cause of type 1 Usher syndrome, responsible for over 15% of such cases in both Spanish [7] and North American [25] cohorts, but were identified in just a single family in this study.

Eighty-one distinct genetic variants in 25 different genes were identified, 22 of which are novel. For USH2A-associated Usher syndrome, the most prevalent disease-causing variant was c.920_923dup p.(His308Glnfs*16), previously reported in multiple European cohorts [32‐34]. The frameshift variant c.397dup p.(His133Profs*7), first reported by Bonnet et al. [35], was the most prevalent cause of MYO7A-associated Usher syndrome. The ADGRV1 gene contained the most novel variants (n = 7), all of which were disease-causing, i.e., ACMG class IV or V. The remaining novel variants were distributed across 13 different genes (Table 3).

We found that most patients (61.5%) experience a symptomatic onset of vision loss during the first 20 years of age, with Bardet-Biedl syndrome patients reporting the earliest visual symptom onset, i.e., within the first decade of life (Table 1). Although a direct comparison cannot be established, this appears to be before than most cases of non-syndromic RP, where a mean age of onset of 19.5 ± 12.6 years and 23.2 ± 16.6 years has been reported by Colombo et al. for autosomal dominant and autosomal recessive non-syndromic RP, respectively, in a large Italian cohort [36]. A mean loss of 11.6 ETDRS letters (p < 0.001) was observed over a follow-up period of 62.0 months, corresponding to an annual reduction in BCVA of 2.24 letters. A similar reduction (2.3 letters) was previously reported by Iftikhar et al. [37] in their cohort of non-syndromic RP patients, illustrating the slowly progressive nature of the disease. Cystoid macular edema was present in 13.6% of eyes. The previously reported prevalence for this comorbidity is widely variable, ranging from ~ 5% [38] to 50.9% [39] of eyes (in non-syndromic RP), and has been noticed not to differ significantly between syndromic or non-syndromic RP [20]. Regardless, ophthalmologists should be aware of the importance of screening patients for the presence of this potentially treatable condition [20, 39].

Our study presents some limitations. First, the absence of standardization in multimodal retinal imaging across different contributing HCPs may have led to differences in the reporting of comorbidities such as cystoid macular edema and epiretinal membrane, as patients were not required to have performed regular optical coherence tomography (OCT) imaging to be included in the cohort. Also, not all Portuguese regions were represented in this cohort, as there were 4 districts for which no patients were included (Fig. 1). Naturally, there is a selection bias toward patients who can visit the ophthalmology clinics of the contributing HCPs. Patients with severe comorbidities and those living in more remote areas may have difficulties accessing these specialized centers and may be underrepresented in this sample. Nevertheless, we were able to enroll a large number of syndromic RP patients from six different HCPs, providing genetic data from 100 families.

In conclusion, as ophthalmology takes a deep dive into precision medicine, nationwide efforts to improve knowledge of the genetic background of IRDs are of utmost importance. The present study illustrates the diverse genetic landscape and provides reference data for syndromic RP in Portugal. Twenty-two novel variants in syndromic RP-associated genes are herein reported for the first time, thus contributing to expand the mutational spectrum of syndromic RP.