Retinal dystrophies (RD), a group of heterogeneous hereditary diseases, are caused by perturbed photoreceptor function. It presents significant genetic heterogeneity and contribute notably to the etiology of blindness around the world (den Hollander et al., 2010). To date, more than 300 disease associated genes have been implicated in the pathogenesis of the disease, most of which are involved in the development and normal function of photoreceptor and cells from the retinal pigment epithelium (RPE) (den Hollander et al., 2008, 2010). Clinical symptoms vary widely among different RD subtypes and disease genes (Ellingford et al., 2016). RD are generally classified based on the type of photoreceptor cells affected, i.e., rods or cones, and thus on the location, macula or peripheral retina (Nash et al., 2015).

The described modes of inheritance in RD are recessive (ar) or dominant (ad) and autosomal or X-linked. A few rare cases of RD exist, in which mitochondrial mutations, digenic inheritance, and pseudodominant transmission have been documented (Lewis et al., 1999; Liu et al., 2019).

In this study, we used whole exome sequencing (WES) and Sanger sequencing to investigate four consanguineous families with several members affected by RD, inherited in an apparent ad pattern. We aimed to identify potential gene variants underlining these cases and described the genotypic and phenotypic findings in these complicated RD pedigrees. Distinct inheritance patterns and disease-causing variants were identified in all families.

In this report, molecular testing revealed the coexistence of pathogenic variants affecting distinct RD causing genes in four RD families. Among the five pathogenic variants identified in these families, one was novel (RDH12 c.105delG, p.Q36Sfs^*6) and four were recurrent (RHO p.G51A, NR2E3 p.R311Q and IMPG2 p.W758^*, p.R964^*) (Macke et al., 1993; Haider et al., 2000; Bandah-Rozenfeld et al., 2010; Patel et al., 2016).

Two families, F1 and F2, carried the most frequent variant p.R311Q in NR2E3. Gerber et al. have previously identified this in patients who may have Enhanced S-cone syndrome (ESCS) (Gerber et al., 2000). Later, Haider et al. reported the same p.R311Q variant with 44.8% frequency in patients with ESCS (Haider et al., 2000) and it was also reported in a patient with Goldmann-Favre syndrome (GFS) (Bernal et al., 2008). The particularity in F1 is the presence of a second heterozygous variant p.G51A in RHO. Two diseases are associated with RHO variants, RP and congenital stationary night blindness (CSNB). However, in F1 the index patient presented with ESCS, which is more related to NR2E3. Interesting, and although RHO: p.G51A has been involved in retinitis pigmentosa (Macke et al., 1993), GnomAD mentions a frequency of 0.1% in the general population and ClinVar classifies this variant as benign/likely benign. Family F1 would favor the latter classification.

New pathogenic homozygous variant c.[105delG];[105delG], p.[Q36Sfs^*6;[Q36Sfs^*6] in RDH12 was detected in F3. Variants in RDH12 have been previously linked to Leber congenital amaurosis (LCA) and ad-RP. Retinal pathologies resulting from variants in RDH12 gene can be inherited in either ad or ar mode (Kumaran et al., 2017; Sarkar et al., 2020). Ar biallelic variants in RDH12 were first identified in three consanguineous Austrian families with severe retinal dystrophy (Janecke et al., 2004).

F4 harbored compound heterozygous variants p.[W758^*];[R964^*] in IMPG2. This is the first time the association of these two variants is described. Biallelic variants in IMPG2 have been shown to underlie recessive childhood-onset rod-cone dystrophy with early macular involvement in several families (Khan and Al Teneiji, 2019). At the heterozygous state, IMPG2 variants have been associated with dominant vitelliform macular dystrophy (Brandl et al., 2017). In F4, the affected mother carried compound likely pathogenic variants, the unaffected father carried a p.W758^* heterozygous variant and the affected children a combination of these variants. Pseudodominant inheritance occurs when an individual with a known recessive disorder has a clinically unaffected partner, but then unexpectedly gives birth to children who are affected with the same recessive disorder as the affected parent (Thompson and Thompson, 1986). Although all the parents in the described families were first cousins, each disease followed a dominant pattern of inheritance, with one affected parent and one or several affected children. Molecular analyses allowed us to correctly classify the mode of inheritance as pseudo-dominant, a mode usually associated with a high mutant frequency, like in hemochromatosis.

The datasets presented in this study can be found in online repositories. The names of the repository/repositories and accession number(s) can be found in the article/supplementary material.

The studies involving human participants were reviewed and approved by Local Ethics Committee of the Hedi Rais Institute in Tunisia. Written informed consent to participate in this study was provided by the participants' legal guardian/next of kin. Written informed consent was obtained from the individual(s), and minor(s)' legal guardian/next of kin, for the publication of any potentially identifiable images or data included in this article.

DS and LE designed the research protocol. IH and DS performed the analyses. YF, HT, KE, AC, and LE performed the clinical evaluation. IH, YF, LE, and DS wrote the manuscript. All authors reviewed the manuscript.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.