Heterozygous mutation in OTX2 associated with early-onset retinal dystrophy with atypical maculopathy

• Introduction
• Methods
• Results
• Discussion

Abstract

Purpose: Heterozygous mutations in OTX2 have been associated with a range of ocular and pituitary abnormalities. We report a novel heterozygous deletion in OTX2 underlying early-onset retinal dystrophy with atypical maculopathy.

Methods: Clinical examination included electroretinography and multimodal retinal imaging. Molecular genetic testing was composed of next-generation sequencing of a panel of retinal dystrophy genes.

Results: A now 17-year-old boy presented 12 years earlier with a history of progressively poor vision since birth, nyctalopia, and early-onset retinal dystrophy with atypical maculopathy. He also had bilateral microphthalmos and a slim prepubertal appearance; growth hormone levels were within normal ranges. Next-generation sequencing of a retinal dystrophy gene panel revealed a heterozygous deletion c.485delC (p.Pro162G.Infs*24) in exon 5 of OTX2.

Conclusions: This second report of maculopathy associated with a heterozygous mutation in OTX2 confirms that mutations in OTX2 should be considered in the differential diagnosis of atypical hereditary maculopathy, with or without rod-cone dystrophy.

Macular pattern dystrophies are a genetically and clinically heterogeneous group of autosomal dominant eye diseases characterized by abnormal bilateral accumulation of lipofuscin-containing pigment in the RPE in the macular area. Macular pattern dystrophy-1 (MDPT1; OMIM 169150) is caused by a heterozygous mutation in the photoreceptor peripherin gene (PRPH2; OMIM 179605) on chromosome 6p21.1 [1,2]. Macular pattern dystrophy-2 (MDPT2; OMIM 608970) is caused by heterozygous mutations in the alpha-E-catenin-cadherin associated protein (CTNNA1; OMIM 116805) on chromosome 5q31 [3]. Adult onset foveomacular dystrophy (OMIM 608161), which is caused by heterozygous mutations in PRPH2 or in BEST1 (OMIM 607854) is sometimes also included among the macular pattern dystrophies [4-6].

In addition, in a historical context, several other morphological patterns have been described, which may have specific genetic associations, such as reticular (fishnet-like) dystrophy, macroreticular (spider-shaped) dystrophy, and butterfly-shaped pigment dystrophy. The last has been described in association with PRPH2, as well as mutations in CTNNA1 [3,7].

A recent study reported two families whose members presented with either “grouped pigmentation macular dystrophy” or macular pattern dystrophy, associated with heterozygous mutations in the homeobox gene Orthodenticle, Drosophila, homolog of, 2 (OTX2; OMIM 600037) [8]. OTX2 encodes a transcription factor that plays a critical role in forebrain and eye development. Heterozygous mutations in OTX2 were originally linked to a heterogeneous phenotype associated with severe ocular malformations or abnormalities with or without brain and pituitary abnormalities. The latter is considered to result from lack of OTX2-mediated regulation of HESX1 (homeobox gene expressed in ES cells 1, OMIM 601802), which is one of the transcription factors involved in pituitary development. In addition, there is a role for OTX2 in the mature retina as evidenced by the fact that loss of Otx2 protein causes slow degeneration of photoreceptor cells [9]. It is thought that bipolar cells import Otx2 protein which is relocated to the mitochondria to support ATP synthesis, highlighting a therapeutic potential of Otx2 protein transduction in retinal dystrophy [10]. In addition, recent studies have even established the OTX2 gene as an oncogene for medulloblastoma [11].

Several malformations of the eye have been associated with heterozygous mutations in OTX2, including bilateral anophthalmia, optic nerve hypoplasia, and ocular coloboma. The retinal phenotypes are summarized im Appendix 1 and have been described as early-onset retinal dystrophy (one patient) [12], Leber congenital amaurosis (one patient) [13], pigmentary retinopathy (one patient) [13], and pattern dystrophy (seven patients) [8]. In the current study, we describe a case of early onset retinal dystrophy with atypical maculopathy, microphthalmos, and small stature without growth hormone abnormality.

Informed consent was obtained for this case report. Institutional Review Board (IRB)/Ethics Committee approval was obtained at King Khaled Eye Specialist Hospital, Riyadh, Saudi Arabia. The research adhered to the tenets of the Declaration of Helsinki. The study adhered to the ARVO statement on human subjects.

A now 17-year-old boy was referred 12 years previously at age 5 years with a history of retinal dystrophy from birth which was progressive and accompanied by nyctalopia. There was no family history of any ophthalmic disorders, and the patient was the youngest of 12 siblings. His parents were first cousins. The patient’s best-corrected visual acuity was 20/200 oculi uterque (OU), and he was bilaterally hyperopic at +6.50 diopters (D). Macular pigmentary changes were noted in both fundi, but fundus imaging was not performed at this stage. There was a history of congenital hip dislocation, and he had undergone surgery with titanium screws in the left side of the hip. He started to walk at about 3 years of age, but no cognitive impairment was reported. On examination at age 17 years, the patient had microphthalmic eyes with axial length 17.98 mm and 17.88 mm in the right and left eyes, respectively. Best-corrected visual acuity was 20/300 OU. Fundus photography revealed dense intraretinal macular pigmentation as can be seen in pattern dystrophy and mid-peripheral pigmentation with mild arteriolar attenuation (Figure 1A,B). OCT transfoveal scans showed unusual intraretinal pigment scars, which may represent RPE hyperplasia and migration; however, there was no sign of lipofuscin deposition such as that seen in macular pattern dystrophy (Figure 1C, lower panel). FfERG performed at age 5 years (under sedation) and at 14 years showed non-recordable rod responses and severely reduced and delayed cone responses, atypical for pattern dystrophy, compatible with rod-cone dystrophy with atypical maculopathy (Figure 2). At the most recent follow-up at age 17 years, his height was 165 cm (8^th percentile), weight was 60 kg (32^nd percentile), and head circumference was 53.5 cm. The human growth hormone (0.8 µg/l), insulin-like growth factor 1 (IGF-1; 254 ng/ml), and insulin-like growth factor binding protein 3 (IGFBP-3; 5,251 ng/ml) levels were all normal. Brain magnetic resonance imaging (MRI) was not performed, because of the potential risks associated with applying a strong magnetic field in the presence of titanium screws in the patient’s hip.

NGS analysis revealed the heterozygous deletion c.485delC (p.Pro162G.Infs*24) in exon 5 of OTX2, leading to a frameshift and predicted premature protein truncation. This variant was not annotated in the Single Nucleotide Polymorphism Database (dbSNP) or the literature; there was no allele frequency. The variant was not listed in the Exac Browser, access date November 17, 2016. A literature review of OTX2-related pathology (Appendix 1), such as rod-cone dystrophy with maculopathy and microphthalmos, and the fact that the OTX2 mutation is a frameshift mutation support pathogenicity.

The primers used for the sequencing of exon 5 of OTX2 and the chromatogram of the region that includes the mutation in OTX2 are presented in Appendix 3. Additional variants that were detected with NGS are given in Table 1.

We provide the second report of atypical maculopathy associated with a heterozygous mutation in OTX2. The phenotype included strikingly dense macular pattern-like intraretinal pigment abnormalities, midperipheral retinal pigment clumping, microphthalmos, and rod-cone dystrophy. Brain MRI was not performed; however, there was no accompanying growth hormone deficiency, or abnormal IGF-1, or insulin-like growth factor binding protein 3 (IGFBP-3) levels.

This case supports the role of OTX2 in retinal development and highlights the retinal dystrophy phenotype that should raise suspicion of a mutation in the gene. Vincent et al. described seven cases, with ages ranging from 8 to 46 years, from two different families with the same heterozygous missense mutation c.235G>A in exon 4 of OTX2. They presented with grouped macular pigments or macular pattern dystrophy [8]. These cases described by Vincent et al. were less severe than the present case and differed from the present case in several aspects [8]. First, all of the cases described by Vincent et al. were either mildly or highly myopic and had normal to long eyes, whereas the present case was hyperopic and had microphthalmos. All the cases had recordable rod and cone responses, and most had completely normal ffERGs, whereas the present case had a nearly extinguished ffERG compatible with rod-cone dystrophy. Finally, all the cases presented with discrete areas of RPE–photoreceptor outer segment separation, whereas this patient had pigmented intraretinal scars and advanced atrophy in the maculas and no apparent separation between the RPE and the photoreceptor outer segments (Figure 1C).

As noted by Ragge et al. [13], genetic counseling for families with mutations in OTX2 may be challenging, due to factors such as the possibility of gonosomal mosaicism, de novo mutations, and uncertainty regarding the factors that determine the penetrance. A limitation of this study is that the NGS panel did not include the membrane-type frizzled-related protein (MFRP, MIM #606227) gene, in which biallelic mutations also have been associated with short axial length and a rod-cone dystrophy [16]. Further limitations are that no relatives were available for examination; thus, segregation analysis was not possible. Whole exome sequencing was not performed; thus, we cannot rule out the possibility of the presence of a homozygous mutation in a different untested gene. However, a literature review of OTX2-related pathology (Appendix 1), such as rod-cone dystrophy with maculopathy and microphthalmos, and the fact that the mutation in OTX2 in this study was a frameshift mutation support pathogenicity. However, although the frameshift variant in OTX2 is likely pathogenic, functional studies are needed to definitively prove this. To summarize, the presence of microphthalmos, early-onset retinal dystrophy, and atypical maculopathy support OTX2 as a putative causative gene, although the presentation differs in many aspects from the classical pattern dystrophy associated with mutations in OTX2 as described by Vincent et al. [8].