Melanopsin (OPN4) is a blue light-sensitive opsin-type G-protein coupled receptor (Provencio et al., 2000). It is highly expressed in photosensitive retinal ganglion cells that mediate responses to light including regulation of sleep, circadian photoentrainment, and pupillary light response (Panda et al., 2002; Rodgers et al., 2018). Consequential variants in OPN4 were previously associated with an increased risk of developing the seasonal affective disorder (Chellappa, 2021). Melanopsin-dependent phototransduction was reported to be impaired in DSWPD and sighted non-24-hour sleep-wake rhythm disorder (Abbott et al., 2021). DSWPD is the most commonly diagnosed circadian rhythm sleep-wake disorder, with an estimated prevalence of 0.2%–10% (Nesbitt, 2018; Zee et al., 2013). It is characterized by a persistent and intractable delay in sleep onset and offset times relative to the societal norm (Zee et al., 2013). In this case report, we describe a case study of a DSWPD patient with a consequential damaging variant in OPN4.

We report a carrier of a rare variant: rs143641898 [NM_033282.4:c.502C>T p.(Arg168Cys)], a rare missense variant (gnomAD (Karczewski et al., 2019) MAF 0.0002). It is most common among European (non-Finnish) and Ashkenazi Jewish populations and extremely rare among East Asian (0.00005) and not detected in South Asian populations, suggesting the highest conservation of this region of the gene across these populations. It is in silico predicted to be damaging and has a Combined Annotation Dependent Depletion (CADD) score of 34. It is a highly conserved variant, and it is a part of the E/DRY motif found in nearly all GPCRs (positive charge to polar, increased hydrophobicity). Interestingly, this variant (rs143641898) was tested using an in vitro expression system as part of a study aiming to determine the functional phenotypes of missense human OPN4 variants (Rodgers et al., 2018). The authors selected 16 potentially deleterious variants for functional characterization using calcium imaging of melanopsin-driven light responses in HEK293T cells (Rodgers et al., 2018). This variant was shown to be incapable of binding retinal chromophore, suggesting that it renders the OPN4 protein non-functional (Rodgers et al., 2018). The introduction of rs143641898 in OPN4 abolished responses to light.

We do not detect this variant in our healthy sleeping super control set of whole genome sequencing (WGS) samples (n = 300) as well as in our control set of WGS (n = 1900). In our whole genome sequencing study of DSWPD patients, we report one other rare stop-gain in OPN4 [NM_033282.4:c.1086_1087insTAGCGG p.(Gln363*)] variant within the OPN4 gene. This variant has been detected in an unrelated individual. This variant requires functional confirmation similar to the one that was carried out for rs143641898.

The patient is a 58-year-old male manifesting DSWPD symptoms since high school. The patient meets DSWPD diagnostic criteria (DSWPD—ICSD-3 (Sateia, 2014) based on a clinical interview with a board-certified sleep physician). The patients’ medical records have an earlier diagnosis of insomnia (per medical history). The patient has been enrolled in a 12-week diary study (Figure 1 raster plot below). The reported sleep onset is consistent and significantly delayed in comparison to a control population. The patient reported an average bedtime of 2:48 a.m., average sleep onset of 3:29 a.m., and average wake-up time of 11:42 a.m. (average over 12 weeks of electronic time-stamped sleep diary). The patient reported a consistent total sleep time (TST) of 8:14 (average over 12 weeks). Confirmatory of the delayed phenotype is also the timing of the dim light melatonin onset (DLMO) occurring at 23:20 (quantified with the salivary melatonin assay) presented in Figure 2. The patient has no other known pLOF mutations reported in the core circadian clock genes including no variants or copy number variants detected in the CRY1 gene or in the AANAT gene. Variants within both were previously associated with DSWPD, and despite the fact they are highly penetrant, they do not explain a large proportion of individuals with DSWPD of mixed ancestry (Patke et al., 2017; Hohjoh et al., 2003; Smieszek et al., 2021).

These data provide valuable insights into the phenotype-genotype consequences of human OPN4 variants. Additional studies focusing on a set of carriers of this and other damaging variants in OPN4 are warranted to confirm this finding. One example would be looking at the lab-based assessment of plasma melatonin suppression under 460 nm (blue) and 555 nm monochromatic light would be very interesting for this patient, as well as other carriers of this variant. Given melanopsin is not functional, there would be little or no suppression in the last half to quarter of the 460 nm light, which is all melanopsin driven; however, the 555 nm (cone system) would look normal (and the opposite to the blind man who only has melanopsin). Additionally, studies looking at pupil constriction dynamics could be helpful confirmatory studies (Gooley et al., 2012). The identification of consequential OPN4 variants leading to disruption of protein function such as rs143641898 leading to disrupted melanopsin-based light perception will help with the identification of patients with an increased risk of sleep disturbances and circadian dysfunction, who may need early interventions.