Large-conductance calcium-activated potassium channel haploinsufficiency leads to sensory deficits in the visual system: a case report

The large-conductance calcium (Ca²⁺)-activated potassium channel (BKCa, MaxiK, KCa1.1, slo1) is a unique member of the K channel family that is characterized by a large single-channel conductance that can be activated by both membrane depolarization (voltage) and elevation of intracellular free calcium [1, 2]. The BKCa channel comprises a homotetramer of pore-forming α-subunits encoded by the KCNMA1 gene that are localized to the plasma membrane. The structure of each pore-forming α-subunit comprises seven membrane-spanning segments (S0–S6) that include a voltage-sensing domain (S0–S4), a pore-gate domain (S5-P-S6), and a cytosolic C-terminal domain involved in Ca²⁺ sensitivity [1, 2]. BKCa channel activity is involved in the integration of various cellular and molecular signaling events via modulation of membrane excitability and Ca²⁺ homeostasis, including modulating neurovascular coupling and regulating vascular, respiratory tone, and especially neurotransmitter release, since BKCa channels are located at axon terminals and dendrites [1, 2].

KCNMA1 mutations have increasingly been linked to several neurological disorders, including epilepsy, cerebellar ataxia, paroxysmal movement disorders, intellectual disability (ID), or autism, underscoring the important role of BKCa channels in controlling neuronal excitability [1, 2]. Since 2005, more than 20 different mutations of KCNMA1 have been reported, with a spectrum of clinically defined pathological and behavioral phenotypes collectively referred to as “KCNMA1-linked channelopathy” [1‐3]. This mutational spectrum encompasses both gain-of-function and loss-of-function alteration of BKCa channel activity, as well as several variants of unknown significance [1, 4]. Interestingly, an indirect BKCa channelopathy has been described in the literature, presenting with an ID phenotype associated with a nonsense mutation (R419X) in the CRBN gene, which encodes a cytosolic protein (CRBN, cereblon) that promotes the cell surface expression of BKCa channels in neurons [5, 6]. In addition, decreased protein expression of KCNMA1 resulting in reduced BKCa channel activity has also been observed in fragile X syndrome (FXS) [7]. Moreover, a novel missense mutation c.413G > A (R138Q) in the fragile X mental retardation 1 locus (FMR1) leads to an FXS-like phenotype with a principal hallmark of reduced BKCa activity [8].

Genetic inactivation of the BKCa α-subunit in murine models leads to neurodevelopmental disorders characterized by the absence of experience-dependent increase in prepulse inhibition in electrophysiological measurements and slowed spatial learning in the Morris water maze [9]. Taken together, the human observations and animal findings suggest that BKCa channels are key contributors to neurodevelopment and normal neuronal communication.

BKCa channels play an important role in the first, neuronal signaling stage of visual processing. BKCa currents are expressed in key retinal cell types, and genetic knockout of the BKCa channel in mice (Kcnma1^−/−) revealed impairments in the visual retinal pathway with a hallmark of reduced retinal response to light [10]. These findings, along with the observation of disturbances in sensory processing, are a prominent feature of neurodevelopmental disorders [11], prompting us to investigate the visual processing pathway in a subject with KCNMA1 haploinsufficiency using electroretinography (ERG) and visual contrast sensitivity (CS). These novel and complementary biomarkers of sensory deficits in the visual system have recently been shown to be practical for assaying visual system function in subjects presenting neurobehavioral phenotypes and have excellent translational utility between a genetic mouse model (Fmr1^−/y) of FXS and human FXS subjects [12].

BKCa channels are expressed in a variety of retinal cells such as rod and cone photoreceptors [16], bipolar cells [17], amacrine cells [18], and ganglion cells [19]. This expression pattern suggests a pivotal role of BKCa channels in retinal function and light perception as highlighted in Kcnma1^−/− mice, where the absence of BKCa leads to a significant reduction of ERG signals at high scotopic and low mesopic stimulus intensities [10]. Indeed, ERGs in Kcnma1^−/− mice are characterized by a reduced b-wave amplitude in dark-adapted conditions for a nocturnal animal without modification of either the a- or b-wave implicit time [10]. This phenotype is consistent with the role of BKCa in retinal neurons since BKCa channels control glutamate release at the photoreceptor synapse [16] and regulate the flow of excitatory synaptic transmission from the photoreceptor pathway to the ganglion cells via the amacrine/bipolar cell pathway [20]. Thus, BKCa channels are involved in signaling of light capture and downstream regulation of neurotransmission of light signals.

Interestingly, investigation of ERG and CS in the Kcnma1^−/− mouse model yielded similar findings to those observed in this KCNMA1^−/+ subject. The substantial reduction in ERG b-wave amplitude (60.4% decrease) shows a bipolar cell signaling deficit in this subject as these cells are mainly involved in b-wave genesis. LA flicker stimulation also resulted in a substantial (63.9%) decrease in flicker wave amplitude, implying sensitivity of the retinal signaling to rapidly changing stimuli that emphasize the processing and communication pathways that lead out of the retina. Although background light conditions were different in the clinical investigation (light-adapted) versus the experimental conditions in Kcnma1^−/− mice (dark-adapted), the commonality of the bipolar cell dysfunction is supported by the decreased flicker response in the KCNMA1^−/+ subject since this response is dominated by post-light-receptor circuit elements, particularly the ON and OFF bipolar cells that interact to shape the steady-state flicker ERG response [21]. Overall, the retinal electrophysiology abnormalities observed in the KCNMA1^−/+ subject are similar to those observed in Kcnma1^−/− mice. BKCa channels are pivotal for normal regulation of retinal neurotransmission and thus visual pathway function, as proposed in murine models [10, 16, 20].

Using both ERG and CS, we have established that electrophysiologic abnormalities in the KCNMA1^-/+ subject presented here are associated with a functional visual impairment that further implicates a role for BKCa channels in visual perception in humans. The subject exhibited a higher contrast sensitivity threshold and lower performance (55.7% decreased) to discriminate several contrast intensities starting at a distance of 3 m or greater. CS changes have direct behavioral consequences on an individual’s interaction and perception of their environment since it is defined by the threshold between the visible and invisible, and thus the ability to detect subtle differences in shading, patterns, in detecting objects without clear outlines, and discriminating objects or details from their background [12]. Interestingly, this KCNMA1^−/+ subject’s caregiver reported that the subject exhibited avoidance behavior in front of a bright light source or during sunny days. The subject avoids bright light by staying inside the home with the curtains closed or wears sunglasses at home, suggesting a photophobia phenotype. Photophobia can be defined as a reduction of contrast sensitivity caused by glare light and thus a general aversion to light [22‐24]. A link between contrast discrimination impairments and photophobia phenotypes has been described previously and suggested to have origins in retinal cell function, especially in cone-driven retinal pathways [22‐25]. Interestingly, the specific b-wave ERG response abnormalities in this case also suggest involvement of cone-driven retinal pathways.

BKCa impairments have been described in FXS [7] as well as in the Fmr1^−/y genetic mouse model of FXS. In both Fmr1^−/y mice and FXS patients, genetic loss of FMRP leads to a reduction of BKCa α-subunit protein expression [7, 26‐28]. In the murine Fmr1^−/y model, protein expression of BKCa α-subunit is reduced by as much as 50%, resulting in a corresponding 50% decrease in channel activity [7].

All these data mimic the BKCa haploinsufficiency phenotype found in our KCNMA1^−/+ subject. It is important to note that modulating the BKCa activity with small-molecule channel openers has been shown to rescue a broad spectrum of behavioral phenotypes in the Fmr1^−/y mouse model of FXS [7, 28, 29]. Interestingly, using the combination of ERG and CS (ERG-CS) in a FXS cohort, we recently identified visual system electrophysiological and functional impairments characterized by decreased ERG b-wave and flicker wave amplitudes and lower CS discrimination ability [12]. The data obtained from that same use of the ERG-CS biomarker strategy in this KCNMA1^−/+ subject are remarkably similar to those obtained in the FXS patients. Therefore, based on the BKCa expression pattern and role in the mammalian retina and the abnormalities of FMRP expression in retina, we suggest that part of the ERG-CS phenotype of the FXS population is due to a functional BKCa deficit linked to loss of FMRP in retina. In other words, BKCa impairments observed in FXS should be the main origin of the ERG-CS deficits, reinforcing the role of the BKCa channel in the FXS phenotype.

In summary, our study shows that BKCa haploinsufficiency leads to a visual pathway deficit associated with a light-capturing deficit at the retinal level, and with a functional alteration in contrast sensitivity associated with photophobia symptoms. The data from this case report suggest that BKCa channels play a pivotal role in visual pathway physiology. Interestingly, visual pathway data from the Fmr1^-/y murine model of FXS [7] and human FXS patients [12], both implicating BKCa channel dysfunction, suggest that BKCa channels play a major role in the functional and behavioral visual pathway impairment in the FXS phenotype. Further studies would be helpful to confirm these findings in broader FXS and KCNMA1-linked channelopathy populations such as Liang–Wang syndrome (LIWAS) (OMIM: 618729). Finally, ERG-CS appears to be good complementary biomarker to identify and characterize visual processing deficits in BKCa channelopathies and has potential utility in the development of therapeutics for conditions involving BKCa dysfunction.