Mitochondrial dysfunction in hyperkalemic periodic paralysis: A case report and literature review

Abstract

Background

Hyperkalemic periodic paralysis is a rare autosomal dominant disorder of sodium channels in skeletal muscles caused by pathogenic variants in the SCN4A gene. Emerging evidence suggests an association between this disorder and mitochondrial dysfunction, observed in mice. To our knowledge, there are still no reported cases demonstrating this link in humans.

Case presentation

We presented a case of hyperkalemic periodic paralysis caused by a heterozygous, de novo, and rare variant, NM_000334.4: c.2078T > C (p.Ile693Thr) in the SCN4A gene, reported for the first time in the Arab region. Our patient experienced intermittent episodes of weakness most frequently in the morning, with sudden onset of limb paralysis and inability to walk. She also exhibited facial myotonia with difficulty opening her mouth and eyelids. She was treated with preventive recommendations to avoid triggers and is now clinically stable. Additionally, the patient presented with mitochondrial dysfunction reported for the first time with hyperkalemic periodic paralysis.

Conclusion

This study is the first clinical report of mitochondrial respiratory chain enzymes dysfunction in a patient with hyperkalemic periodic paralysis caused by a rare variant, c.2078T > C (p.Ile693Thr), in the SCN4A gene.

Background

Periodic paralysis is defined as episodes of muscle weakness occurring at irregular intervals and caused by ion channelopathies in skeletal muscles. To be considered primary, periodic paralysis should be a genetic disorder often with more than one generation affected and occurring usually before the age of 20, with the exclusion of other disorders that could alter serum potassium concentrations [1]. One type of primary periodic paralysis is hyperkalemic periodic paralysis (OMIM:170500) which is an autosomal dominant neuromuscular disorder caused by pathogenic variants in the SCN4A gene. Variable manifestations are caused by different mutations in the SCN4A gene such as nondystrophic myotonias and periodic paralysis, due to either gain of function or inactivation of the channel, respectively [2]. This gene encodes a 260 kDa glycoprotein forming the alpha subunit of the voltage-dependent sodium channels in skeletal muscle [3]. With a reported minimum point prevalence of 0.17/100,000 in England and an estimated prevalence of 1/200,000 cases worldwide, this condition is classified as an ultra-rare disorder [4, 5]. Clinical manifestations include episodes of flaccid paralysis, with or without myotonia, typically associated with hyperkalemia during the attacks [3, 6, 10]. The age of onset is usually within the first decade of life. Attacks may be triggered by several factors like potassium-rich diet, cold temperature, stress, and fasting [11].

Emerging evidence suggests an association between hyperkalemic periodic paralysis and mitochondrial dysfunction. A recent study proposed a possible link between these two entities, mainly through sodium driven membrane excitability, in mice [12]. To our knowledge, there are still no reported cases demonstrating this link in humans.

This study highlights the presence of mitochondrial dysfunction in a patient with hyperkalemic periodic paralysis caused by a rare variant, NM_000334.4: c.2078T > C (p.Ile693Thr) in the SCN4A gene, reported for the first time in an Arab patient. This is the first clinical report of a possible link between hyperkalemic periodic paralysis and mitochondrial dysfunction in humans.

Case presentation

The proband is a three-year-old female who presented to our medical center for a history of recurrent episodes of muscle weakness. She was born full term to non-consanguineous Lebanese parents, with unremarkable perinatal complications. Family history was negative for neuromuscular or metabolic disorders. Her psychomotor development was normal. She first began experiencing intermittent episodes of weakness at 2 years and 7 months of age. These episodes typically occurred in the morning, with sudden onset of limb paralysis and inability to walk. During the attacks, she also exhibited facial myotonia characterized by difficulty opening her mouth and eyelids. Over a five-month period, she experienced three similar episodes, each lasting approximately 15 min, occurring during viral respiratory infections along with poor feeding. Between episodes, her physical examination, including muscle tone, was normal. Additionally, the patient did not exhibit muscle wasting or myopathy.

Serial blood tests revealed a mild increase in creatine phosphokinase (CPK), only during the episodes, with peak value reaching 730 IU/L (reference range: 25–195 IU/L), and mild hyperkalemia with serum potassium level ranging between 5.2 and 5.5 mmol/L (reference range 3.5-5 mmol/L).

A comprehensive work-up to rule out a metabolic or endocrine myopathy was normal. This included: plasma amino acids chromatography, total and free carnitine, acylcarnitine profile, urine organic acids chromatography in addition to adrenal and thyroid hormonal evaluations (glucose, electrolytes, cortisol, ACTH, renin, angiotensin, parathormone, TSH, and free T3). Electromyography and nerve conduction studies performed outside the attacks were normal. Brain MRI was also normal.

A skeletal muscle biopsy was obtained outside of symptomatic attacks and analyzed in the Department of Pathology and Laboratory Medicine at our institution. Histopathological analysis revealed a mild variation in myofibers size and shape, with minimal fibrosis, and occasional fibers with internalized nuclei as noted on modified Gomori’s trichrome stain. Stains of Oxidative enzymes, performed at as described in the literature [13], revealed a significant reduction in the activities of all assessed respiratory chain enzymes: NADH dehydrogenase, cytochrome c oxidase, and succinate dehydrogenase (Fig. 1), with preserved ATP synthase expression by immunohistochemistry (Fig. 2).

Fig. 1.

Muscle histology in a patient with hyperkalemic periodic paralysis. (A) Modified Gomori’s Trichrome (GT) stain: mild variation in myofibers size and shape, with minimal fibrosis, and sparse muscle fibers with nuclear internalization. (B-D) Oxidative enzymes stains: significant reduction in the activities of all respiratory chain enzymes, succinate dehydrogenase (SDH) (B), cytochrome c oxidase (COX) (C), and combined COX-SDH staining (D), with the most pronounced decrease observed in COX activity. The insert in panel C represents COX activity in an age-matched control. No ragged red fibers or COX-negative/SDH-positive fibers are seen. Upper and lower panels are shown at 200x magnification. Scale bars: 50 μm

Fig. 2.

ATP synthase expression by immunohistochemistry on muscle biopsy of a patient with hyperkalemic periodic paralysis. The expression of ATP synthase was preserved as shown by immunohistochemistry. The image represents 200x magnification. Scale bar: 50 μm

Respiratory chain enzyme activities were further assessed in skeletal muscle homogenate using spectrophotometric assays in our reference laboratory, as previously described [13]. Results were compared with laboratory-specific reference ranges and normalized to citrate synthase activity [14]. Biochemical analysis showed a mild reduction in the mitochondrial content as reflected by the decreased citrate synthase reference enzyme activity. The isolated activities of respiratory chain complexes I, II, III, IV and combined activity of complexes II + III, were below their reference ranges. Normalized activities, expressed in terms of the citrate synthase level, didn’t reach statistical significance for complexes III and IV indicating a potential combined deficiency. In addition, NADP-dependent isocitrate dehydrogenase activity was significantly reduced, whereas cytosolic lactate dehydrogenase activity remained within normal range (Table 1).

Table 1.

Biochemical assessment of mitochondrial respiratory chain complexes activities on a muscle biopsy of the patient

	Activities (nmol.min− 1.mg− 1)	Ref. range (nmol.min− 1.mg− 1)
CI	8.16*	[10.42–47.30]
CII	10.51*	[18.73–47.70]
CIII	42.69*	[81.28-210.86]
CIV	49.98*	[82.01-237.59]
CII + III	13.28*	[16.21–33.24]
Citrate Synthase	105.92*	[110.86-288.65]
LDH	5808	[2084–7317]

Values represent enzymatic activities and are expressed in nanomoles of substrate transformed per minute per milligrams of proteins (nmol.min-1.mg-1)

CI: NADH dehydrogenase, CII: succinate dehydrogenase, CIII: coenzyme Q cytochrome c-oxidoreductase, CIV: cytochrome c oxidase, LDH: Lactate dehydrogenase

Isolated activities of respiratory chain complexes I, II, III, IV and combined activity CII + III are below their reference ranges

* indicates activities that do not fall within normal reference values

Whole exome with mitochondrial DNA sequencing was performed at a reference commercial laboratory, Centogene GmbH- Germany, on an Illumina platform using short reads sequencing to obtain an average coverage depth of ~ 100x. It revealed a heterozygous pathogenic variant: NM_000334.4: c.2078T > C (p.Ile693Thr) in the SCN4A gene, confirming the diagnosis of hyperkalemic periodic paralysis. The de-novo nature of this variant was confirmed by parental testing.

Preventive therapeutic recommendations were implemented to avoid known triggers of attacks, mainly cold, exercise, potassium-rich food, and fasting. The patient was instructed to drink glucose solution during intercurrent illnesses or after exercise. At her most recent follow-up, at six years six months of age, the patient remained clinically stable although she continued to experience two to three attacks per year, which were aborted with prompt glucose intake.

Discussion and conclusions

This study describes a case of hyperkalemic periodic paralysis harboring a rare variant, NM_000334.4: c.2078T > C (p.Ile693Thr), in the SCN4A gene. To our knowledge, this is the first reported case of this pathogenic variant of Arab origin. Previously, this variant has predominantly been identified in patients of European descent [3, 6, 8, 10, 15], with few cases from East Asia [7, 9, 16].

Mutations in the SCN4A gene have been associated with a spectrum of neuromuscular phenotypes, including hyperkalemic periodic paralysis with or without paramyotonia congenita, potassium-aggravated myotonia, and, more rarely, hypokalemic periodic paralysis [17]. The de novo nature of this mutation was confirmed by parental genetic testing in addition to the negative family history. Although hyperkalemic periodic paralysis is typically inherited in an autosomal dominant manner, the proportion of “de novo” cases is still unknown [11]. The clinical manifestations associated with hyperkalemic periodic paralysis are highly variable among patients, with no genotype-phenotype correlation established to date [11]. Therefore, the differential diagnosis can be wide, encompassing other forms of periodic paralysis, adrenal disorders, and inherited metabolic diseases such as glycogen storage disease type V (McArdle disorder) and fatty acid oxidation defects. In the present case, the age of onset and the clinical manifestations were non-specific and the extensive metabolic/endocrine work-up was negative. Notably, this patient exhibited paramyotonia affecting the facial muscles and eyelids.

Facial paramyotonia and eyelid involvement have been previously reported in association with SCN4A-related channelopathies, particularly in paramyotonia congenita and overlap phenotypes. In the first description by Plassart et al., affected members of a French family carrying the p.Ile693Thr SCN4A mutation were diagnosed with paramyotonia congenita, while one individual additionally exhibited hyperkalemic periodic paralysis, highlighting early evidence of intrafamilial phenotypic overlap [3]. This de novo SCN4A mutation causing combined hyperkalemic periodic paralysis and paramyotonia congenita was also described by Lee et al., further reinforcing that facial paramyotonia and eyelid myotonia fall within the recognized clinical spectrum of sodium channelopathies [18]. Subsequent reports have further supported facial muscle involvement in SCN4A disorders having the p.Ile693Thr variant. Mankodi et al. described a child presenting with eyelid myotonia, grip myotonia, cold-induced symptoms, and transient weakness, with attacks triggered by rest after exercise and metabolic stressors [6]. Similarly, Yuan et al. reported eyelid myotonia, grip myotonia, cold sensitivity, and transient weakness in an SCN4A-mutated patient [7]. Taken together, these observations suggest that facial and eyelid involvement, although not typical of classic hyperkalemic periodic paralysis, may represent part of the phenotypic variability associated with SCN4A mutations and could reflect clinical overlap with paramyotonia congenita in certain cases.

Management was primarily preventive, focusing on avoidance of known triggers and correction of hyperkalemia during acute attacks. Although the patient’s condition was stabilized with preventive measures, the long-term prognosis remains uncertain, as symptom severity may increase with age [11].

At the molecular level, this rare SCN4A variant, c.2078T > C (p.Ile693Thr) is classified as pathogenic according to the American College of Medical Genetics and Genomics and ClinVar. This missense mutation results in the substitution of a neutral non-polar amino acid, Isoleucine, with a neutral and polar one, Threonine, at position 693. This change disrupts the protein’s hydrophobic interactions and hydrogen bonding likely altering the sodium channel’s structure and function. The resulting dysfunction of the sodium channel in muscle increases ion permeability, allowing excess sodium influx into skeletal muscle cells [3].

At the cellular level, the mutation in voltage-dependent sodium channels leads to prolonged depolarization which presents clinically as myotonic stiffness, followed by episodes of flaccid paralysis due to the subsequent muscles inexcitability. The resulting potassium ions release outside the cells exacerbates these effects by further activating the mutated channels [19].

Notably, the patient described in this report had a potential concomitant mitochondrial dysfunction as illustrated by the biochemical and histological studies of the mitochondrial respiratory chain enzymes in the muscle biopsy. A recent study suggests an association between hyperkalemic periodic paralysis and mitochondrial dysfunction in mice by reinforcing the close links among membrane excitability, ryanodine receptor 1 (RYR1), and mitochondrial function [12]. It also cites the work of Matthews et al., in which RYR1 mutations are causally linked to late-onset atypical periodic paralysis, both with and without associated myopathy [20]. Furthermore, incorporating findings from Chung et al., who demonstrated reversible inhibition of mitochondrial ATP synthesis and increased free radical production in cardiac muscle upon sodium overload [21], Suetterlin et al. proposed a negative spiral mechanism to explain this association. Free radicals’ accumulation causes this negative spiral by increasing the RYR1 leak which in turn increases mitochondrial dysfunction. This cycle has been shown to be exaggerated in mice with hyperkalemic periodic paralysis, displaying an impaired oxidative phosphorylation and an acquired RYR1 leak due to the genetic ion channel mutation [22]. This association seen in mice may be extrapolated to human cases of hyperkalemic periodic paralysis showing mitochondrial dysfunction. Accordingly, it has been demonstrated that uptake of sodium into intracellular compartments led to inhibition of mitochondrial respiration in human macrophages [23]. However, the direct applicability of these findings to human skeletal muscle in hyperkalemic periodic paralysis remains uncertain. Further investigations are certainly needed to confirm whether this sodium overload would also lead to the same effects in human skeletal muscles.

It is recognized that a significant proportion of patients with mitochondrial disease, particularly in childhood, may exhibit minimal or even normal muscle histology, and therefore absence of ragged red fibers or COX-negative fibers does not exclude primary mitochondrial pathology [24]. However, in our patient, several features favor a secondary mitochondrial dysfunction. Citrate synthase activity was mildly reduced, suggesting decreased mitochondrial content rather than a compensatory mitochondrial proliferation typically observed in primary mitochondrial disorders. In addition, muscle architecture was preserved with only mild nonspecific changes, diffuse reduction of oxidative enzymes activities, and preserved ATP synthase expression, without focal COX deficiency, subsarcolemmal mitochondrial accumulation, or ragged red fibers. The combined reduction across respiratory chain complexes I–IV without a dominant isolated defect further supports secondary impairment of oxidative phosphorylation. In this context, and supported by experimental evidence linking SCN4A-related sodium overload to mitochondrial dysfunction, we interpret our findings as most consistent with secondary mitochondrial impairment associated with hyperkalemic periodic paralysis.

Additionally, the absence of muscle wasting and a clear myopathic phenotype in our patient is consistent with a secondary mitochondrial dysfunction. It is associated with an impairment of mitochondrial oxidative phosphorylation, rather than inherited defects in mitochondrial DNA or oxidative phosphorylation genes [25].

The relationship between hyperkalemic periodic paralysis and mitochondrial dysfunction in this case remains uncertain, and a causal link cannot be established based on a single observation. A suggested illustration of the molecular pathogenesis of mitochondrial dysfunction in the patient’s hyperkalemic periodic paralysis is provided (Fig. 3).

Fig. 3.

Proposed mechanism of mitochondrial dysfunction in SCN4A-related hyperkalemic periodic paralysis. (1) The SCN4A p.Ile693Thr mutation alters Nav1.4 sodium channel function in skeletal muscle fibers. (2) Altered Nav1.4 channel activity results in persistent sodium conductance and abnormal membrane excitability. (3) Increased sodium influx into skeletal muscle fibers leads to (4) membrane depolarization, clinically manifesting as myotonia and episodic paralysis, and contributes to (5) intracellular sodium overload with disrupted ionic homeostasis. (6) Intracellular sodium overload impairs mitochondrial ATP synthesis and increases reactive oxygen species (ROS) production. (7) ROS accumulation promotes oxidative stress within the muscle fiber and (8) enhances RYR1-mediated calcium leak from the sarcoplasmic reticulum. (9) Increased cytosolic calcium and ROS further aggravate mitochondrial dysfunction, creating a self-perpetuating cycle of oxidative stress, calcium dysregulation, and mitochondrial impairment. (10) These mechanisms contribute to impaired mitochondrial respiratory chain activity, reflected by reduced respiratory chain complexes I–IV activities, diffuse reduction in oxidative enzyme staining, and preserved ATP synthase expression observed in the present patient

Several alternative explanations should be considered. First, mitochondrial dysfunction may be coincidental and unrelated to the underlying channelopathy. Second, mitochondrial impairment may be secondary to repeated episodes of muscle depolarization, metabolic stress, or illness-related catabolic states, which could lead to secondary mitochondrial dysfunction through increased energetic demand and oxidative stress [26]. Finally, mitochondrial dysfunction may act as a disease modifier, potentially influencing the severity or frequency of hyperkalemic periodic paralysis attacks without serving as a primary etiologic factor.

In conclusion, this study describes the first patient from Arab descent carrying the rare de novo variant NM_000334.4: c.2078T > C (p.Ile693Thr), in the SCN4A gene associated with hyperkalemic periodic paralysis. To our knowledge, this is the first study reporting mitochondrial respiratory chain enzymes dysfunction in a patient with this neuromuscular disorder. As a limitation, this report highlights a potential association rather than a direct causality allowing for the consideration of alternative explanations. Inherently, case reports are unable to establish causality nor generalizability. Thus, further larger studies are needed to elucidate a potential association, which may possibly have implications for future therapeutic modalities.

Authors’ contributions

FM and AY wrote the first draft of the manuscript and contributed equally to this work. RK contributed to data collection, writing and analysis. PK contributed to conceptualization, data collection, writing and revision of the manuscript. All authors read and approved the final manuscript.

Funding

Not applicable.

Data availability

All data supporting the findings of this study are available within the paper.

Declarations

Ethics approval and consent to participate

This study was conducted in accordance with the Declaration of Helsinki. Ethical approval was obtained by the Institutional Review Board of the American University of Beirut, Lebanon. Written informed consent to participate in this study was obtained from the legal guardians of the patient as per the Institutional Review Board of the American University of Beirut, Lebanon.

Consent for publication

Written informed consent to for publication of clinical details and/or clinical images was obtained from the legal guardians of the patient as per the Institutional Review Board of the American University of Beirut, Lebanon.

Competing interests

The authors declare no competing interests.