It was a complete surprise, when we initially discovered the first human genetic variant in a calmodulin-encoding gene.
Geneticist Mette Nyegaard had come across a large family with rare cardiac arrhythmia that was clearly Mendelian inherited. It was in the early 2000s, and genetic sequencing was still costly and labor‑intensive. But through genotyping and linkage analysis she narrowed the locus to chromosome 14. Sequencing revealed a novel variant causing an asparagine‑to‑isoleucine substitution in calmodulin.
At the time, calmodulin was already recognized as an extraordinarily conserved calcium sensor protein. Yet a search of the literature revealed no reports of calmodulin variants in cardiac arrhythmia — or in humans at all.
To explore the physiological implications of this unexpected finding, Nyegaard and protein biophysicist Michael Overgaard attended for a calcium conference. In one of the program breaks, Overgaard had the chance to talk to an influential scientist in calcium signaling. He mentioned that they had discovered a calmodulin variant in a patient, and he could not reveal what the disease was. The immediate reply from the other scientist was: “Is it the brain?”
The rest is history – or?
The discovery of the calmodulin variant in cardiac arrhythmia was published in 20121 and quickly followed by other reports of calmodulin variants in cardiac patients2–4. Mechanistic studies described how these variants impaired the ability of calmodulin to act as a calcium sensor and dampened its interaction with key calcium channels that drive cardiac contraction.
Today, all cardiac cases are described in the International Calmodulinopathy Registry (ICalmR), anchored with prof. Peter Schwartz and assoc. prof. Lia Crotti in Milan, Italy5,6 – each entry tells a dramatic story of cardiac fibrillation, cardiac arrest or even sudden cardiac death in childhood. The variants remain ultra-rare, with approximately 140 patients registered at present.
While we have participated in these efforts, primarily to describe the mechanistic effects of calmodulin variants, the idea of a neurological effect has remained a persistent question.
Calmodulin is abundantly expressed in the brain and has hundreds of interaction partners. It is among others an upstream regulator of a number of kinases, where the calcum/calmodulin-dependent kinase II (CaMKII) is potentially the most well-known.
So, could there in fact be an effect of calmodulin variants in the brain? If there was an effect, what would it be?
Finding the neurological patients
The ICalmR provided an initial starting point, though entries were limited. There were reports of patients with neurological impairments, and it was unclear if these impairments were secondary to cardiac events. As the registry expanded, however, it became evident that some calmodulin variant carriers exhibited neurological phenotypes independent of cardiac disease. Remarkably, one carrier was reported with neurological impairments only.
Although diagnosed with different diseases, there was a pattern of neurodevelopmental disorders among these patients: autism, attention-deficit/hyperactivity disorder, intellectual disability, epilepsy, and developmental delay. This observation probed a hypothesis that calmodulin variants could contribute to development of neurodevelopmental disorders – also in patients with no cardiac disease.
But where could we find neurological patients? Calmodulin variants are extremely rare (we estimate ~1:5000), and no one was looking for them in neurological patients.
Then in 2022, Singh et al published a large exome sequencing study of 24,248 schizophrenia patients and 97,322 control individuals7 – enough people for us to look for calmodulin variants in patients with a neurodevelopmental disorder.
Surprisingly low effects of neurological variants
We found seven calmodulin variants in schizophrenia patients and 20 in control individuals8. Excitingly, the variants in schizophrenia patients were in the C-terminal part of the protein, similarly to variants from cardiac patients9. Encouraged by this observation, Malene Brohus and I went to the laboratory to study the functional effects of these variants.
The effects were modest.
For cardiac arrhythmia patients, calmodulin variants typically cause severe impairment of calcium sensing, with affinity reductions in the range ~10‑fold9. By contrast, the schizophrenia variants showed only 1.5–2‑fold decreases, and some even increased calcium affinity.
We moved on to measure the effect of these variants on calmodulin’s regulation of the voltage-gated calcium channel 1.2 (CaV1.2). Dysregulation of this channel is central for explaining the cardiac phenotypes10,11. In the brain, it is also expressed and implicated in schizophrenia12. We hypothesized that mechanism might be conserved between the two tissues.
Again for CaV1.2 interaction, we found small effects – and again, some variants increased the affinity. Functionally, we could see significant effects of one variant.
Interpretation came with a new mindset
“Disappointment” is a wrong word to use. However, we found it difficult to interpret these low effect variants and what their mechanism might be.
So we went back to our training in physiology and genetics and thought about the differences between cardiac tissue and neurological tissues and their diseases.
Cardiac myocytes express a limited set of specialized ion channels, and pathogenic variants in arrhythmia are typically strong, monogenic drivers of disease. Neurons, by contrast, express a broad and complex repertoire of ion channels. In neurodevelopmental disorders, disease risk often arises from the cumulative impact of many small‑effect variants.
This framework aligned precisely with our findings: calmodulin variants in schizophrenia patients showed small effect sizes. In patients, they likely interact with other genetic variants that increase risk of schizophrenia to cause disease.
Our model now is that the functional effect size of calmodulin variants is (at least in part) predictive of diagnosis. If the variant has strong effects on calcium binding and CaV1.2 regulation, there is a high likelihood of cardiac arrythmia. If the variant has small effects, there is smaller risk of cardiac arrythmia. In all cases there is probably a risk of neurological impairments.
Finding the mechanism
Having established a link between calmodulin variants and neurological impairments, our next challenge is to uncover the underlying mechanisms. Some insights can be drawn from cardiac arrhythmia, yet it is clear that we must also explore new directions.
It is particularly intriguing that some variants have increased calcium sensing. These gain‑of‑function changes enable calmodulin to respond more rapidly and to smaller signals, potentially engaging targets beyond the ion channels we have studied to date. Such effects may aggregate to produce significant signaling imbalances, especially within pathways that operate through feed‑forward mechanisms.
We are approaching these questions from multiple angles. At the molecular level, we focus on selected neurological targets to assess their interaction with calmodulin. At the cellular and organismal level, we examine physiological consequences in patient‑derived cells and model animals. In parallel, we have deepened our engagement with the neuroscience community, fostering discussions around bold hypotheses that may illuminate unexpected molecular mechanisms.
References
1 Nyegaard M, Overgaard MT, Søndergaard MT, Vranas M, Behr ER, Hildebrandt LL et al. Mutations in Calmodulin Cause Ventricular Tachycardia and Sudden Cardiac Death. The American Journal of Human Genetics 2012; 91: 703–712.
2 Crotti L, Johnson CN, Graf E, De Ferrari GM, Cuneo BF, Ovadia M et al. Calmodulin mutations associated with recurrent cardiac arrest in infants. Circulation 2013; 127: 1009–1017.
3 Makita N, Yagihara N, Crotti L, Johnson CN, Beckmann BM, Roh MS et al. Novel calmodulin mutations associated with congenital arrhythmia susceptibility. Circ Cardiovasc Genet 2014; 7: 466–474.
4 Reed GJ, Boczek NJ, Etheridge SP, Ackerman MJ. CALM3 mutation associated with long QT syndrome. 2015 doi:10.1016/j.hrthm.2014.10.035.
5 Crotti L, Spazzolini C, Tester DJ, Ghidoni A, Baruteau A, Beckmann B et al. Calmodulin mutations and life-threatening cardiac arrhythmias: insights from the International Calmodulinopathy Registry. Eur Heart J 2019; 40: 2964–2975.
6 Crotti L, Spazzolini C, Nyegaard M, Overgaard MT, Kotta M, Dagradi F et al. Clinical presentation of calmodulin mutations: the International Calmodulinopathy Registry. Eur Heart J 2023; 44: 3357–3370.
7 Singh T, Poterba T, Curtis D, Akil H, Al Eissa M, Barchas JD et al. Rare coding variants in ten genes confer substantial risk for schizophrenia. Nature 2022; 604: 509–516.
8 Jensen HH, Brohus M, Hussey JW, Busuioc A-O, Iversen ED, Darki F et al. Functional consequences of calmodulin variants identified among schizophrenia patients and controls. Transl Psychiatry 2025. doi:10.1038/s41398-025-03735-3.
9 Jensen HH, Brohus M, Nyegaard M, Overgaard MT. Human Calmodulin Mutations. Front Mol Neurosci 2018; 11. doi:10.3389/fnmol.2018.00396.
10 Limpitikul WB, Dick IE, Joshi-Mukherjee R, Overgaard MT, George AL, Yue DT. Calmodulin mutations associated with long QT syndrome prevent inactivation of cardiac L-type Ca2+ currents and promote proarrhythmic behavior in ventricular myocytes. J Mol Cell Cardiol 2014; 74: 115–124.
11 Limpitikul WB, Dick IE, Tester DJ, Boczek NJ, Limphong P, Yang W et al. A Precision Medicine Approach to the Rescue of Function on Malignant Calmodulinopathic Long-QT Syndrome. Circ Res 2017; 120: 39–48.
12 Nyegaard M, Demontis D, Foldager L, Hedemand A, Flint TJ, Sørensen KM et al. CACNA1C (rs1006737) is associated with schizophrenia. Mol Psychiatry 2010; 15: 119–121.