Introduction: Niemann-Pick disease Type C (NPC) is an autosomal recessive disease that is caused by mutations in two genes, NPC1 or NPC2 that encode lysosomal proteins. Mutations in either gene result in the accumulation of cholesterol, sphingomyelin and glycosphingolipids in lysosomes[1]. The principal clinical manifestations include hepatosplenomegaly, ataxia, seizures and neurodegeneration. No in-depth studies have been performed on cardiac function in these patients. Recent studies have shown that Ca²⁺ released from acidic stores such as lysosomes can take a part in cytosolic Ca²⁺ transients, enhancing contraction amplitude and speed[2]. NPC cells have reduced levels of lysosomal Ca²⁺[3], thus, it is possible that altered Ca²⁺ signalling in cardiomyocytes can affect the lysosome mediated Ca²⁺ signalling cascade and may be potentially arrhythmogenic.

Aims: To characterise the NPC mouse heart both structurally and at the molecular level, and to investigate its susceptibility to arrhythmia. 

Methods: All experiments complied with the United Kingdom Home Office Guide on the Operation of Animal (Scientific Procedures) Act of 1986. Wild-type (WT) and Npc1^-/- mice were bred from heterozygous BALBc/cNctr- Npc1^m1n/J. Masson’s trichrome and picrosirius red staining was performed on heart sections to assess the level of fibrosis and further validated by western blotting of the fibrosis marker collagen I and vimentin. Npc1^-/- heart electrophysiology (ECG) was performed on a Langendorff perfusion system using electrical (burst pacing) and pharmacological (50nM isoprenaline) protocols. We also performed a transcriptomic analysis on Npc1^-/- hearts at 3, 7 and 9 weeks of age to investigate the molecular mechanism involved in Npc1^-/- cardiac pathophysiology development at the transcriptional level.

Result: Histological staining revealed significantly increased fibrosis in Npc1^-/- heart sections at 7 weeks of age (WT 17.1%±0.3 vs NPC 19.4%±0.28, p<0.0001, n=3). Western blot for collagen I and vimentin supported the histology results, with increased levels of collagen I and vimentin in NPC tissue (p=0.0261 or 0.0435 respectively, n=6). Compared to WT, Npc1^-/- hearts had significantly pro-longed QT interval for 11.7 ms (p=0.0029, n=7), suggesting a change in ion channel function. Also, Npc1^-/- hearts presented ECG abnormalities including arrhythmias under pharmacological stress. Transcriptomic analysis found 3240, 2190 and 5300 differential expressed genes (DEGs) comparing hearts at 3, 7 and 9 weeks of age (p<0.05, n=5). Several significantly altered cellular pathways were identified including immune/inflammation response, lysosome, calcium signalling, arrhythmogenic right ventricular cardiomyopathy, and adrenergic signalling, which support our hypothesise that Npc1^-/-heart is more susceptible to arrhythmia.

Conclusion: Our studies highlight in Npc1-/- hearts fibrotic remodelling, inflammation and ion signalling alterations.  We provide the first evidence that NPC hearts have a higher tendency to display structural and functional disturbances including arrhythmias as a consequence of lysosomal Ca²⁺ signalling defects in the heart.