Role of TRPM4 ion channel in pediatric arrhythmic syndromes

Table 1 TRPM4 variants electrophysiologically studied

Substitution	Variant	Location	Effect	Underlying mechanism	Protein expression level on membrane	Current density	Ca2+ dependence	Voltage-dependence	Electrophysiological results	Classification	ECG morphology; phenotype	Ref.
p.E7K	c.19G>A	N-terminus. Putative CaM binding site	GoF	Impaired SUMOylation (attenuated deSUMOylation). Impaired endocytosis Enhanced interaction of PIP2 and TRPM4 protein (increased risk of generating triggered activities)	↑	↑			Prolonged AP (due to facilitated open state condition). Depolarizing shifts of the resting membrane potential (depending on the channel density or maximal activity). Insensitivityto PIP2 depletion	Pathogenic	PFHB1B, RBBB, progressive conduction block	[30,49]
p. R164W	c.490C>T	N-terminus	GoF		No changes	↑	No changes		No effect on SUMOylation stimulation. Dynamitin-insensitive	Pathogenic	PFHB1B	[48]
p.D198G		Putative CaM binding site			No changes	No changes				Uncertain significance		[50,51]
p.A432T	c.1294G>A	N-terminus	GoF	Likely to be caused by protein misfolding and retention in the endoplasmic reticulum	↓	↓	No changes		Cotransfection experiments of TRPM4 mutants with Ubc9 showed an increase of current density (stimulating effect on SUMOylation). Dynamitin-insensitive. Lowering the incubation temperature of cells expressing the variant to 28 °C for 24 hours increased their expression both at the total level (A432T 78% ± 3% of WT); and at the cell surface level (A432T 72% ± 8% of WT). A significant increase in the current density of A432T was observed (599 ± 104 pA/pF at 28 °C vs 230 ± 27 pA/pF at 37 °C), whereas WT showed no significant change (639 ± 101 pA/pF at 28 °C vs 553 ± 74 pA/pF at 37 °C;P > 0.05)	Pathogenic	PFHB1A. PFHB1B. Atrioventricular block. Brugada syndrome	[49]
p.A432T/G582S		N-terminus	LoF	Likely to be caused by protein misfolding and retention in the endoplasmic reticulum	↓	No changes			Lowering the incubation temperature of cells expressing the variant to 28 °C for 24 hours increased their expression both at the total level (A432T/G582S 69% ± 5% of WT) and at the cell surface level (A432T/G582S 65% ± 7% of WT)
p.V441M			LoF			↓						[49]
p.R499W			LoF			↓						[48]
p.G582S	c.1744G>A	N-terminus	GoF	Direct SUMOylation of TRPM4 is unlikely to be linked to the increased expression and gain of function of the variant	↑	↑ (171% ± 20% of WT)				Pathogenic	PFHB1A. PFHB1B. Brugada syndrome	[48]
p.T677I		N-terminus			No changes	No changes				Uncertain significance
p. P779R	c.2336C>G	TM2			↓	↓	↓	V1/2 ↑		Uncertain significance	Brugada syndrome. Right bundle brunch block
p.G844D	c.2531G>A	ABC-motif	GoF		No changes	↑	No changes		Cotransfection experiments of TRPM4 mutants with Ubc9 showed an increase of current density (stimulating effect on SUMOylation). Dynamitin-insensitive	Pathogenic	PFHB1B right bundle-branch block. Brugada syndrome. Long QT syndrome	[48]
p.T873I	c.2618C>T	TM3			↑		↓	No changes		Uncertain significance		[48]
p.K914X	c.2740A>T	Putative SUMOylation site		Non-functional channel	↓	No current	↓	No changes		Likely benign	PFHB1B right bundle-branch block	[52]
p.V921I	c.2761G>A	TM4-TM5			No changes	No significant changes				Likely benign	PFHB1B	[49,53]
p.L1075P	c.3224T>C	C-terminus			↑	↓	↓	No changes		Uncertain significance	PFHB1B

AP: Action potential; GoF: Gain-of-function mutation; LoF: Loss-of-function mutation; PFHBIA: Progressive familial conduction block type IA; PFHBIB: Progressive familial conduction block type IB; RBBB: Right bundle branch block; ECG: Electrocardiography.

Table 2 Genetic variants identified in patients

Patients	Gene	GRCh38	Nucleotide change	Amino acid change	db SNP, MAF%	CADD	SIFTcat	PolyPhenCat
Patient 1	TRPM4	Chr19:49200722,	NM 017636.4: C.C2890A	p.Arg964Ser	rs749078579, 0.001592%	26	Deleterious	PD
Patient 2	TRPM4	Chr19:49210799	NM 017636.4: C.A3418T	p.Lys1140Ter	No data	45	NA	NA
Patient 2	MYPN	Chr10: 68201958	NM 032578.4: C.A3623T	p.Asp1208Val	No data	32	Deleterious	PD

CADD: Combined annotation dependent depletion (GRCh38-v1.6); NA: Not available; PD: Probably damaging.