Conclusion
The Q1491H and G1481V mutations are both located near the IFM motif,
which is involved in fast inactivation. These mutations changed the
inactivation state, shifting it to a more positive voltage. They also
increased the window current, which can result in heart malfunctions
such as LQT3. Full biophysical characterization constitutes a strong
benefit, providing useful supplemental criterion in the variant
classification procedure. These two infants may have benefited from
class 1 antiarrhythmic drug therapy, but the lack of effect these drugs
is not well understood and warrants further study.