Supplemental Table 1: Dogs (Beagle, Golden Retriever and Labrador Retriever Breeds) Used

Dog ID* Signalment Source CNM T-tubule morphometry Cryosections for status count (myofibers) histology/IHC Ctl1 32mo M Beagle UK colony WT 130 (6) Ctl2 32mo M Beagle UK colony WT 153 (5) Ctl3 17mo F Beagle UK colony WT 160 (6) Ctl4 17mo F Beagle UK colony WT 223 (6) Ctl5 Dzer 17mo M Golden Alfort WT 216 (8) TS and LS Ctl6 Dingo 40mo M Golden Alfort WT 152 (6) TS and LS Het1 Elipse 11mo F Labrador Alfort +/- 152 (6) CNM1 11mo M Labrador Alfort CNM 136 (6) TS and LS Elios CNM2 Elvis 11mo M Labrador Alfort CNM 173 (6) TS and LS CNM3 11mo F Labrador Alfort CNM 204 (6) Electra CNM4 11mo F Labrador Alfort CNM 173 (6) Elisa Het1-2 30mo F Labrador Alfort +/- 186 (6) CNM1-2 30mo M Labrador Alfort CNM 212 (8) TS and LS CNM2-2 30mo M Labrador Alfort CNM 190 (8) TS and LS CNM3-2 30mo F Labrador Alfort CNM 205 (6) CNM4-2 30mo F Labrador Alfort CNM 253 (6) Het2 Momo 14y M Labrador Alfort +/- 240 (6) TS CNM5 14y M Labrador Alfort CNM 259 (6) TS Moustic

Supplemental table 1: Dogs (Beagle, Golden Retriever and Labrador Retriever breeds) used for ultrastructural and histopathological evaluations in this study. T-tubule morphometry was performed from electron micrographs of longitudinal ultrathin sections, images were taken in a standard manner and we planned to take measurements from at least 125 t-tubules per dog over a minimum of 5 myofibers – the actual number of t-tubules evaluated for each dog is listed in the table. TS: transverse sections; LS: longitudinal sections; N/A: not applicable *The suffix -2 designates the same dog biopsied on a second occasion at 30 months of age to evaluate temporal progression.