Thymidine Kinase 2 (TK2 ) Deficiency

The purpose of this clinical trial is to evaluate the efficacy and safety of Doxecitin and Doxribtimine (dC+dT) in adult participants with thymidine kinase 2 (TK2) deficiency attended in the Neuromuscular Unit of '12 de Octubre' Hospital. The main questions it aims to answer are: * Is dT+dC effective in the treatment of the adult participants with TK2 deficiency? * Is dT+dC safe in the treatment of adult participants with TK2 deficiency? Researchers will evaluate the effectiveness of the treatment doxecitin and doxribthymine in adult participants with TK2 deficiency. In addition, the mitochondrial DNA levels before and after treatment (extracted from the muscle and from uroepithelial cells) of these participants will be also studied.