ALPL and hyperinsulinemic hypoglycemia, familial, 4: Although the etiology of muscle weakness and fatigue in HPP is poorly understood, TNSALP deficiency is associated with elevated PLP levels, mitochondrial hyperfunction, diminished neural function, and high adenosine triphosphate (ATP) levels, which have been posited to cause muscle weakness and/or fatigue by impairing energy metabolism [17, 18].