Figure 1. The mutant mouse had intermittent dystonic behaviors affecting hindlimbs and tail.
(A) During ambulation, hyperextension could affect either hindlimb or both, sometimes with inversion of the foot. When hyperextension was unilateral, some mice had a preferred side et al. switched sides. Hyperextension of hindlimbs was seen at the youngest age during locomotion, but by 4 months of age was sometimes seen at rest and sometimes was bilateral. A bilateral, maximally extended posture is within a WT mouse’s normal repertoire because it is shown by nursing dams straddling a large litter. (B) Hyperextension often continued when the animals sat. (C) Briefly curved tail was sometimes the first symptom in weanlings but was seen in older adults mainly when stressed. The curvature, in the plane of the floor, utilizes lateral muscle groups, and Straub tail was seldom if ever seen. (D) While WT mice sometimes have brief periods of rigidity and tilting when dropped in water, the mutant mice adopted an upright posture with extreme hyperextension and spread toes. They soon recovered and swam. (E) The normal WT reflex when suspended by the tail. (F) Mutants exhibited caudal hyperextensions involving one or both hindlimbs. This is also within the normal repertoire because WT exhibit a hindlimb posture like this when suspended just out of reach of an object and reaching with the forelimbs. (G) The mutants also exhibited transient hyperflexions of one or both hindlimbs. This was not a coordinated 'clasped' posture (limbs held together at the midline). Vibration stimulation of the knee joint in awake, hand-held mutant mice sometimes elicited strong dystonic movements when mice were released (not shown).