Validity of a Neurological Scoring System for Canine X-Linked Myotubular Myopathy

Abstract

A simple clinical neurological test was developed to evaluate response to gene therapy in a preclinical canine model of X-linked myotubular myopathy (XLMTM). This devastating congenital myopathy is caused by mutation in the myotubularin (MTM1) gene. Clinical signs include muscle weakness, early respiratory failure, and ventilator dependence. A spontaneously occurring canine model has a similar clinical picture and histological abnormalities on muscle biopsy compared with patients. We developed a neuromuscular assessment score, graded on a scale from 10 (normal) to 1 (unable to maintain sternal recumbency). We hypothesize that this neurological assessment score correlates with genotype and established measures of disease severity and is reliable when performed by an independent observer. At 17 weeks of age, there was strong correlation between neurological assessment scores and established methods of severity testing. The neurological severity score correctly differentiated between XLMTM and wild-type dogs with good interobserver reliability, on the basis of strong agreement between neurological scores assigned by independent observers. Together, these data indicate that the neurological scoring system developed for this canine congenital neuromuscular disorder is reliable and valid. This scoring system may be helpful in evaluating response to therapy in preclinical testing in this disease model, such as response to gene therapy.

FIG. 1.

Example of the neurological assessment form completed for all dogs at each evaluation. This form reflects a normal neurological examination.

FIG. 2.

Clinical signs observed in wild type as compared with XLMTM dogs. (A) Wild-type dog with normal masticatory muscles. (B) XLMTM dog with pronounced temporal muscle atrophy (arrows). (C) Wild-type dog able to hold mouth in a normal, closed position. (D) XLMTM dog unable to maintain mouth in closed position (dropped jaw). XLMTM, X-linked myotubular myopathy. Color images available online at www.liebertpub.com/humc

FIG. 3.

Validity results at 10 weeks of age in untreated XLMTM and wild-type (WT) littermate dogs. Open circles denote WT dogs, while closed circles denote affected dogs. (A) Box plot of scores for WT and affected dogs. Black horizontal lines indicate mean neurological assessment score. (B–F) Comparison of neurological assessment scores to traditional severity data available at the time of analysis (B, gait velocity; C, gait step length; D, gait stride length; E, hind limb torque extension; F, hind limb torque flexion). Dashed line indicates the estimated linear trend. Estimated correlation coefficients are indicated on all plots. All points are jittered to avoid overplotting.

FIG. 4.

Validity results at 17 weeks of age in untreated XLMTM and wild-type (WT) littermate dogs. Open circles denote WT dogs, while closed circles denote affected dogs. (A) Box plot of scores for WT and affected dogs. Black horizontal lines indicate mean neurological assessment score. (B–F) Comparison of neurological assessment scores to traditional severity data available at the time of analysis (B, gait velocity; C, gait step length; D, gait stride length; E, hind limb torque extension; F, hind limb torque flexion). Dashed line indicates the estimated linear trend. Estimated correlation coefficients are indicated on all plots. All points are jittered to avoid overplotting.

FIG. 5.

Reliability results at 17 weeks. Open circles denote wild-type (WT, n=6) dogs, while closed circles denote XLMTM dogs. Graphs A, C, and E present scatterplots with estimated correlation coefficients. The dashed line indicates the estimated linear trend, and the solid line shows the line of agreement. Graphs B, D, and F present Bland–Altman plots. The gray line shows the zero line, the solid black line shows the average of the differences, and the dotted black line shows the limits of agreement. All points were jittered to avoid overplotting. Original, original score provided by neurologist; Student, score provided by second independent observer based on written neurological examination record; Video, score provided by second independent observer based on videotaped neurological examination.