Myofiber size correlates with MTM1 mutation type and outcome in X-linked myotubular myopathy

Abstract

We aimed to correlate pathologic findings with MTM1 mutation type in a series of molecularly defined XLMTM cases. Clinical data from 15 XLMTM patients and their corresponding 16 muscle biopsies were studied. All patients were infants (range: 6-217 days old) when initially biopsied. The proportion of myofibers with central nuclei did not correlate with clinical outcome, however, morphometric studies showed that survivors had larger myofiber diameters in infancy than those who died (10.4+/-3.9microm versus 8.9+/-3microm; p<0.001). As a corollary, patients with MTM1 missense mutations had larger myofiber diameters (11.1+/-4microm), than those with truncation/deletion mutations (8.6+/-2.7microm) (controls 11.7+/-2.5microm) (p<0.0001). These data indicate that differences in myofiber size correlate with MTM1 mutation type and patient outcome. Failure to attain and/or maintain myofiber size, along with fiber type perturbations and the misplacement of myofiber nuclei and other organelles, are important components of XLMTM muscle pathology.

Figures

Figure 1

(A) H&E stained frozen section of the quadriceps biopsy performed at 6 days of age from patient 447-1, who has an MTM1 mutation that truncates myotubularin shows scattered centrally nucleated hypotrophic myofibers with the characteristic appearance of myotubes (arrowhead). Note the presence of many extremely small myofibers (arrows). (B) H&E stained frozen sections from patient 171-1a with R69C missense mutation and had a quadriceps biopsy at 8 days. Note that there are no extremely small myofibers like those depicted in A. (C) The contralateral quadriceps was biopsied in patient 171-1b at 3.3 years and it shows a population of considerably larger fibers. (D) Electron micrographs show small rounded myofibers with central nuclei and glycogen, which was qualitatively similar whether a patient had an MTM1 missense mutation or truncation/deletion mutation. Note the redundant basal lamina suggestive of atrophy (arrowhead). The bar is 20 μm in A, B and C and 2 μm in D.

Figure 2

(A) Histogram of morphometry data from patient 462-1, shows a shift toward smaller sized fibers with a mean myofiber size of 6.1±1.8 μm. (B) Histogram from patient 171-1 comparing morphometry data from quadriceps biopsies taken at 8 days of age (black) and at 3.3 years of age (grey). There is an overall decrease in mean myofiber size, from 14.9±3.8 μm to 12.0±6.1 μm, in the interval between biopsies (p

Figure 3

Box plots of percent myofibers with central nuclei/hpf. The mean percent of myofibers with central nuclei ranged from 4.7% to 39.7% with wide variability from field-to-field in some biopsies. The black horizontal line in each box denotes the mean. The top of the box denotes the 75th percentile, while the bottom indicates the 25th percentile. The top bar shows the 95th percentile and the bottom bar shows the 5th percentile. The dots outside of the bars represent outlying data points.