Myofiber degeneration in autosomal dominant Emery-Dreifuss muscular dystrophy (AD-EDMD) (LGMD1B)

Abstract

Autosomal dominant Emery-Dreifuss muscular dystrophy is caused by mutations in the LMNA gene that code for the nuclear membrane protein lamin A/C. We investigated skeletal muscle fibers from several muscles for cytoplasmic degenerative changes in three patients with genetically confirmed Emery-Dreifuss muscular dystrophy. Methods included quantitative light and electron microscopy and PCR-based mutational analysis.

Results: The degenerative pathway was characterized by the gradual replacement of individual myofibers by connective tissue. Early stages of degeneration typically involved only a segment of the cross-sectional area of a myofiber. Intermediate stages consisted of myofiber shrinkage due to "shedding" of peripheral cytoplasmic portions into the endomysial space, and fragmentation of the myofibers by interposed collagen fibrils. Empty basement membrane sheaths surrounded by abundant deposits of extracellular matrix marked the end stage of the degenerative process. The nuclear number-to-cytoplasmic area in myofibers of one patient increased with increasing cross-sectional area, suggesting that satellite cell fusion with myofibers may have compensated for myofiber shrinkage. The pattern of degeneration described herein differs from muscular dystrophies with plasma membrane defects (dystrophinopathy, dysferlinopathy) and explains the frequently found absence of highly elevated serum creatine kinase levels in autosomal dominant Emery-Dreifuss muscular dystrophy.

Figure 1

Case 1. Myofibers in early (A,B,F,G), intermediate (C,D,H) and late (E,I) stages of degeneration. A,F. Biopsy at 4 years. Right vastus medialis muscle. B–E,G–I. Autopsy specimens. B,G–I. Left biceps muscle. C–E. Right deltoid muscle. A. The cytoplasm of the myofiber is fragmented at the periphery (arrow). B. The myofiber’s surface is indented on the left side and replaced by extracellular matrix. C,D. Intermediate stages. The cross‐sectional area is reduced. The myofibers are surrounded by deposits of extracellular matrix. Note small cytoplasmic fragments separated from the parent myofiber by extracellular matrix in (D) (arrow). E. End stage. Deposits of collagen and fibroblasts occupy endomysial space among myofibers. F,G.“Shedding” of cytoplasmic fragments (arrowheads). H. Myofiber fragmented by streaks of extracellular matrix. I. End stage. Basement membrane sheath containing few cytoplasmic fragments is surrounded by deposits of extracellular matrix.

Figure 2

Case 2. Right biceps brachii muscle. Early (A–C), intermediate (D–E) and late stages (F–H) of myofiber degeneration. A,B. The cytoplasm of the myofibers is fragmented at the periphery over a small segment of the fibers’ circumference (arrows). C. Higher magnification of (B). Note empty basement membrane sheaths in (C) (arrows). D,E. The cross‐sectional area of these myofibers is fragmented by interposed collagen fibrils. F–H. Collagen fibrils have replaced most of the myofibers’ cross‐sectional area. Scale bars 30 µm (A) and 3 µm (C,E).

Figure 3

Case 3. Right quadriceps muscle. Intermediate stage of myofiber degeneration. A. There is abundant “shedding” of cytoplasmic fragments in this myofiber. B. This myofiber is fragmented. Scale bars 5 µm (A) and 3 µm (B).

Figure 4

Case 3. Spaced serial sections of the myofiber depicted in Figure 3A to demonstrate gradual replacement of the myofiber by connective tissue. Scale bars 30 µm.

Figure 5

Case 1. The cross‐sectional area of an individual myofiber is plotted against the total number of its nuclei to compare the nuclear number‐to‐cytoplasmic area between myofibers of different sizes. Each dot represents a muscle fiber (n = 200). Regression lines indicate the slope trend in each muscle. The nuclear number‐to‐cytoplasmic area increases with increasing myofiber cross‐sectional area.