Objective:

To identify biomarkers distinguishing acquired sporadic late onset nemaline myopathy (SLONM) from inherited nemaline myopathy (iNM) and investigate the pathophysiology leading to nemaline rod formation in these different disorders. 

Background:

SLONM and iNM both feature accumulation of nemaline rods in muscle fibers. Unlike iNM, SLONM is amenable to therapy.  The distinction between these disorders is therefore crucial, especially when the diagnosis remains ambiguous after initial clinical assessment and investigations. 

Design/Methods:

Twenty-two muscle samples from patients affected by SLONM or genetically characterized iNM underwent quantitative histological analysis, laser capture microdissection for proteomic analysis of nemaline rod areas and rod-free areas, and transcriptomic analysis.

Results:

In all iNM samples, nemaline rods were found in subsarcolemmal or central aggregates, whereas they were diffusely distributed within muscle fibers in 70% of SLONM samples. In SLONM, muscle fibers harboring nemaline rods were smaller than those without rods, while the opposite was seen in iNM. Necrotic fibers, increased endomysial connective tissue, and atrophic fibers filled with nemaline rods were significantly more common in SLONM. Proteomic analysis detected 142 differentially expressed proteins between nemaline rod areas and rod-free areas across all samples. Comparing SLONM and iNM, fifty-one proteins were differentially expressed in nemaline rod areas and 10 proteins were differentially expressed in rod-free areas. These differentially expressed proteins implicated immune, structural, metabolic, and cellular processes in disease pathophysiology. Notably, immunoglobulin overexpression with accumulation in nemaline rod areas was detected in SLONM, supporting use of plasma cell-targeting chemotherapy for SLONM treatment. Transcriptomic analysis corroborated the proteomic findings and further revealed substantial gene expression differences between SLONM and iNM. 

Conclusions:

We identified unique pathological and molecular signatures associated with SLONM and iNM, suggesting distinct underlying pathophysiological mechanisms. These findings are a critical step towards optimizing diagnostic tools that can better distinguish these disorders and accelerating SLONM treatment.