Krebs von den Lungen 6 decreased in the serum and muscle of GNE myopathy patients

Abstract

UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (GNE) is necessary for sialic acid biosynthesis. GNE myopathy is caused by a defect in GNE, and hyposialylation is a key factor in the pathomechanism of GNE myopathy. Although candidates for evaluating hyposialylation have been reported, it is difficult to measure them in routine clinical practice. Sialylation is necessary for synthesis of various glycoproteins, including Krebs von den Lungen-6 (KL-6)/mucin 1 (MUC1). Here we report that KL-6/MUC1 is decreased in GNE myopathy. We observed that KL-6 levels were decreased in the serum of patients with GNE myopathy, and that KL-6 and MUC1-C were also decreased in muscle biopsy specimens from these patients. An immunofluorescent study revealed that KL-6 and MUC1-C were not present in the sarcolemma but were, instead, localized in rimmed vacuoles in specimens from patients with GNE myopathy. KL-6 is already used to detect lung diseases in clinical practice, and this glycoprotein may be a novel candidate for evaluating hyposialylation in GNE myopathy.

Keywords: GNE myopathy; KL-6; MUC1; hyposialylation; rimmed vacuoles.

Fig 1

Serum KL‐6 levels in patients of the GNEM, GNE‐negative DM, sIBM, IIM, and NC groups. The levels are the lowest in the GNEM group among all the examined groups. No significant difference in serum KL‐6 levels was found among the groups other than the GNEM group. *P < 0.03.

Fig 2

Histological findings in sections of the muscle biopsy specimens. Serial sections show that RVs of GNEM (A) and sIBM (C) cases are positively stained for KL‐6 (B, D). In contrast, IIM (E) and NC (D) cases are negatively stained for KL‐6 in the cytoplasm. Scale bars: 50 μm (A‐F).

Fig 3

Microphotographs of the muscle biopsy specimen sections stained by the double‐labeling immunofluorescence method. RVs (red), which are positive for phosphorylated TDP‐43 (pTDP‐43) in GNEM and sIBM, are positively stained for MUC1‐C and KL‐6 (green, arrows). In IIM and NC cases, cytoplasmic accumulations are undetectable. Scale bars: 50 μm.

Fig 4

Results of Western blotting (A, C) and densitometry (B, D) for MUC‐C (A, B) and KL‐6 (C, D) in muscle biopsy specimens. (A) MUC1‐C‐immunoreactive bands are observed in the GNEM, sIBM, IIM, and NC groups at the same mobility as the positive control ileum. (B) The normalized MUC1‐C‐immunoreactive signal values are significantly decreased in the GNEM group at approximately 50% of those in the sIBM and NC groups. (C) KL‐6‐immunoreactive bands are observed at a mobility above 250 kDa in the muscles and the positive control ileum. (D) The normalized KL‐6‐immunoreactive signal values are significantly decreased in the GNEM group at approximately 50% of those in other groups. **P < 0.01.