MLIP causes recessive myopathy with rhabdomyolysis, myalgia and baseline elevated serum creatine kinase

Abstract

Striated muscle needs to maintain cellular homeostasis in adaptation to increases in physiological and metabolic demands. Failure to do so can result in rhabdomyolysis. The identification of novel genetic conditions associated with rhabdomyolysis helps to shed light on hitherto unrecognized homeostatic mechanisms. Here we report seven individuals in six families from different ethnic backgrounds with biallelic variants in MLIP, which encodes the muscular lamin A/C-interacting protein, MLIP. Patients presented with a consistent phenotype characterized by mild muscle weakness, exercise-induced muscle pain, variable susceptibility to episodes of rhabdomyolysis, and persistent basal elevated serum creatine kinase levels. The biallelic truncating variants were predicted to result in disruption of the nuclear localizing signal of MLIP. Additionally, reduced overall RNA expression levels of the predominant MLIP isoform were observed in patients' skeletal muscle. Collectively, our data increase the understanding of the genetic landscape of rhabdomyolysis to now include MLIP as a novel disease gene in humans and solidifies MLIP's role in normal and diseased skeletal muscle homeostasis.

Keywords: MLIP; cardiomyopathy; hyperCKemia; myopathy; rhabdomyolysis.

Figure 1

Histological and imaging findings in patients with MLIP pathogenic variants. Muscle biopsies varied from showing normal to minimal findings in Patients P1 and P3 (A and D, respectively; haematoxylin and eosin, ×200) to revealing focal areas of necrosis and regeneration of muscle fibres in Patients P2, P5 and P4 (B, C and E, respectively; haematoxylin and eosin, ×200). Electron microscopy imaging performed on Patient P6 (F) did not reveal ultrastructural abnormalities. MRI of thighs in Patients P1 and P3 show mild signal hyperintensity in the vastus lateralis (G and H, respectively; G: STIR sequence; H: TIRM sequence).

Figure 2

Localization and effect of MLIP variants. (A) MLIP genomic map with identified position of each pathogenic variant. (B) Expected effects of the MLIP pathogenic variants on the size of MLIP protein for the various patients. Star: Non-canonical 3′ splice site in exon 4. (C and D) Alignment of MLIP between species in the AT-Hook domain (C) and GSK3 phosphorylation motif (D). The consensus sequence for C and D is denoted by: asterisk as identical; a single dot as conserved substitutions; and double dot as semi-conservative substitutions.

Figure 3

RNA expression and protein localization experiments for MLIP. (A) Droplet digital PCR performed on muscle samples for Patients P1, P3 and P4, compared to control muscle (Ctrl). Amplicon concentration in copies/µl is shown normalized to B2M. Concentrations are shown for the boundaries between exons 1 and 2 (E1-E2), exons 3 and 4 (E3-E4), and exons 4 and 5 (E4-E5) of the transcript MLIP-213. (B) Immunofluorescence studies on muscle biopsy samples of Patients P1, P3 and P4. Confocal microscope pictures show a similar pattern of staining for MLIP and emerin in the control tissue compared to patient samples. Emerin (green) stains the nuclear envelope, while MLIP (red) is decorated mostly in the nucleus, with some cytoplasmic staining. DAPI (blue) is used for nuclear staining control. Top: ×200 magnification; bottom: ×630 magnification.