De novo RYR1 heterozygous mutation (I4898T) causing lethal core-rod myopathy in twins.

"Core-rod myopathy" is a rare congenital myopathy characterized by the presence of "cores" and "rods" in distinct locations in the same or different muscle fibres. This association is linked currently to mutations in RYR1, NEB and ACTA1 genes. We report identical twins who presented with polyhydramnios and loss of fetal motility during pregnancy; hypotonia, arthrogryposis and swallowing impairment at birth; need of immediate respiratory support and death at 27 and 50 days of life.

"Necklace" fibers, a new histological marker of late-onset MTM1-related centronuclear myopathy.

Mutations in the gene encoding the phosphoinositide phosphatase myotubularin 1 protein (MTM1) are usually associated with severe neonatal X-linked myotubular myopathy (XLMTM). However, mutations in MTM1 have also been recognized as the underlying cause of "atypical" forms of XLMTM in newborn boys, female infants, female manifesting carriers and adult men. We reviewed systematically the biopsies of a cohort of patients with an unclassified form of centronuclear myopathy (CNM) and identified four patients presenting a peculiar histological alteration in some muscle fibers that resembled a necklace ("necklace fibers").