How does a g993t mutation in the emerin gene cause Emery-Dreifuss muscular dystrophy?

X-linked Emery-Dreifuss muscular dystrophy is usually caused by absence of the nuclear membrane protein, emerin, due to nonsense mutations or deletions, but a few missense mutations also exist. A pathogenic g993t mutation causes a Q133H change in the nuclear targeting region of emerin, but it may also reduce emerin levels by affecting mRNA splicing. We have introduced the g993t mutation by in vitro mutagenesis and studied the effect of Q133H on nuclear targeting by transfection of COS-7 cells. No qualitative or quantitative differences in nuclear targeting were observed between normal and mutant emerin. Quantitative BIAcore analysis showed no significant change in lamin A binding to emerin when the mutation was present. We conclude that Q133 is not essential for nuclear targeting of emerin or its interaction with lamin A. Reduced emerin levels due to altered splicing or defective interaction with an unidentified binding partner remain possible pathogenic mechanisms.