Neurology -- Correspondence for Nishino et al., 59 (11) 1689-1693

Correspondence published:

Reply to Letter to the Editor: Ichizo Nishino, Satoru Noguchi (7 February 2003)

Distal myopathy with rimmed vacuoles is allelic to hereditary inclusion body myopathy: Stephan Hinderlich, Ilan Salama, Iris Eisenberg and Stella Mitrani-Rosenbaum (7 February 2003)

Reply to Letter to the Editor 7 February 2003

Ichizo Nishino
National Institute of Neuroscience National Center of Neurology and Psychiatry Tokyo Japan,
Satoru Noguchi
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Re: Reply to Letter to the Editor

nishino{at}ncnp.go.jp Ichizo Nishino, et al.

We thank Hinderlich et al. for their interest in our paper [1] and for their observations. We agree that our method is technically limited because it provides only a rough measurement of the UDP-GlcNAc 2-epimerase activity. This limitation is mainly due to the low amount, and therefore, the low enzyme activity, of the GNE gene product in lymphocytes. Our measurements of UDP-GlcNAc 2-epimerase function demonstrated a wide range of normal enzyme activities that overlapped with those of the DMRV patients. For more accurate enzymatic activity measurements, other cell types or different methods must be used. Accordingly, the use of lymphoblastoid cell lines, as described by Dr. Hinderlich et al., may provide more reliable results. Our preliminary data from recombinant proteins with various mutants show that UDP-GlcNAc 2-epimerase activity is most dramatically decreased when the mutations are in the UDP-GlcNAc 2- epimerase domain (manuscript in preparation). Therefore, we assume that the preserved enzymatic activity in lymphoblastoid cells with the M712T mutation, which is in the ManNAc kinase domain, might not significantly affect UDP-GlcNAc 2-epimerase activity. Additional studies are necessary to clarify the pathomechanism of DMRV/HIBM including the establishment of a sensitive assay for ManNAc kinase activity.
Reference
1. Nishino I, Noguchi S, Murayama K, et al. Distal myopathy with rimmed vacuoles is allelic to hereditary inclusion body myopathy. Neurology 2002;59:1689-1693.

Distal myopathy with rimmed vacuoles is allelic to hereditary inclusion body myopathy 7 February 2003

Stephan Hinderlich
Freie Universitat Berlin Germany,
Ilan Salama, Iris Eisenberg and Stella Mitrani-Rosenbaum
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Re: Distal myopathy with rimmed vacuoles is allelic to hereditary inclusion body myopathy

hinderli{at}zedat.fu-berlin.de Stephan Hinderlich, et al.

It is with interest that read the article by Nishino et al. [1], showing the allelic unity of DMRV and HIBM. Our comment merely aims at the enzyme activites of GNE, the protein affected by point mutations in HIBM/DMRV, detected in patients� lymphocytes. GNE consists of two functional domains, an UDP-GlcNAc 2-epimerase domain and a ManNAc kinase domain. The authors� decision to detect UDP-GlcNAc 2-epimerase activity and not ManNAc kinase activity in lymphocytes, although most mutations affect the kinase domain of the enzyme, was in accordance to our experience, because the ManNAc kinase activity is hardly detectable in lymphocytes, due to very high background activities. But the authors described an almost total loss of UDP-GlcNAc 2-epimerase activity in patients� lymphocytes. Formerly published results about UDP-GlcNAc 2- epimerase show, that a lack of the enzyme, at least in mice, results in early embryonal lethality [2]. Furthermore, a loss of UDP-GlcNAc 2- epimerase in lymphoblastoid cell lines results in drastic effects in immune functions of the cells, due to a loss of the generation of sialic acids [3]. Therefore, if indeed there is a drastic decrease of the enzyme activity as presented in the Nishino paper, the phenotype of HIBM/DMRV patients should be much more severe than the observed myopathy.
We assume, that the results of UDP-GlcNAc 2-epimerase activities described in the paper are due to technical problems with the assay. We have determined the enzyme activites in lymphoblastoid cell lines derived from HIBM patients carrying the M712T mutation [4], and in control cells. Our, so far unpublished, results show UDP-GlcNAc 2-epimerase activities of 255 � 20 pmol/mg protein/min (n = 3) for control cells and 165 � 25 pmol/mg protein/min (n = 3) for patient-derived cells. These results fit well with published results of UDP-GlcNAc 2-epimerase activity in lymphoblastoid cell lines [3] and are about 10.000-fold higher than the activities detected by Nishino et al. [1], underlining their technical problems during the assay which should be clarified. Our data show that HIBM patients absolutely have an active UDP-GlcNAc 2-epimerase, in accordance with the essential role of the enzyme for the biosynthesis of sialic acids [2,3]. Nevertheless, cells derived from HIBM patients seem to have a slightly reduced UDP-GlcNAc 2-epimerase activity, although the mutation occurs in the kinase domain of the enzyme. Further investigations have to show if this may have an effect on the biosynthesis of sialic acids in cells and patients, and if this is a clue for the pathological mechanism of the disease.
References
[1] Nishino I, Noguchi S, Murayama K, et al. Distal myopathy with rimmed vacuoles is allelic to hereditary inclusion body myopathy. Neurology 2002;59:1689-1693.
[2] Schwarzkopf M, Knobeloch KP, Rohde E, et al. Sialylation is essential for early development in mice. Proc Natl Acad Sci U S A 2002;99:5267-5270.
[3] Keppler OT, Hinderlich S, Langner J, Schwartz-Albiez R, Reutter W, Pawlita M. UDP-GlcNAc 2-epimerase: a regulator of cell surface sialylation. Science 1999;284:1372-1376.
[4] Eisenberg I, Avidan N, Potikha T, et al. The UDP-N- acetylglucosamine 2-epimerase/N-acetylmannosamine kinase gene is mutated in recessive hereditary inclusion body myopathy. Nat Genet 2001;29:83-87.