Date:   4/19/2005 1:40:42 PM ( 20 y ago)
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URL:   https://www.curezone.org/forums/fm.asp?i=711209

SUBJ (06/00): HOMEOBOX GENE

I am in a genetics course in school, and we are doing research on muscular dystrophy. I know that researchers studying the homeobox genes find that a clone of such a gene hybridizes near the marker found for facioscapulohumeral muscular dystrophy (FSH or FSHD or FSHMD). What is some evidence that that might implicate the homeobox gene as a candidate gene for FSHMD?


REPLY from MDA: Sharon Hesterlee, Ph.D., Research Program Coordinator, Research Department, Muscular Dystrophy Association, Tucson, AZ

The FSHD defect, localized to a region at the far end of chromosome 4 containing a marker called D4Z4, is unusual because it does not occur in the coding region of a gene, but rather in a series of apparently noncoding DNA sequences called "repeats." People with FSHD have fewer of these repeats than other people, but researchers are still trying to understand how this loss leads to a muscular dystrophy.

These D4Z4 repeats have been shown to contain homeobox sequences, but no "expressed" sequences from these repeats have been identified—that is, they do not seem to have the appropriate equipment (promoters) to make a protein product.

Recently, however, a Belgium/British group (Ref. 1) found evidence that the remaining repeats in the D4Z4 region of a person with FSHD might each contain an expressible gene after all (they found an obscure promoter), and that this gene appears to be capable of making a protein containing two homeodomains (they named this putative gene DUX4). As you probably know from your genetics class, a homeodomain protein is a special DNA binding protein with a helix-turn-helix motif that binds to the DNA of other genes and regulates their transcription. (The term "homeobox" refers to that portion of the DNA sequence that codes for the "homeodomain" of the resulting protein.)

Although still very much in the speculative phase, this group suspects that the loss of some D4Z4 repeat sequences in those with FSHD allows the remaining DUX4 genes to be active when they should not be. Thus you might have a homeodomain protein that is capable of regulating a lot of other genes (possibly muscle-specific genes) being active at an inappropriate time. If muscle gene expression is being scrambled, this in turn could lead to muscle cell death. Keep in mind that this is just one theory of several that have been put forth to explain FSHD and there are still plenty of missing pieces to this puzzle.

On the other hand, you might ask (and this is probably the answer you were really looking for) what is the evidence that muscle gene expression is being misregulated in FSHD? It turns out that another group (from the U.S. and Italy) recently discovered that muscle-specific genes are "profoundly misregulated" in people with FSHD (ref. 2). The group compared mRNA populations from normal muscle and that of people with FSHD and found severe over- or underexpression of genes in the FSHD muscles. Several of the genes with altered regulation were themselves muscle-specific transcription factors (responsible for activating other muscle genes). Importantly, this sort of global muscle gene misregulation was only found in samples from those with FSHD, and not in samples from those with other kinds of muscular dystrophy. All of this evidence points toward a problem at the very top of the muscle gene cascade, and homeobox-containing genes tend to be in such positions. It's all very speculative, but intriguing nonetheless.

Ref. 1: Gabriels J, Beckers MC, Ding H, De Vriese A, Plaisance S, van der Maarel SM, Padberg GW, Frants RR, Hewitt JE, Collen D, Belayew A. Nucleotide sequence of the partially deleted D4Z4 locus in a patient with FSHD identifies a putative gene within each 3.3 kb element. Gene. 1999 Aug 5;236(1):25-32.
Ref. 2: Tupler R, Perini G, Pellegrino MA, Green MR. Profound misregulation of muscle-specific gene expression in facioscapulohumeral muscular dystrophy. Proc Natl Acad Sci U S A. 1999 Oct 26;96(22):12650-4.





 

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