FSHD: copy number variations on the theme of muscular dystrophy

In humans, copy number variations (CNVs) are a common source of phenotypic diversity and disease susceptibility. Facioscapulohumeral muscular dystrophy (FSHD) is an important genetic disease caused by CNVs. It is an autosomal-dominant myopathy caused by a reduction in the copy number of the D4Z4 macrosatellite repeat located at chromosome 4q35. Interestingly, the reduction of D4Z4 copy number is not sufficient by itself to cause FSHD. A number of epigenetic events appear to affect the severity of the disease, its rate of progression, and the distribution of muscle weakness. Indeed, recent findings suggest that virtually all levels of epigenetic regulation, from DNA methylation to higher order chromosomal architecture, are altered at the disease locus, causing the de-regulation of 4q35 gene expression and ultimately FSHD

Expression of a mutated ras gene in Dictyostelium discoideum alters the binding of cyclic AMP to its chemotactic receptor

Dictyostelium discoideum cells contain a ras gene that codes for a polypeptide that is highly homologous to the human ras proteins. Extra copies of the wild-type gene or a gene carrying a missense mutation in codon 12 (ras-Gly12 and ras-Thr12, respectively) have been introduced into Dictyostelium cells by transformation. We have investigated the properties of the chemotactic cell surface cyclic AMP receptor in crude membrane preparations of wild-type Dictyostelium cells and ras-Gly12 and ras-Thr12 transformants. In vitro, an ATP- and Ca2+-dependent reduction of the number of cyclic AMP receptors was observed in membranes from all three cell types. The number of available receptors was decreased maximally by about 50%. In the presence of ATP the half-maximal Ca2+ concentration required for this process was about 10-5 M in wild-type and ras-Gly12 membranes, and less than 10-7M in ras-Thr12 membranes. Addition of GTP (but not GDP) or the phorbol ester PMA (phorbol-12-myristate-13-acetate) reduced the Ca2+ requirement of the process in wild-type and ras-Gly12 membranes to the physiological level of less than 10-7 M. In membranes derived from ras-Thr12 cells addition of GTP or PMA had no effect. The results indicate that D. discoideum cells contain a cyclic AMP receptor-controlling pathway that can be activated in vitro and involves a GTP-binding protein and a Ca2+ plus ATP-dependent activity, possibly protein kinase C. It is concluded that the ras protein specifically interacts with this pathway; the pathway appears to be constitutively activated by the mutated ras gene product.

Binding of matrix attachment regions to lamin polymers involves single-stranded regions and the minor groove.

Chromatin in eukaryotic nuclei is thought to be partitioned into functional loop domains that are generated by the binding of defined DNA sequences, named MARs (matrix attachment regions), to the nuclear matrix. We have previously identified B-type lamins as MAR-binding matrix components (M. E. E. Ludérus, A. de Graaf, E. Mattia, J. L. den Blaauwen, M. A. Grande, L. de Jong, and R. van Driel, Cell 70:949-959, 1992). Here we show that A-type lamins and the structurally related proteins desmin and NuMA also specifically bind MARs in vitro. We studied the interaction between MARs and lamin polymers in molecular detail and found that the interaction is saturable, of high affinity, and evolutionarily conserved. Competition studies revealed the existence of two different types of interaction related to different structural features of MARs: one involving the minor groove of double-stranded MAR DNA and one involving single-stranded regions. We obtained similar results for the interaction of MARs with intact nuclear matrices from rat liver. A model in which the interaction of nuclear matrix proteins with single-stranded MAR regions serves to stabilize the transcriptionally active state of chromatin is discussed