Mutations in GFAP, encoding glial fibrillary acidic protein, are associated with Alexander disease

Alexander disease is a rare disorder of the central nervous system of unknown etiology. Infants with Alexander disease develop a leukoencephalopathy with macrocephaly, seizures and psychomotor retardation, leading to death usually within the first decade; patients with juvenile or adult forms typically experience ataxia, bulbar signs and spasticity, and a more slowly progressive course. The pathological hallmark of all forms of Alexander disease is the presence of Rosenthal fibers, cytoplasmic inclusions in astrocytes that contain the intermediate filament protein GFAP in association with small heat-shock proteins. We previously found that overexpression of human GFAP in astrocytes of transgenic mice is fatal and accompanied by the presence of inclusion bodies indistinguishable from human Rosenthal fibers. These results suggested that a primary alteration in GFAP may be responsible for Alexander disease. Sequence analysis of DNA samples from patients representing different Alexander disease phenotypes revealed that most cases are associated with non-conservative mutations in the coding region of GFAP. Alexander disease therefore represents the first example of a primary genetic disorder of astrocytes, one of the major cell types in the vertebrate CNS.     

Failure to confirm NOTCH4 association with schizophrenia in a large population-based sample from Scotland

The NOTCH4 gene was recently reported to be associated with schizophrenia based on TDT analysis of 80 British trios. The strongest evidence for association derived from two microsatellites. We genotyped both loci in a large sample of unrelated Scottish schizophrenics and controls, but failed to replicate the reported association, finding instead that each putative schizophrenia-associated allele had a somewhat lower frequency in schizophrenics than in controls.     

Regulatory defects in liver and intestine implicate abnormal hepcidin and Cybrd1 expression in mouse hemochromatosis

Individuals with hereditary hemochromatosis suffer from systemic iron overload due to duodenal hyperabsorption. Most cases arise from a founder mutation in HFE (845G-->A; ref. 2) that results in the amino-acid substitution C282Y and prevents the association of HFE with beta2-microglobulin. Mice homozygous with respect to a null allele of Hfe (Hfe-/-) or homozygous with respect to the orthologous 882G-->A mutation (Hfe(845A/845A)) develop iron overload that recapitulates hereditary hemochromatosis in humans, confirming that hereditary hemochromatosis arises from loss of HFE function. Much work has focused on an exclusive role for the intestine in hereditary hemochromatosis. HFE deficiency in intestinal crypt cells is thought to cause intestinal iron deficiency and greater expression of iron transporters such as SLC11A2 (also called DMT1, DCT1 and NRAMP2) and SLC11A3 (also called IREG1, ferroportin and MTP1; ref. 3). Published data on the expression of these transporters in the duodenum of HFE-deficient mice and humans are contradictory. In this report, we used a custom microarray to assay changes in duodenal and hepatic gene expression in Hfe-deficient mice. We found unexpected alterations in the expression of Slc39a1 (mouse ortholog of SLC11A3) and Cybrd1, which encode key iron transport proteins, and Hamp (hepcidin antimicrobial peptide), a hepatic regulator of iron transport. We propose that inappropriate regulatory cues from the liver underlie greater duodenal iron absorption, possibly involving the ferric reductase Cybrd1.     

Localized mutations in the gene encoding the cytoskeletal protein filamin A cause diverse malformations in humans

Remodeling of the cytoskeleton is central to the modulation of cell shape and migration. Filamin A, encoded by the gene FLNA, is a widely expressed protein that regulates re-organization of the actin cytoskeleton by interacting with integrins, transmembrane receptor complexes and second messengers. We identified localized mutations in FLNA that conserve the reading frame and lead to a broad range of congenital malformations, affecting craniofacial structures, skeleton, brain, viscera and urogenital tract, in four X-linked human disorders: otopalatodigital syndrome types 1 (OPD1; OMIM 311300) and 2 (OPD2; OMIM 304120), frontometaphyseal dysplasia (FMD; OMIM 305620) and Melnick-Needles syndrome (MNS; OMIM 309350). Several mutations are recurrent, and all are clustered into four regions of the gene: the actin-binding domain and rod domain repeats 3, 10 and 14/15. Our findings contrast with previous observations that loss of function of FLNA is embryonic lethal in males but manifests in females as a localized neuronal migration disorder, called periventricular nodular heterotopia (PVNH; refs. 3-6). The patterns of mutation, X-chromosome inactivation and phenotypic manifestations in the newly described mutations indicate that they have gain-of-function effects, implicating filamin A in signaling pathways that mediate organogenesis in multiple systems during embryonic development.     

Transport of lipids from golgi to plasma membrane is defective in tangier disease patients and Abc1-deficient mice

Mutations in the gene encoding ATP-binding cassette transporter 1 ( ABC1) have been reported in Tangier disease (TD), an autosomal recessive disorder that is characterized by almost complete absence of plasma high-density lipoprotein (HDL), deposition of cholesteryl esters in the reticulo-endothelial system (RES) and aberrant cellular lipid trafficking. We demonstrate here that mice with a targeted inactivation of Abc1 display morphologic abnormalities and perturbations in their lipoprotein metabolism concordant with TD. ABC1 is expressed on the plasma membrane and the Golgi complex, mediates apo-AI associated export of cholesterol and phospholipids from the cell, and is regulated by cholesterol flux. Structural and functional abnormalities in caveolar processing and the trans-Golgi secretory pathway of cells lacking functional ABC1 indicate that lipid export processes involving vesicular budding between the Golgi and the plasma membrane are severely disturbed.     

Limb-girdle muscular dystrophy type 2G is caused by mutations in the gene encoding the sarcomeric protein telethonin

Autosomal recessive limb-girdle muscular dystrophies (AR LGMDs) are a genetically heterogeneous group of disorders that affect mainly the proximal musculature. There are eight genetically distinct forms of AR LGMD, LGMD 2A-H (refs 2-10), and the genetic lesions underlying these forms, except for LGMD 2G and 2H, have been identified. LGMD 2A and LGMD 2B are caused by mutations in the genes encoding calpain 3 (ref. 11) and dysferlin, respectively, and are usually associated with a mild phenotype. Mutations in the genes encoding gamma-(ref. 14), alpha-(ref. 5), beta-(refs 6,7) and delta (ref. 15)-sarcoglycans are responsible for LGMD 2C to 2F, respectively. Sarcoglycans, together with sarcospan, dystroglycans, syntrophins and dystrobrevin, constitute the dystrophin-glycoprotein complex (DGC). Patients with LGMD 2C-F predominantly have a severe clinical course. The LGMD 2G locus maps to a 3-cM interval in 17q11-12 in two Brazilian families with a relatively mild form of AR LGMD (ref. 9). To positionally clone the LGMD 2G gene, we constructed a physical map of the 17q11-12 region and refined its localization to an interval of 1.2 Mb. The gene encoding telethonin, a sarcomeric protein, lies within this candidate region. We have found that mutations in the telethonin gene cause LGMD 2G, identifying a new molecular mechanism for AR LGMD.     

Conditional inactivation of Fgf4 reveals complexity of signalling during limb bud development

Development of the vertebrate limb bud depends on reciprocal interactions between the zone of polarizing activity (ZPA) and the apical ectodermal ridge (AER). Sonic hedgehog (SHH) and fibroblast growth factors (FGFs) are key signalling molecules produced in the ZPA and AER, respectively. Experiments in chicks suggested that SHH expression in the ZPA is maintained by FGF4 expression in the AER, and vice versa, providing a molecular mechanism for coordinating the activities of these two signalling centres. This SHH/FGF4 feedback loop model is supported by genetic evidence showing that Fgf4 expression is not maintained in Shh-/- mouse limbs. We report here that Shh expression is maintained and limb formation is normal when Fgf4 is inactivated in mouse limbs, thus contradicting the model. We also found that maintenance of Fgf9 and Fgf17 expression is dependent on Shh, whereas Fgf8 expression is not. We discuss a model in which no individual Fgf expressed in the AER (AER-Fgf) is solely necessary to maintain Shh expression, but, instead, the combined activities of two or more AER-Fgfs function in a positive feedback loop with Shh to control limb development.