Cytoskeletal catastrophe causes brain degeneration

Archetypal and new families with Alexander disease and novel mutations in GFAP

OBJECTIVE

To describe genetic analyses of the 2 most thoroughly studied, historically seminal multigenerational families with Alexander disease described prior to the identification of GFAP as the related gene, as well as 1 newly discovered family.

DESIGN

Clinical histories were obtained and DNA was analyzed from blood, cheek epithelial cells, or fixed paraffin-embedded surgical samples.

SUBJECTS

Affected and unaffected adult members of 3 families and affected children were included.

MAIN OUTCOME MEASURES

Mutations in GFAP and behavior of mutant protein in cellular transfection assays.

RESULTS

Family A contains 4 siblings in whom we found a novel p.Ser247Pro mutation that was paternally inherited. The phenotypes of these siblings include 1 unaffected adult, 1 individual with juvenile-onset disease, and 2 individuals with adult-onset disease. Family B spans 4 generations, including the first described patient with adult-onset disease originally reported in 1968. Analysis of members of the later generations revealed a novel p.Asp417Ala mutation. Family C contains 3 generations. We detected a novel p.Gln426Leu mutation that, to our knowledge, is the farthest C-terminal mutation known.

CONCLUSIONS

These families display clear evidence of variable phenotypes but do not support recessive inheritance. While germline mosaicism cannot be excluded for 1 family (A), we propose that for genetic counseling purposes the risk of germline mosaicism should be described as less than 1%.

Very-high-dose alpha-tocopherol supplementation increases blood pressure and causes possible adverse central nervous system effects in stroke-prone spontaneously hypertensive rats

Tocopherols and tocotrienols constitute the vitamin E family. Although alpha-tocotrienol is the most neuroprotective form of vitamin E proved to be effective against stroke, alpha-tocopherol is the most abundant in nature and is used most often for disease prevention/treatment. A recent metaanalysis of human studies suggested that alpha-tocopherol supplementation increases all-cause mortality. Therefore, we investigated the effects of alpha-tocopherol ( approximately 44 mg/kg body weight; equivalent to 2,600 mg/human/day) on the central nervous system (CNS) of stroke-prone spontaneously hypertensive rats (SHRSP). SHRSP treated with high dose alpha-tocopherol had significantly higher blood pressure than untreated controls fed a basal diet that contained approximately 4 mg tocopherols/kg body weight, but neither group experienced a change in degree of lipid peroxidation in serum or CNS tissue. Biochemical/immunohistochemical analyses demonstrated that expressions of phosphorylated neurofilament H protein, glial fibrillary acidic protein and cathepsin D in the CNS tissue were significantly enhanced in alpha-tocopherol-supplemented rats, whereas expressions of SOD2 and Bcl-xL were diminished in response to alpha-tocopherol supplementation. Similarly, the frequency of cathepsin D-positive cells, corresponding mostly to microglial cells, was significantly increased in alpha-tocopherol-supplemented rats. Alpha-tocopherol supplementation also increased the number of lysosomes and lipofuscin granules in perikarya of both hippocampal pyramidal and Purkinje cells. Furthermore, alpha-tocopherol supplementation increased the frequency of glial filaments and lipofuscin granules in astrocytes and lysosomes in microglial cells that were frequently occupied with phagocytosed inclusion structures. The present results are the first to suggest that a very high dose of alpha-tocopherol supplementation increases blood pressure in SHRSP rats and influences the CNS tissue in a manner that seems adverse.

The functional alteration of mutant GFAP depends on the location of the domain: morphological and functional studies using astrocytoma-derived cells

To clarify the functional effects of mutant glial fibrillary acidic protein (GFAP), we examined the expression patterns of mutant GFAPs (V87G, R88C, and R416W) in astrocytoma-derived cells and performed migration assay. The morphological change was found in mutant GFAP cells, although the number of changes was small. On migration assay, the migration rate in cells with the V87G or R88C mutation, which are located in the helical rod domain in GFAP, was significantly higher than those of wild-type and R416W. These findings suggest that the functional abnormalities of astrocytes might be induced prior to aggregation of GFAP in Alexander disease and that the functional alteration depends on the location of the domain.

Crystallins in the eye: Function and pathology

Crystallins are the predominant structural proteins in the lens that are evolutionarily related to stress proteins. They were first discovered outside the vertebrate eye lens by Bhat and colleagues in 1989 who found alphaB-crystallin expression in the retina, heart, skeletal muscles, skin, brain and other tissues. With the advent of microarray and proteome analysis, there is a clearer demonstration that crystallins are prominent proteins both in the normal retina and in retinal pathologies, emphasizing the importance of understanding crystallin functions outside of the lens. There are two main crystallin gene families: alpha-crystallins, and betagamma-crystallins. alpha-crystallins are molecular chaperones that prevent aberrant protein interactions. The chaperone properties of alpha-crystallin are thought to allow the lens to tolerate aging-induced deterioration of the lens proteins without showing signs of cataracts until older age. alpha-crystallins not only possess chaperone-like activity in vitro, but can also remodel and protect the cytoskeleton, inhibit apoptosis, and enhance the resistance of cells to stress. Recent advances in the field of structure-function relationships of alpha-crystallins have provided the first clues to their underlying roles in tissues outside the lens. Proteins of the betagamma-crystallin family have been suggested to affect lens development, and are also expressed in tissues outside the lens. The goal of this paper is to highlight recent work with lens epithelial cells from alphaA- and alphaB-crystallin knockout mice. The use of lens epithelial cells suggests that crystallins have important cellular functions in the lens epithelium and not just the lens fiber cells as previously thought. These studies may be directly relevant to understanding the general cellular functions of crystallins.

Dominantly-inherited adult-onset leukodystrophy with palatal tremor caused by a mutation in the glial fibrillary acidic protein gene

We report on a pedigree of dominantly-inherited, adult-onset Alexander disease caused by the glial fibrillary acidic protein (GFAP) gene mutation, R416W. This pedigree highlights the importance of genetic analysis of the GFAP gene in leukodystrophy with palatal tremor.

Functional complexity of intermediate filament cytoskeletons: from structure to assembly to gene ablation

The cell biology of intermediate filament (IF) proteins and their filaments is complicated by the fact that the members of the gene family, which in humans amount to at least 65, are differentially expressed in very complex patterns during embryonic development. Thus, different tissues and cells express entirely different sets and amounts of IF proteins, the only exception being the nuclear B-type lamins, which are found in every cell. Moreover, in the course of evolution the individual members of this family have, within one species, diverged so much from each other with regard to sequence and thus molecular properties that it is hard to envision a unifying kind of function for them. The known epidermolytic diseases, caused by single point mutations in keratins, have been used as an argument for a role of IFs in mechanical "stress resistance," something one would not have easily ascribed to the beaded chain filaments, a special type of IF in the eye lens, or to nuclear lamins. Therefore, the power of plastic dish cell biology may be limited in revealing functional clues for these structural elements, and it may therefore be of interest to go to the extreme ends of the life sciences, i.e., from the molecular properties of individual molecules including their structure at the atomic level to targeted inactivation of their genes in living animals, mouse, and worm to define their role more precisely in metazoan cell physiology.

Identification of GFAP gene mutation in hereditary adult-onset Alexander's disease

Alexander's disease, a leukodystrophy characterized by Rosenthal fibers (RFs) in the brain, is categorized into three subtypes: infantile, juvenile, and adult. Although most are sporadic, occasional familial Alexander's disease cases have been reported for each subtype. Hereditary adult-onset Alexander's disease shows progressive spastic paresis, bulbar or pseudobulbar palsy, palatal myoclonus symptomatologically, and prominent atrophy of the medulla oblongata and upper spinal cord on magnetic resonance imaging. Recent identification of GFAP gene mutations in the sporadic infantile- and juvenile-onset Alexander's disease prompted us to examine the GFAP gene in two Japanese hereditary adult-onset Alexander's disease brothers with autopsy in one case. Both had spastic paresis without palatal myoclonus, and magnetic resonance imaging showed marked atrophy of the medulla oblongata and cervicothoracic cord. The autopsy showed severely involved shrunken pyramids, but scarce Rosenthal fibers (RFs). Moderate numbers of Rosenthal fibers (RFs) were observed in the stratum subcallosum and hippocampal fimbria. In both cases, we found a novel missense mutation of a G-to-T transition at nucleotide 841 in the GFAP gene that results in the substitution of arginine for leucine at amino acid residue 276 (R276L). This is the first report of identification of the causative mutation of the GFAP gene for neuropathologically proven hereditary adult-onset Alexander's disease, suggesting a common molecular mechanism underlies the three Alexander's disease subtypes.

An update on the leukodsytrophies

This review centers on important recent advances in the understanding of the role of glial fibrillary acidic protein in Alexander disease and of proteolipid protein in hypomyelinating disorders such as Pelizaeus-Merzbacher and spastic paraplegia. We also describe seven novel leukodystrophies. These include childhood ataxia with central nervous system hypomyelination, a relatively common leukodystrophy syndrome with linkage to chromosome 3 in some patients, and megalencephalic leukoencephalopathy with subcortical cysts whose gene has recently been cloned. These, along with five other disorders, including leukodystrophy with polyol metabolism abnormality, demonstrate that an increasing number of protein and metabolic abnormalities can cause primary myelin disorders.

Infantile Alexander disease: spectrum of GFAP mutations and genotype-phenotype correlation

Heterozygous, de novo mutations in the glial fibrillary acidic protein (GFAP) gene have recently been reported in 12 patients affected by neuropathologically proved Alexander disease. We searched for GFAP mutations in a series of patients who had heterogeneous clinical symptoms but were candidates for Alexander disease on the basis of suggestive neuroimaging abnormalities. Missense, heterozygous, de novo GFAP mutations were found in exons 1 or 4 for 14 of the 15 patients analyzed, including patients without macrocephaly. Nine patients carried arginine mutations (four had R79H; four had R239C; and one had R239H) that have been described elsewhere, whereas the other five had one of four novel mutations, of which two affect arginine (2R88C and 1R88S) and two affect nonarginine residues (1L76F and 1N77Y). All mutations were located in the rod domain of GFAP, and there is a correlation between clinical severity and the affected amino acid. These results confirm that GFAP mutations are a reliable molecular marker for the diagnosis of infantile Alexander disease, and they also form a basis for the recommendation of GFAP analysis for prenatal diagnosis to detect potential cases of germinal mosaicism.

Genes for intermediate filament proteins and the draft sequence of the human genome

We screened the draft sequence of the human genome for genes that encode intermediate filament (IF) proteins in general, and keratins in particular. The draft covers nearly all previously established IF genes including the recent cDNA and gene additions, such as pancreatic keratin 23, synemin and the novel muscle protein syncoilin. In the draft, seven novel type II keratins were identified, presumably expressed in the hair follicle/epidermal appendages. In summary, 65 IF genes were detected, placing IF among the 100 largest gene families in humans. All functional keratin genes map to the two known keratin clusters on chromosomes 12 (type II plus keratin 18) and 17 (type I), whereas other IF genes are not clustered. Of the 208 keratin-related DNA sequences, only 49 reflect true keratin genes, whereas the majority describe inactive gene fragments and processed pseudogenes. Surprisingly, nearly 90% of these inactive genes relate specifically to the genes of keratins 8 and 18. Other keratin genes, as well as those that encode non-keratin IF proteins, lack either gene fragments/pseudogenes or have only a few derivatives. As parasitic derivatives of mature mRNAs, the processed pseudogenes of keratins 8 and 18 have invaded most chromosomes, often at several positions. We describe the limits of our analysis and discuss the striking unevenness of pseudogene derivation in the IF multigene family. Finally, we propose to extend the nomenclature of Moll and colleagues to any novel keratin.

Alexander disease: new insights from genetics

Prior to finding that GFAP mutations underlie many cases of Alexander disease, it was unclear whether the disease originated in astrocytes or if the formation of Rosenthal fibers was a response to an external insult. It was also unclear whether the etiology of the disease was environmental or genetic. For many cases of Alexander disease, these questions have now been answered. An immediate clinical benefit of this discovery is the possibility of diagnosing most cases of Alexander disease through analysis of patient DNA samples, rather than resorting to brain biopsy. In addition, fetal testing is now an option for parents who have had an Alexander disease child with an identified mutation and who wish to have additional children. For the future, these mutations should provide a unique window for illuminating the mechanism of the disease.