Periventricular heterotopia (PH) occurs when collections of neurons lay along the lateral ventricles or just beneath. Human Filamin A gene (FLNA) mutations are associated with classical X-linked bilateral periventricular nodular heterotopia (PNH), featuring contiguous heterotopic nodules, mega cisterna magna, cardiovascular malformations and epilepsy. FLNA encodes an F-actin-binding cytoplasmic phosphoprotein and is involved in early brain neurogenesis and neuronal migration. A rare, recessive form of bilateral PNH with microcephaly and severe delay is associated with mutations of the ADP-ribosylation factor guanine nucleotide-exchange factor-2 (ARFGEF2) gene, required for vesicle and membrane trafficking from the trans-Golgi. However, PH is a heterogeneous disorder. We studied clinical and brain MRI of 182 patients with PH and, based on its anatomic distribution and associated birth defects, identified 15 subtypes. Classical bilateral PNH represented the largest group (98 patients: 54%). The 14 additional phenotypes (84 patients: 46%) included PNH with Ehlers-Danlos syndrome (EDS), temporo-occipital PNH with hippocampal malformation and cerebellar hypoplasia, PNH with fronto-perisylvian or temporo-occipital polymicrogyria, posterior PNH with hydrocephalus, PNH with microcephaly, PNH with frontonasal dysplasia, PNH with limb abnormalities, PNH with fragile-X syndrome, PNH with ambiguous genitalia, micronodular PH, unilateral PNH, laminar ribbon-like and linear PH. We performed mutation analysis of FLNA in 120 patients, of whom 72 (60%) had classical bilateral PNH and 48 (40%) other PH phenotypes, and identified 25 mutations in 40 individuals. Sixteen mutations had not been reported previously. Mutations were found in 35 patients with classical bilateral PNH, in three with PNH with EDS and in two with unilateral PNH. Twenty one mutations were nonsense and frame-shift and four missense. The high prevalence of mutations causing protein truncations confirms that loss of function is the major cause of the disorder. FLNA mutations were found in 100% of familial cases with X-linked PNH (10 families: 8 with classical bilateral PNH, 1 with EDS and 1 with unilateral PH) and in 26% of sporadic patients with classical bilateral PNH. Overall, mutations occurred in 49% of individuals with classical bilateral PNH irrespective of their being familial or sporadic. However, the chances of finding a mutation were exceedingly gender biased with 93% of mutations occurring in females and 7% in males. The probability of finding FLNA mutations in other phenotypes was 4% but was limited to the minor variants of PNH with EDS and unilateral PNH. Statistical analysis considering all 42 mutations described so far identifies a hotspot region for PNH in the actin-binding domain (P < 0.05).
Objective Neurodegeneration with brain iron accumulation (NBIA) represents a distinctive phenotype of neurodegenerative disease for which several causative genes have been identified. The spectrum of neurologic disease associated with mutations in NBIA genes is broad, with phenotypes that range from infantile neurodegeneration and death in childhood to adult-onset parkinsonism-dystonia. Here we report the discovery of a novel gene that leads to a distinct form of NBIA. Methods Using autozygosity mapping and candidate gene sequencing, we identified mutations in the fatty acid hydroxylase gene FA2H, newly implicating abnormalities of ceramide metabolism in the pathogenesis of NBIA. Results Neuroimaging demonstrated T2 hypointensity in the globus pallidus, confluent T2 white matter hyperintensities, and profound pontocerebellar atrophy in affected members of two families. Phenotypically, affected family members exhibited spastic quadriparesis, ataxia, and dystonia with onset in childhood and episodic neurological decline. Analogous to what has been reported previously for PLA2G6, the phenotypic spectrum of FA2H mutations is diverse based on our findings and those of prior investigators, because FA2H mutations have been identified in both a form of hereditary spastic paraplegia (SPG35) and a progressive familial leukodystrophy. Interpretation These findings link white matter degeneration and NBIA for the first time and implicate new signaling pathways in the genesis of NBIA.
Hypomorphic mutations of the MRE11 gene are the hallmark of the radiosensitive ataxia-telangiectasia-like disorder (ATLD). Here, we describe a new family with two affected siblings, ATLD5 and ATLD6, now aged 37 and 36, respectively. They presented with late onset cerebellar degeneration slowly progressing until puberty and absence of telangiectasias, and were cancer-free. Both patients were wild-type for ATM and NBS1, but compound heterozygotes for MRE11 gene mutations [1422C-->A, T481K; 1714C-->T, R571X]. The 1422C-->A allele was inherited from the mother, whereas the 1714C-->T, allele paternally inherited, was apparently null as a result of nonsense-mediated mRNA decay (NMD). Interestingly, the 1714C-->T mutation is the same as previously identified in an unrelated English ATLD family (probands ATLD3 and ATLD4), suggesting an important role for NMD in saving potentially lethal mutations. Lymphoblastoid cell lines (LCLs) derived from ATLD5 and ATLD6 were normal for ATM, but defective for Mre11, Rad50 and Nbs1 (the MRN complex) protein expression. Their response to gamma-radiation was abnormal, as evidenced by the enhanced radiosensitivity, attenuated autophosphorylation of ATM-S1981 and phosphorylation of the ATM targets p53-S15 and Smc1-S966, failure to form Mre11 nuclear foci and defective G1 checkpoint arrest. The fibroblasts, but not LCLs, from ATLD5 and ATLD6 showed an impaired ATM-dependent Chk2 phosphorylation. These findings further underscore the interconnection between ATM activity and MRN function, which rationalizes the clinical similarity between ataxia-telangiectasia (A-T) and ATLD.
Sialidosis is an autosomal recessive disease caused by the genetic deficiency of lysosomal sialidase, which catalyzes the hydrolysis of sialoglycoconjugates. The disease is associated with progressive impaired vision, macular cherry-red spots and myoclonus (sialidosis type I) or with skeletal dysplasia, Hurler-like phenotype, dysostosis multiplex, mental retardation and hepatosplenomegaly (sialidosis type II). We have analyzed the genomic DNA from nine sialidosis patients of multiple ethnic origin in order to find mutations responsible for the enzyme deficiency. The activity of the identified variants was studied by transgenic expression. One patient had a frameshift mutation (G623delG deletion), which introduced a stop codon, truncating 113 amino acids. All others had missense mutations: G679G-->A (Gly227Arg), C893C-->T (Ala298Val), G203G-->T (Gly68Val), A544A-->G (Ser182Gly) C808C-->T (Leu270Phe) and G982G-->A (Gly328Ser). We have modeled the three-dimensional structure of sialidase based on the atomic coordinates of the homologous bacterial sialidases, located the positions of mutations and estimated their potential effect. This analysis showed that five mutations are clustered in one region on the surface of the sialidase molecule. These mutations dramatically reduce the enzyme activity and cause a rapid intralysosomal degradation of the expressed protein. We hypothesize that this region may be involved in the interface of sialidase binding with lysosomal cathepsin A and/or beta-galactosidase in their high-molecular-weight complex required for the expression of sialidase activity in the lysosome.
Supernatant of homogenized human placenta hardly contains lysosomal neuraminidase activity. It is, however, possible to generate remarkably high activity by concentration of a partially purified glycoprotein fraction. This activity is labile to dilution, but can be stabilized by incubation at 37°C and acid pH. Using β‐galactosidase specific affinity chromatography and immunotitration, we show that the activated and stabilized human lysosomal neuraminidase exists in a complex with β‐galactosidase. Sucrose density gradient centrifugation experiments demonstrate that the neuraminidase activity is exclusively present in a high density multimeric form of β‐galactosidase. The formation of multimeric forms of β‐galactosidase is known to require a 32000‐Mr‘protective’ protein. Monospecific antibodies against this ‘protective’ protein were purified from a conventional antiserum containing a mixture of antibodies against the 64000‐Mrβ‐galactosidase protein and against the 32000‐Mr‘protective’ protein, using a nitrocellulose blot immunoaffinity purification procedure. Immunotitration experiments with these antibodies show that the 32000‐Mr‘protective’ protein is present both in association with the β‐galactosidase multimer and with the high‐density multimeric form together with neuraminidase. Our data further suggest that association of the 32000‐Mr‘protective’ protein and another yet unidentified subunit is essential for the catalytic activity of lysosomal neuraminidase. These results explain the absence of neuraminidase activity in the autosomal recessive human lysosomal storage disorder galactosialidosis, where the 32000‐Mr‘protective’ protein is known to be absent.
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