Barbara K. Stubblefield

A Mutation in the Human Glucocerebrosidase Gene in Neuronopathic Gaucher's Disease

To search for a genetic marker for type 2 Gauchers disease (acute neuronopathic form), we compared the nucleotide sequence of a cloned glucocerebrosidase gene from a patient with Gauchers disease with a normal gene. We found only a single base substitution (T----C) in exon X. This mutation results in the substitution of proline for leucine in position number 444 and produces a new cleavage site for the NciI restriction endonuclease. We analyzed NciI enzymatic digests of genomic DNA from 20 patients with type 1, 5 with type 2, and 11 with type 3 Gauchers disease, and 29 normal controls for a restriction-fragment-length polymorphism (RFLP). Four of 5 patients with type 2 disease and all 11 with type 3 disease had at least one allele with the mutation. Two of 5 patients with type 2 disease and 7 of 11 with type 3 were homozygous for this mutation. Only 4 of 20 patients with type 1 Gauchers disease had the mutant allele and were heterozygous for it. None of the 29 normal controls had the mutant allele. The high frequency of this mutation (444leucine----proline) in patients with neuronopathic Gauchers disease, detectable by the NciI RFLP, may be of value in the identification of patients who will have the neurologic sequelae of Gauchers disease.

Glucocerebrosidase gene mutations in patients with type 2 Gaucher disease

Gaucher disease, the most common lysosomal storage disorder, results from the inherited deficiency of the enzyme glucocerebrosidase. Three clinical types are recognized: type 1, non‐neuronopathic; type 2, acute neuronopathic; and type 3, subacute neuronopathic. Type 2 Gaucher disease, the rarest type, is progressive and fatal. We have performed molecular analyses of a cohort of 31 patients with type 2 Gaucher disease. The cases studied included fetuses presenting prenatally with hydrops fetalis, infants with the collodion baby phenotype, and infants diagnosed after several months of life. All 62 mutant glucocerebrosidase (GBA) alleles were identified. Thirty‐three different mutant alleles were found, including point mutations, splice junction mutations, deletions, fusion alleles and recombinant alleles. Eleven novel mutations were identified in these patients: R131L, H255Q, R285H, S196P, H311R, c.330delA, V398F, F259L, c.533delC, Y304C and A190E. Mutation L444P was found on 25 patient alleles. Southern blots and direct sequencing demonstrated that mutation L444P occurred alone on 9 alleles, with E326K on one allele and as part of a recombinant allele on 15 alleles. There were no homozygotes for point mutation L444P. The recombinant alleles that included L444P resulted from either reciprocal recombination or gene conversion with the nearby glucocerebrosidase pseudogene, and seven different sites of recombination were identified. Homozygosity for a recombinant allele was associated with early lethality. We have also summarized the literature describing mutations associated with type 2 disease, and list 50 different mutations. This report constitutes the most comprehensive molecular study to date of type 2 Gaucher disease, and it demonstrates that there is significant phenotypic and genotypic heterogeneity among patients with type 2 Gaucher disease. Hum Mutat 15:181–188, 2000. Published 2000 Wiley‐Liss, Inc.

Phenotypic continuum in neuronopathic Gaucher disease: an intermediate phenotype between type 2 and type 3.

Neuronopathic Gaucher disease, classically divided into two types, can have a continuum of phenotypes, often defying categorization. Nine children had an intermediate phenotype characterized by a delayed age of onset but rapidly progressive neurological disease, including refractory seizures and oculomotor abnormalities. There was genotypic heterogeneity among these patients.

Myoclonic Epilepsy in Gaucher Disease: Genotype-Phenotype Insights from a Rare Patient Subgroup

Gaucher disease, the inherited deficiency of lysosomal glucocerebrosidase, presents with a wide spectrum of manifestations. Although Gaucher disease has been divided into three clinical types, patients with atypical presentations continue to be recognized. A careful phenotypic and genotypic assessment of patients with unusual symptoms may help define factors that modify phenotype in this disorder. One such example is a rare subgroup of patients with type 3 Gaucher disease who develop progressive myoclonic epilepsy. We evaluated 16 patients with myoclonic epilepsy, nine of whom were diagnosed by age 4 y with severe visceral involvement and myoclonus, and seven with a more chronic course, who were studied between ages 22 and 40. All of the patients had abnormal horizontal saccadic eye movements. Fourteen different genotypes were encountered, yet there were several shared alleles, including V394L (seen on two alleles), G377S (seen on three alleles), and L444P, N188S, and recombinant alleles (each found on four alleles). V394L, G377S, and N188S are mutations that have previously been associated with non-neuronopathic Gaucher disease. The spectrum of genotypes differed significantly from other patients with type 3 Gaucher disease, where genotypes L444P/L444P and R463C/null allele predominated. Northern blot studies revealed a normal glucocerebrosidase transcript, whereas Western studies showed that the patients studied lacked the processed 56 kD isoform of the enzyme, consistent with neuronopathic Gaucher disease. Brain autopsy samples from two patients demonstrated elevated levels of glucosylsphingosine, a toxic glycolipid, which could contribute to the development of myoclonus. Thus, although there were certain shared mutant alleles found in these patients, both the lack of a shared genotype and the variability in clinical presentations suggest that other modifiers must contribute to this rare phenotype.

Reciprocal and Nonreciprocal Recombination at the Glucocerebrosidase Gene Region: Implications for Complexity in Gaucher Disease

Gaucher disease results from an autosomal recessive deficiency of the lysosomal enzyme glucocerebrosidase. The glucocerebrosidase gene is located in a gene-rich region of 1q21 that contains six genes and two pseudogenes within 75 kb. The presence of contiguous, highly homologous pseudogenes for both glucocerebrosidase and metaxin at the locus increases the likelihood of DNA rearrangements in this region. These recombinations can complicate genotyping in patients with Gaucher disease and contribute to the difficulty in interpreting genotype-phenotype correlations in this disorder. In the present study, DNA samples from 240 patients with Gaucher disease were examined using several complementary approaches to identify and characterize recombinant alleles, including direct sequencing, long-template polymerase chain reaction, polymorphic microsatellite repeats, and Southern blots. Among the 480 alleles studied, 59 recombinant alleles were identified, including 34 gene conversions, 18 fusions, and 7 downstream duplications. Twenty-two percent of the patients evaluated had at least one recombinant allele. Twenty-six recombinant alleles were found among 310 alleles from patients with type 1 disease, 18 among 74 alleles from patients with type 2 disease, and 15 among 96 alleles from patients with type 3 disease. Several patients carried two recombinations or mutations on the same allele. Generally, alleles resulting from nonreciprocal recombination (gene conversion) could be distinguished from those arising by reciprocal recombination (crossover and exchange), and the length of the converted sequence was determined. Homozygosity for a recombinant allele was associated with early lethality. Ten different sites of crossover and a shared pentamer motif sequence (CACCA) that could be a hotspot for recombination were identified. These findings contribute to a better understanding of genotype-phenotype relationships in Gaucher disease and may provide insights into the mechanisms of DNA rearrangement in other disorders.

Induced pluripotent stem cell model recapitulates pathologic hallmarks of Gaucher disease

Gaucher disease (GD) is an autosomal recessive disorder caused by mutations in the acid β-glucocerebrosidase gene. To model GD, we generated human induced pluripotent stem cells (hiPSC), by reprogramming skin fibroblasts from patients with type 1 (N370S/N370S), type 2 (L444P/RecNciI), and type 3 (L444P/L444P) GD. Pluripotency was demonstrated by the ability of GD hiPSC to differentiate to all three germ layers and to form teratomas in vivo. GD hiPSC differentiated efficiently to the cell types most affected in GD, i.e., macrophages and neuronal cells. GD hiPSC-macrophages expressed macrophage-specific markers, were phagocytic, and were capable of releasing inflammatory mediators in response to LPS. Moreover, GD hiPSC-macrophages recapitulated the phenotypic hallmarks of the disease. They exhibited low glucocerebrosidase (GC) enzymatic activity and accumulated sphingolipids, and their lysosomal functions were severely compromised. GD hiPSC-macrophages had a defect in their ability to clear phagocytosed RBC, a phenotype of tissue-infiltrating GD macrophages. The kinetics of RBC clearance by types 1, 2, and 3 GD hiPSC-macrophages correlated with the severity of the mutations. Incubation with recombinant GC completely reversed the delay in RBC clearance from all three types of GD hiPSC-macrophages, indicating that their functional defects were indeed caused by GC deficiency. However, treatment of induced macrophages with the chaperone isofagomine restored phagocytosed RBC clearance only partially, regardless of genotype. These findings are consistent with the known clinical efficacies of recombinant GC and isofagomine. We conclude that cell types derived from GD hiPSC can effectively recapitulate pathologic hallmarks of the disease.

Regional Distribution of the GABAA/Benzodiazepine Receptor (α Subunit) mRNA in Rat Brain

Abstract: A human cDNA clone containing the 5’coding region of the GABAA/benzodiazepine receptor α subunit was used to quantify and visualize receptor mRNA in various regions of the rat brain. Using a [32P]CTP‐labelled antisense RNA probe (860 bases) prepared from the α subunit cDNA, multiple mRNA species were detected in Northern blots using total and poly A rat brain RNA. In all brain regions, mRNAs of 4.4 and 4.8 kb were observed, and an additional mRNA of 3.0 kb was detected in the cerebellum and hippocampus. The level of GABAA/benzodiazepine receptor mRNA was highest in the cerebellum followed by the thalamus = frontal cortex = hippocampus = parietal cortex = hypothalamus ≫ pons = striatum = medulla. In situ hybridization revealed high levels of α subunit mRNA in cerebellar gray matter, olfactory bulb, thalamus, hippocampus/dentate gyrus, and the arcuate nucleus of the hypothalamus. These data suggest the presence of multiple GABAA/benzodiazepine receptor α subunit mRNAs in rat brain and demonstrate the feasibility of studying the expression of genes encoding the GABAA/benzodiazepine receptor after pharmacological and/or environmental manipulation.

Evaluation of quinazoline analogues as glucocerebrosidase inhibitors with chaperone activity.

Gaucher disease is a lysosomal storage disorder (LSD) caused by deficiency in the enzyme glucocerebrosidase (GC). Small molecule chaperones of protein folding and translocation have been proposed as a promising therapeutic approach to this LSD. Most small molecule chaperones described in the literature contain an iminosugar scaffold. Here we present the discovery and evaluation of a new series of GC inhibitors with a quinazoline core. We demonstrate that this series can improve the translocation of GC to the lysosome in patient-derived cells. To optimize this chemical series, systematic synthetic modifications were performed and the SAR was evaluated and compared using three different readouts of compound activity: enzymatic inhibition, enzyme thermostabilization, and lysosomal translocation of GC.

Isolation of cDNA clones for human β-glucocerebrosidase using the λgtll expression system

Two cDNA clones (λGC-1 and λGC-2) for human β-glucocerebrosidase [EC 3.2.1.45] have been isolated from a human hepatoma library in λgtll by immunological screening using monospecific polyclonal antibody for β-glucocerebrosidase. Restriction endonuclease mapping indicates that these clones are probably identical in size, each with a 1900 bp insert. The 50 kDa size of the insert-encoded polypeptide produced by these clones in fusion with β-galactosidase of λgtll in E.coli BNN103 is consistent with the size of the nascent form of β-glucocerebrosidase. These fusion proteins are shown by Western blotting to react with antibody to β-glucocerebrosidase. Amino acid sequence deduced from the nucleotide sequence of the insert ir pGC-1 is identical to known amino acid sequence of β-glucocerebrosidase, and thus, confirms that the clones are specific for β-glucocerebrosidase.

Association of an X-chromosome dodecamer insertional variant allele with mental retardation

Mental retardation is a prominent feature of many neurodevelopmental syndromes. In an attempt to identify genetic components of these illnesses, we isolated and sequenced a large number of human genomic cosmid inserts containing large trinucleotide repeats. One of these cosmids, Cos-4, maps to the X-chromosome and contains the sequence of a 7.3-kb mRNA. Initial polymorphism analysis across a region of repetitive DNA in this gene revealed a rare 12-bp exonic variation (1% in non-ill males) having an increased prevalence in non-Fragile X males with mental retardation (4%, P <0.04, n = 81). This variant was not present in the highly conserved mouse homologue that has 100% amino acid identity to the human sequence near the polymorphism. Subsequent screening of two additional independent cohorts of non-Fragile X mentally retarded patients and ethnically matched controls demonstrated an even higher prevalence of the 12-bp variant in males with mental retardation (8%, P <0.0003, n = 125, and 14%, P <0.10, n = 36) vs the controls. Multivariate analysis was conducted in an effort to identify other phenotypic components in affected individuals, and the findings suggested an increased incidence of histories of hypothyroidism (P <0.001) and treatment with antidepressants (P <0.001). we conclude that the presence of this 12-bp variant confers significant susceptibility for mental retardation.