Investigations within this project concern the cell biology of rare human genetic disorders and normal and abnormal intracellular processes. The research goal is to gain insight into changes in molecular function that underlie various genetic metabolic disorders and work towards treatments for these illnesses. The research focuses on four groups of rare disorders:? ? 1. Disorders of sialic acid metabolism. The key enzyme in the sialic acid biosynthesis pathway is UDP-GlcNAc 2-epimerase/ManNAc kinase (GNE) (reviewed in (6)). Dominant mutations in the allosteric site of GNE cause sialuria, characterized by overproduction of sialic acid. Recessive mutations in GNE cause the neuromuscular disorder hereditary inclusion body myopathy (HIBM). Our group found that polymorphisms in dystroglycan were not responsible for the variation in clinical phenotype in HIBM (4) and that patients have normal sialylation of serum N-linked and O-GalNAc-linked glycans (12). We developed GNE-epimerase and GNE-kinase enzyme assays and demonstrated impaired GNE enzymatic activities in HIBM patients? cells (14). We have characterized a knock-in HIBM mouse model and demonstrated that N-acetylmannosamine (ManNAc) rescues the phenotype of the homozygous mutant mice (manuscript in preparation) and is a promising treatment option for human patients. Other treatments (sialic acid, galactose) are currently being tested on this murine HIBM model. We developed an allele specific real-time PCR method (Provisional Patent No. 60/718,321 ?Use of real time PCR for detection of allelic expression?) to measure GNE allelic expression levels in both HIBM and sialuria. In addition, we are performing in vitro siRNA silencing experiments of the dominant, mutated allele in sialuria cells (manuscript in preparation). ? ? 2. Disorders of 3-methylglutaconic aciduria (3MGA) presenting with or without optic atrophy. We reported that optic atrophy is a common, under-recognized, clinical feature in many metabolic disorders (7,8). In 2001, our group isolated OPA3, a gene of unknown function responsible for Costeff syndrome, which is characterized by 3MGA and optic atrophy. We tested DNA from patients with/without 3MGA and/or isolated optic atrophy for mutations in OPA3 (15). We are currently investigating OPA3 function, and we have identified a novel OPA3 isoform with a rare dual mitochondrial and peroxisomal localization (manuscript in preparation). We also created zebrafish models for Costeff syndrome using antisense morpholino technology (manuscript in preparation). ? ? 3. Disorders of intracellular vesicle sorting and formation. These disorders include Hermansky-Pudlak syndrome (HPS), Chediak-Higashi syndrome, Griscelli syndrome, and other genetically unclassified disorders. Common clinical features are albinism due to defects in melanosomes and bleeding due to platelet defects (reviewed in (2)). Our group investigates known and unknown HPS-causing genes, with the goal of better understanding the biology of the disease. Our group also catalogues the clinical and genetic characteristics of the seven distinct subgroups of HPS (3,9,10,13). To study the effects of HPS mutations, we perform cell biological studies on patients? material (employing immuno-fluorescence, immmuno-EM, and live cell imaging) to examine defective intracellular trafficking and sorting of proteins and organelles in HPS cells. Such cells fail to transport certain lysosomal proteins to their correct destinations, and HPS gene products are involved in recognizing the specific vesicles that give rise to lysosome-like organelles (5,11). ? ? 4. Genetic interstitial deletion syndromes. Routine mutation screening commonly involves PCR-based approaches followed by direct sequencing. Occurrence of larger genomic deletions may be missed by these approaches if the deletion breakpoints extend beyond the position of the PCR primers. In recessive disorders, this can lead to mistaking homozygosity for hemizygosity. Our group applied quantitative real-time PCR to detect hemizygosity and deletion breakpoints in a variety of rare disorders. ? - Hermansky-Pudlak syndrome: We identified patients with a large genomic deletion on the HPS1 locus (Griffin et al. Clin Genet (2005) 68, 23-30) and the HPS6 locus (manuscript in preparation). ? - Holoprosencephaly: We tested patients by quantitative real-time PCR for submicroscopic deletions in candidate gene regions (1), supplementing multicolor FISH results. ? - Smith-Magenis syndrome (SMS): This disorder is mainly (greater than 95%) caused by an interstitial deletion of 17p11.2. Our group is currently performing quantitative real-time PCR to identify hemizygosity in key genes on 17p11.2 in 98 patients with SMS. Our results, in combination with FISH analysis and future tiling arrays performed by collaborating groups, will shed light on the variable phenotype of SMS patients and genotype-phenotype correlations. ? In the future, these quantitative real-time PCR methods can be applied to other deletion syndromes (e.g., Jacobsen syndrome).
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