Paul V. Nelson

A new molecular mechanism for severe myoclonic epilepsy of infancy : Exonic deletions in SCN1A

We examined cases of severe myoclonic epilepsy of infancy (SMEI) for exon deletions or duplications within the sodium channel SCN1A gene by multiplex ligation-dependent probe amplification. Two of 13 patients (15%) who fulfilled the strict clinical definition of SMEI but without SCN1A coding or splicing mutations had exonic deletions of SCN1A.

Frequent deletions at Xq28 indicate genetic heterogeneity in Hunter syndrome

SummaryHunter syndrome is a human X-linked disorder caused by deficiency of the lysosomal exohydrolase iduronate-2-sulphatase (IDS). The consequent accumulation of the mucopolysaccharides dermatan sulphate and heparan sulphate, in the brain and other tissues, often results in death before adulthood. There is, however, a broad spectrum of severity that has been attributed to different mutations of the Hunter syndrome gene. We have used an IDS cDNA clone to localise the IDS gene to Xq28, distal to the fragile X mutation (FRAXA). One-third of Hunter syndrome patients had various deletions or rearrangements of their IDS gene, proving that different mutations are common in this condition. Deletions of the IDS gene can include a conserved locus that is tightly linked to FRAXA, suggesting that deletion of nearby genes may contribute to the variable clinical severity noted in Hunter syndrome. The cDNA clone was also shown to span the X chromosome breakpoint in a female Hunter syndrome patient with an X;autosome translocation.

A common PEX1 frameshift mutation in patients with disorders of peroxisome biogenesis correlates with the severe Zellweger syndrome phenotype.

Peroxisome biogenesis disorders are a heterogeneous group of human neurodegenerative diseases caused by peroxisomal metabolic dysfunction. At the molecular level, these disorders arise from mutations in PEX genes that encode proteins required for the import of proteins into the peroxisomal lumen. The Zellweger syndrome spectrum of diseases is a major sub-set of these disorders and represents a clinical continuum from Zellweger syndrome (the most severe) through neonatal adrenoleukodystrophy to infantile Refsum disease. The PEX1 gene, which encodes a cytoplasmic AAA ATPase, is the responsible gene in more than half of the Zellweger syndrome spectrum patients, and mutations in PEX1 can account for the full spectrum of phenotypes seen in these patients. In these studies, we have undertaken mutation analysis of PEX1 in skin fibroblast cell lines from Australasian Zellweger syndrome spectrum patients. A previously reported common PEX1 mutation that gives rise to a G843D substitution and correlates with the less severe disease phenotypes has been found to be present at high frequency in our patient cohort. We also report a novel PEX1 mutation that occurs at high frequency in Zellweger syndrome spectrum patients. This mutation produces a frameshift in exonxa013, a change that leads to the premature truncation of the PEX1 protein. A Zellweger syndrome patient who was homozygous for this mutation and who survived for less than two months from birth had undetectable levels of PEX1 mRNA. This new common mutation therefore correlates with a severe disease phenotype. We have adopted procedures for the detection of this mutation for successful prenatal diagnosis.

Population frequency of the arylsulphatase A pseudo-deficiency allele

SummaryThe enzymatic diagnosis of metachromatic leukodystrophy is complicated by the frequent occurrence of the pseudo-deficiency of arylsulphatase A (ASA) enzyme activity. An A to G nucleotide transition in the first polyadenylation signal of the ASA gene results in the loss of its major mRNA species and a greatly reduced level of enzyme activity. This nucleotide change (nucleotide 1620 of the ASA cDNA) is the cause of ASA pseudo-deficiency and is closely linked to another A to G transition (nucleotide 1049), within the ASA gene, which changes Asn350 to serine but which does not affect ASA activity. The distribution of these 2 nucleotide changes has been investigated in 73 unrelated individuals from the Australian population. The two transitions were found together on 14 (9.6%) out of 146 chromosomes. The transition at nucleotide 1620 was not found alone; however, the other transition was found alone on 7 (4.8%) out of the 146 chromosomes. The carrier frequency of the ASA pseudo-deficiency mutation in Australia is thus estimated to be about 20%.

Human glucosamine-6-sulfatase cDNA reveals homology with steroid sulfatase

Glucosamine-6-sulfatase is a lysosomal enzyme which degrades glycosaminoglycans and is deficient in mucopolysaccharidosis type IIID. Human liver contains two major active forms of glucosamine-6-sulfatase, form A which has a single 78 kDa polypeptide and form B which has two polypeptides of 48 kDa and 32 kDa. A 1761 base pair cDNA clone encoding the complete 48 kDa polypeptide of form B was isolated. Form A is shown to be processed to form B with the 48 kDa polypeptide C-terminal to the 32 kDa polypeptide, and it is shown that C-terminal processing is limited to a region of thirty amino acids. The glucosamine-6-sulfatase sequence reveals homology with steroid sulfatase, a microsomal enzyme.

Mutations among Italian mucopolysaccharidosis type I patients

A group of 27 Italian patients was screened for α-L-iduronidase mucopolysaccharidosis type I mutations. Mutations were found in 18 patients, with 28 alleles identified. The two most common mutations in northern Europeans (W402X and Q70X) accounted for 11% and 13% of the alleles, respectively. The R89Q mutation, uncommon in Europeans, was found only in one patient, accounting for 1 of 54 alleles (1.9%). The other mutations, P533R, A327P and G51D, accounted for 11%, 5.6% and 9.3% of the total alleles, respectively. Interestingly, the high frequency of the P533R mutation seems to be confined to Sicily and is higher than the 3% reported in a British/Australian study.

The clinical phenotype of two patients with a complete deletion of the iduronate-2-sulphatase gene (mucopolysaccharidosis II--Hunter syndrome).

SummaryTwo patients with a complete deletion of the iduronate-2-sulphatase (IDS) gene are described. In both patients, the resulting phenotype was that of very severe Hunter syndrome (mucopolysaccharidosis II). In addition, both had features not commonly seen in this disorder, e.g. early onset of seizures in one patient and ptosis in the other. It is speculated that loss of adjacent loci may contribute to the unusual findings and that the severe features present in both patients may represent contiguous gene syndromes. Further analysis of IDS cDNA from other patients with Hunters syndrome may eventually enable phenotype to be predicted more accurately.

Carbohydrate-deficient glycoprotein syndrome: Beyond the screen

We report two siblings with carbohydrate-deficient glycoprotein syndrome (CDG) type 1 (McKusick 212065) secondary to phosphomannomutase deficiency, both of whom have repeatedly normal transferrin isoform screening tests.

Prenatal diagnosis of mucolipidosis II - Electron microscopy and biochemical evaluation

Prenatal diagnosis of mucolipidosis type II (I‐cell disease) can be performed quickly and reliably by electron microscopy of chorionic villus tissue. This study reports the results of studies in three prenatal assessments (two families) where the pregnancy was at one in four risk of the disorder. In all three cases, electron microscopy showed marked vacuolation in chorionic villus cells, consistent with the fetus being affected by the disorder. Further studies in cultured chorionic villus cells showed a marked deficiency of a number of lysosomal enzymes. All pregnancies were terminated. Follow‐up studies in fetal tissue (where available) confirmed the prenatal diagnosis as correct. Copyright

Mucopolysaccharidosis type I (Hurler syndrome): linkage disequilibrium indicates the presence of a major allele.

SummaryTwo polymorphisms exist in the α-l-iduronidase (IDUA) gene, the gene that is defective in mucopolysaccharidosis type I (MPS I), viz. aKpnI polymorphism and a variable number of tandem repeats (VNTR) polymorphism with three common alleles. The analysis of allele and haplotype frequencies for these two polymorphisms in the normal population and in MPS I patients revealed the presence of linkage disequilibrium. The frequency of the 2,2 (VNTR,KpnI) allele in MPS I patients was 57% compared with only 37% in the normal population. The implications for the presence of a major MPS I allele and the ability to predict patient phenotype are discussed.