High frequency of missense mutations in glycogen storage disease type VI.
Deficiency of liver glycogen phosphorylase in glycogen storage disease (GSD) type VI results in a reduced ability to mobilize glucose from glycogen. Six mutations of the PYGL gene, which encodes the liver isoform of the enzyme, have been identified in the literature. We have characterized eight patients from seven families with GSD type VI and identified 11 novel PYGL gene defects. The majority of the mutations were missense, resulting in the substitution of highly conserved residues. These could be grouped into those that were predicted to affect substrate binding (p.V456M, p.E673K, p.S675L, p.S675T), pyridoxal phosphate binding (p.R491C, p.K681T), or activation of glycogen phosphorylase (p.Q13P) or that had an unknown effect (p.N632I and p.D634H). Two mutations were predicted to result in null alleles, p.R399X and [c.1964_1969inv6;c.1969+1_+4delGTAC]. Only 7 of the 23 (30%) reported PYGL alleles carry nonsense, splice site or frameshift mutations compared to 68-80% of affected alleles of the highly homologous muscle glycogen phosphorylase gene, PYGM, that underlie McArdle disease. There was heterogeneity in the clinical symptoms observed in affected individuals. These varied from hepatomegaly and subclinical hypoglycaemia, to severe hepatomegaly with recurrent severe hypoglycaemia and postprandial lactic acidosis. We conclude that deficiency of liver glycogen phosphorylase is predominantly the result of missense mutations affecting enzyme activity. There are no common mutations and the severity of clinical symptoms varies significantly.
Beauchamp
N J
NJ
Academic Unit of Child Health, University of Sheffield, Stephenson Wing, Sheffield Children's NHS Trust, Western Bank, Sheffield, S10 2TH, UK. n.j.beauchamp@sheffield.ac.uk
Taybert
J
J
Champion
M P
MP
Layet
V
V
Heinz-Erian
P
P
Dalton
A
A
Tanner
M S
MS
Pronicka
E
E
Sharrard
M J
MJ
D000595:Amino Acid Sequence; D000818:Animals; D001786:Blood Glucose / Q000378:metabolism; D002675:Child, Preschool; D004252:DNA Mutational Analysis; D005091:Exons; D005260:Female; D020022:Genetic Predisposition to Disease; D005838:Genotype; D025001:Glycogen Phosphorylase, Liver Form / Q000737:chemistry / Q000172:deficiency / Q000235:genetics*; D006011:Glycogen Storage Disease Type IV / Q000201:enzymology / Q000235:genetics*; D006801:Humans; D007223:Infant; D007438:Introns; D019344:Lactic Acid / Q000097:blood; D008099:Liver / Q000201:enzymology*; D008297:Male; D008958:Models, Molecular; D008969:Molecular Sequence Data; D020125:Mutation, Missense; D010375:Pedigree; D010641:Phenotype; D011487:Protein Conformation; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid; D012720:Severity of Illness Index
D001786:Blood Glucose; D019344:Lactic Acid; D025001:Glycogen Phosphorylase, Liver Form
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