Mutations in the liver glycogen phosphorylase gene (PYGL) underlying glycogenosis type VI.

Deficiency of glycogen phosphorylase in the liver gives rise to glycogen-storage disease type VI (Hers disease; MIM 232700). We report the identification of the first mutations in PYGL, the gene encoding the liver isoform of glycogen phosphorylase, in three patients with Hers disease. These are two splice-site mutations and two missense mutations. A mutation of the 5' splice-site consensus of intron 14 causes the retention of intron 14 and the utilization of two illegitimate 5' splice sites, whereas a mutation of the 3' splice-site consensus of intron 4 causes the skipping of exon 5. Two missense mutations, N338S and N376K, both cause nonconservative replacements of amino acids that are absolutely conserved even in yeast and bacterial phosphorylases. We also report corrections of the PYGL coding sequence, sequence polymorphisms, and a partial PYGL gene structure with introns in the same positions as in PYGM, the gene of the muscle isoform of phosphorylase. Our findings demonstrate that PYGL mutations cause Hers disease, and they may improve laboratory diagnosis of deficiencies of the liver phosphorylase system.

Identification of a mutation in liver glycogen phosphorylase in glycogen storage disease type VI.

Glycogen storage disease type VI (GSD6) defines a group of disorders that cause hepatomegaly and hypoglycemia with reduced liver phosphorylase activity. The course of these disorders is generally mild, but definitive diagnosis requires invasive procedures. We analyzed a Mennonite kindred with an autosomal recessive form of GSD6 to determine the molecular defect and develop a non-invasive diagnostic test. Linkage analysis was performed using genetic markers flanking the liver glycogen phosphorylase gene ( PYGL ), which was suspected to be the cause of the disorder on biochemical grounds. Mennonite GSD6 was linked to the PYGL locus with a multipoint LOD score of 4.7. The PYGL gene was analyzed for mutations by sequencing genomic DNA. Sequencing of genomic DNA revealed a splice site abnormality of the intron 13 splice donor. Confirmation of the genomic mutation was performed by sequencing RT-PCR products, which showed heterogeneous PYGL mRNA lacking all or part of exon 13 in affected persons. This study is the first to demonstrate that a mutation in the PYGL gene can cause GSD6. This mutation is estimated to be present on 3% of Mennonite chromosomes and the disease affects 0.1% of that population. Determination of this mutation provides a basis for the development of a simple and non-invasive diagnostic test for the disease and the carrier state in this population and confirms biochemical data showing the importance of this gene in glucose homeostasis.

McArdle's & Hers' diseases: glycogen phosphorylase transcriptional expression in human tissues.

We have cloned the cDNA encoding human liver glycogen phosphorylase (glycogenosis type VI) from a fetal brain cDNA library. Liver(L) and muscle(M) phosphorylase cDNA probes were used to determine the relative abundance of mRNA encoding the L- and M-isozymes of phosphorylase in human fetal and adult tissues. The transcript encoding the M-isozyme is 3.4 kb; the L-isozyme transcript is 3.3 kb. Transcriptional expression of the L-isozyme in human and primate tissues was found to differ from the isozyme's reported tissue specificity in non-primate mammals. Furthermore, using degenerate oligonucleotide probes to two different coding regions of M-phosphorylase, a novel 4.1-kb transcript was demonstrated to be present in human fetal and adult brain.

The behavior of hepatic phosphorylase b kinase, phosphorylase a and b after administration of glucagon to patients with glycogen storage disease type VIa.

In the patients with glycogen storage disease (GSD) type VIa and different serum glucose response to glucagon, the activities of hepatic phosphorylase b kinase, phosphorylase a and b were estimated before and after the intravenous administration of glucagon. 3 min after the administration of glucagon an increase in the activities of phosphorylase b kinase and phosphorylase a was found in liver tissue of all patients except one. These enzymatic activities, however, did not exceed the values of these enzymes in the control liver biopsies without glucagon loading. After the intravenous administration of glucagon an unsuspected increase of phosphorylase b activity was observed in the control liver tissues and in patients with GSD type VIa, except one. In vitro investigations revealed that an increase of hepatic phosphorylase b activity occurs during its conversion to phosphorylase a. We suppose that this phosphorylase b represents a partially phosphorylated form of this enzyme (an intermediate form) that is due to the action of the active phosphorylase b kinase. The correlations between the activities of phosphorylase b kinase, phosphorylase a and an intermediate form of phosphorylase b and hepatic glycogen degradation after administration of glucagon has been discussed.

Isoenzyme pattern of phosphorylase in white blood cells and fibroblasts from patients with liver phosphorylase deficiency.

Isoenzyme patterns of phosphorylase in white blood cells and cultured fibroblasts of a patient affected with liver-type phosphorylase deficiency were studied. Three bands were observed with electrofocusing of white blood cells and liver from controls. In the white blood cells of the patient only two bands were observed. Patient and control fibroblasts showed two bands, probably identical to the two bands observed in the patient's white blood cells. These results indicate that the liver-type phosphorylase is not expressed in the cultured fibroblasts.

Glycogen phosphorylase and its converter enzymes in haemolysates of normal human subjects and of patients with type VI glycogen-storage disease. A study of phosphorylase kinase deficiency.

1. The properties of phosphorylase a, phosphorylase b, phosphorylase kinase and phosphorylase phosphatase present in a human haemolysate were investigated. The two forms of phosphorylase have the same affinity for glucose 1-phosphate but greatly differ in Vmax. Phosphorylase b is only partially stimulated by AMP, since, in the presence of the nucleotide, it is about tenfold less active than phosphorylase a. In a fresh human haemolysate phosphorylase is mostly in the b form; it is converted into phosphorylase a by incubation at 20degreesC, and this reaction is stimulated by glycogen and cyclic AMP. Once activated, the enzyme can be inactivated after filtration of the haemolysate on Sephadex G-25. This inactivation is stimulated by caffeine and glucose and inhibited by AMP and fluoride. The phosphorylase kinase present in the haemolysate can also be measured by the rate of activation of added muscle phosphorylase b, on addition of ATP and Mg2+. 2. The activity of phosphorylase kinase was measured in haemolysates obtained from a series of patients who had been classified as suffering from type VI glycogenosis. In nine patients, all boys, an almost complete deficiency of phosphorylase kinase was observed in the haemolysate and, when it could be assayed, in the liver. A residual activity, about 20% of normal, was found in the leucocyte fraction, whereas the enzyme activity was normal in the muscle. These patients suffer from the sex-linked phosphorylase kinase deficiency previously described by others. Two pairs of siblings, each time brother and sister, displayed a partial deficiency of phosphorylase kinase in the haemolysate and leucocytes and an almost complete deficiency in the liver. This is considered as being the autosomal form of phosphorylase kinase deficiency. Other patients were characterized by a low activity of total (a+b) phosphorylase and a normal or high activity of phosphorylase kinase in their haemolysate.