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 thatcause hepatomegaly and hypoglycemia with reduced liver phosphorylaseactivity. The course of these disorders is generally mild, but definitivediagnosis requires invasive procedures. We analyzed a Mennonite kindredwith an autosomal recessive form of GSD6 to determine the molecular defectand develop a non-invasive diagnostic test. Linkage analysis was performedusing genetic markers flanking the liver glycogen phosphorylase gene ( PYGL), which was suspected to be the cause of the disorder on biochemicalgrounds. Mennonite GSD6 was linked to the PYGL locus with a multipoint LODscore of 4.7. The PYGL gene was analyzed for mutations by sequencinggenomic DNA. Sequencing of genomic DNA revealed a splice site abnormalityof the intron 13 splice donor. Confirmation of the genomic mutation wasperformed by sequencing RT-PCR products, which showed heterogeneous PYGLmRNA lacking all or part of exon 13 in affected persons. This study is thefirst to demonstrate that a mutation in the PYGL gene can cause GSD6. Thismutation is estimated to be present on 3% of Mennonite chromosomes and thedisease affects 0.1% of that population. Determination of this mutationprovides a basis for the development of a simple and non- invasivediagnostic test for the disease and the carrier state in this populationand confirms biochemical data showing the importance of this gene inglucose homeostasis.