Shen J, Bao Y, Liu H M, Lee P, Leonard J V, Chen Y T
Department of Pediatrics, Division of Medical Genetics, Duke University Medical Center, North Carolina 27710, USA.
J Clin Invest. 1996 Jul 15;98(2):352-7. doi: 10.1172/JCI118799.
Glycogen storage disease type HI (GSD-III), an autosomal recessive disease, is caused by deficient glycogen debranching enzyme (GDE) activity. Most GSD-III patients are GDE deficient in both liver and muscle (type IIIa), and some GSD-III patients have GDE absent in liver but retained in muscle (type IIIb). The molecular basis for this enzymatic variability is largely unknown. In the present study, the analysis of the GDE gene in three GSD-IIIb patients by single-strand conformation polymorphism (SSCP), DNA sequencing, restriction analysis, and family studies, revealed each of them as being a compound heterozygote for two different mutations. The first mutant alleles in all three patients involved mutations in exon 3 at amino acid codon 6 of the GDE protein. Two had an AG deletion at nucleotides 17 and 18 of the GDE cDNA (17delAG) which resulted in change of subsequent amino acid sequence and a truncated protein (25X); the other had a C to T transition at nucleotide 16 of the cDNA which changed a Glutamine codon to a stop codon (Q6X). The 17delAG mutation was also found in 8 of the 10 additional GSD-IIIb patients. The Q6X mutation was found in one of the remaining two GSD-IIIb patients. These two mutations were not found in any of the 31 GSD-IIIa patients, 2 GSD-IIId patients, nor 28 unrelated normal controls. The second mutant alleles in each of the three GSD-IIIb patients were R864X, R1228X, and W68OX. The R864X and R1228X were not unique for GSD-IIIb as they were also found in GSD-IIIa patients (frequency of 10.3% and 5.2% in Caucasian patients, respectively). Our data demonstrated that both IIIa and IIIb had mutations in the same GDE gene and established for the first time the molecular basis of GSD-III that differentially expressed in liver and muscle. The striking and specific association of exon 3 mutations with GSD-IIIb may provide insight into mechanisms controlling tissue-specific expression of the GDE gene. The identification of exon 3 mutations has clinical significance as well because it distinguished GSD-IIIb from IIIa hence permitting diagnosis from a blood sample rather than a more invasive muscle biopsy.
III型糖原贮积病（GSD-III）是一种常染色体隐性疾病，由糖原脱支酶（GDE）活性缺乏引起。大多数GSD-III患者肝脏和肌肉中的GDE均缺乏（IIIa型），一些GSD-III患者肝脏中GDE缺失但肌肉中保留（IIIb型）。这种酶活性变异性的分子基础很大程度上尚不清楚。在本研究中，通过单链构象多态性（SSCP）、DNA测序、限制性分析以及家系研究，对3例IIIb型GSD患者的GDE基因进行分析，发现他们均为两种不同突变的复合杂合子。所有3例患者的第一个突变等位基因均涉及GDE蛋白第6位氨基酸密码子所在外显子3的突变。其中2例患者的GDE cDNA第17和18位核苷酸处有AG缺失（17delAG），导致后续氨基酸序列改变及蛋白质截短（25X）；另1例患者cDNA第16位核苷酸处有C到T的转换，将谷氨酰胺密码子变为终止密码子（Q6X）。在另外10例IIIb型GSD患者中，有8例也发现了17delAG突变。在其余2例IIIb型GSD患者中的1例发现了Q6X突变。在31例IIIa型GSD患者、2例IIIc型GSD患者及28例无关正常对照中均未发现这两种突变。3例IIIb型GSD患者各自的第二个突变等位基因分别为R864X、R1228X和W680X。R864X和R1228X并非IIIb型GSD所特有，因为在IIIa型GSD患者中也有发现（在白种人患者中的频率分别为10.3%和5.2%）。我们的数据表明，IIIa型和IIIb型GSD在同一GDE基因上均有突变，并首次确立了在肝脏和肌肉中差异表达的GSD-III的分子基础。外显子3突变与IIIb型GSD显著而特异的关联可能为控制GDE基因组织特异性表达的机制提供线索。外显子3突变的鉴定也具有临床意义，因为它可将IIIb型GSD与IIIa型区分开来，从而可通过血样而非更具侵入性的肌肉活检进行诊断。
