A c.3216_3217delGA mutation in AGL gene in Tunisian patients with a glycogen storage disease type III: evidence of a founder effect

Spectrum of amyloglucosidase mutations in Asian Indian patients with Glycogen storage disease type III

Glycogen storage disease type III (GSD III) is a rare autosomal recessive inborn error of glycogen degradation pathway due to deficiency or reduced activity of glycogen debranching enzyme (GDE) that results in accumulation of abnormal glycogen in the liver, muscle, and heart. The cardinal hallmarks are hepatomegaly, fasting hypoglycemia, seizures, growth retardation, progressive skeletal myopathy, and cardiomyopathy in few. To date, 258 mutations in amyloglucosidase (AGL) gene have been identified worldwide. However, the mutation spectrum in the Asian Indian region is yet to be well characterized. We investigated 24 patients of Asian origin from 21 unrelated families with a provisional diagnosis of GSD III based on clinical and biochemical criteria. Molecular diagnosis was assessed by bidirectional sequencing and the impact of novel missense variants on the tertiary (three-dimensional) structure of GDE was evaluated by molecular modeling approach. Eighteen different pathogenic variants were identified, out of which 78% were novel. Novel variants included five nonsense, three small duplications and two small deletions, a splice site variant, and three missense variants. Variations in Exons 4, 14, 19, 24, 27, and 33 accounted for 61% of the total pathogenic variants identified and Allele p.Gly798Alafs*3 showed a high allele frequency of 11%. Molecular modeling study of novel pathogenic missense variants indicated the probable underlying molecular mechanism of adverse impact of variations on the structure and catalytic function of human GDE. Our study is the first large study on GSD III from the Asian subcontinent, which further expands the mutation spectrum of AGL.

Targeted next generation sequencing screening of Lynch syndrome in Tunisian population

A high colorectal cancer (CRC) incidence is observed in Tunisia, with a relatively high proportion of patients developing CRC before the age of 40. While this suggests a genetic susceptibility, only a few Tunisian Lynch Syndrome families have been described. In this study we aimed to identify the underlying genetic cause in 32 patients with early onset CRC and/or a positive family history. Of twenty-four patients' tumor or biopsies could be analyzed with immunohistochemical staining to detect loss of expression of one of the MMR proteins. Ten tumors showed loss of expression, of which one tumor was from a patient where a germline pathogenic MSH2 variant was detected previously with Sanger sequencing. Next generation sequencing of the MMR, POLE and POLD1 genes was performed in leukocyte and tumor DNA of the remaining nine patients, as well as in two patients with MMR-proficient tumors, but with severe family history. In six of 11 patients a germline variant was detected in MLH1 (n = 5) or MSH2 (n = 1). Two of six patients were from the same family and both were found to carry a novel in-frame MLH1 deletion, predicted to affect MLH1 function. All MLH1 variant carriers had loss of heterozygosity with retention of the variant in the tumors, while a somatic pathogenic variant was detected in the patient with the germline MSH2 variant.

A Novel Nonsense Mutation of the AGL Gene in a Romanian Patient with Glycogen Storage Disease Type IIIa

Background. Glycogen storage disease type III (GSDIII) is a rare metabolic disorder with autosomal recessive inheritance, caused by deficiency of the glycogen debranching enzyme. There is a high phenotypic variability due to different mutations in the AGL gene. Methods and Results. We describe a 2.3-year-old boy from a nonconsanguineous Romanian family, who presented with severe hepatomegaly with fibrosis, mild muscle weakness, cardiomyopathy, ketotic fasting hypoglycemia, increased transaminases, creatine phosphokinase, and combined hyperlipoproteinemia. GSD type IIIa was suspected. Accordingly, genomic DNA of the index patient was analyzed by next generation sequencing of the AGL gene. For confirmation of the two mutations found, genetic analysis of the parents and grandparents was also performed. The patient was compound heterozygous for the novel mutation c.3235C>T, p.Gln1079(⁎) (exon 24) and the known mutation c.1589C>G, p.Ser530(⁎) (exon 12). c.3235 >T, p.Gln1079(⁎) was inherited from the father, who inherited it from his mother. c.1589C>G, p.Ser530(⁎) was inherited from the mother, who inherited it from her father. Conclusion. We report the first genetically confirmed case of a Romanian patient with GSDIIIa. We detected a compound heterozygous genotype with a novel mutation, in the context of a severe hepatopathy and an early onset of cardiomyopathy.

A founder AGL mutation causing glycogen storage disease type IIIa in Inuit identified through whole-exome sequencing: a case series

BACKGROUND

Glycogen storage disease type III is caused by mutations in both alleles of the AGL gene, which leads to reduced activity of glycogen-debranching enzyme. The clinical picture encompasses hypoglycemia, with glycogen accumulation leading to hepatomegaly and muscle involvement (skeletal and cardiac). We sought to identify the genetic cause of this disease within the Inuit community of Nunavik, in whom previous DNA sequencing had not identified such mutations.

METHODS

Five Inuit children with a clinical and biochemical diagnosis of glycogen storage disease type IIIa were recruited to undergo genetic testing: 2 underwent whole-exome sequencing and all 5 underwent Sanger sequencing to confirm the identified mutation. Selected DNA regions near the AGL gene were also sequenced to identify a potential founder effect in the community. In addition, control samples from 4 adults of European descent and 7 family members of the affected children were analyzed for the specific mutation by Sanger sequencing.

RESULTS

We identified a homozygous frame-shift deletion, c.4456delT, in exon 33 of the AGL gene in 2 children by whole-exome sequencing. Confirmation by Sanger sequencing showed the same mutation in all 5 patients, and 5 family members were found to be carriers. With the identification of this mutation in 5 probands, the estimated prevalence of genetically confirmed glycogen storage disease type IIIa in this region is among the highest worldwide (1:2500). Despite identical mutations, we saw variations in clinical features of the disease.

INTERPRETATION

Our detection of a homozygous frameshift mutation in 5 Inuit children determines the cause of glycogen storage disease type IIIa and confirms a founder effect.

Molecular and biochemical characterization of a novel intronic single point mutation in a Tunisian family with glycogen storage disease type III

Genetic deficiency of the glycogen debranching enzyme causes glycogen storage disease type III, an autosomal recessive inherited disorder. The gene encoding this enzyme is designated as AGL gene. The disease is characterized by fasting hypoglycemia, hepatomegaly, growth retardation, progressive myopathy and cardiomyopathy. In the present study, we present clinical features and molecular characterization of two consanguineous Tunisian siblings suffering from Glycogen storage disease type III. The full coding exons of the AGL gene and their corresponding exon-intron boundaries were amplified for the patients and their parents. Gene sequencing identified a novel single point mutation at the conserved polypyrimidine tract of intron 21 in a homozygous state (IVS21-8A>G). This variant cosegregated with the disease and was absent in 102 control chromosomes. In silico analysis using online resources showed a decreased score of the acceptor splice site of intron 21. RT-PCR analysis of the AGL splicing pattern revealed a 7 bp sequence insertion between exon 21 and exon 22 due to the creation of a new 3' splice site. The predicted mutant enzyme was truncated by the loss of 637 carboxyl-terminal amino acids as a result of premature termination. This novel mutation is the first mutation identified in the region of Bizerte and the tenth AGL mutation identified in Tunisia. Screening for this mutation can improve the genetic counseling and prenatal diagnosis of GSD III.