Introduction
Glycogen storage disease type III (GSDIII; OMIM #232400) is a rare inborn error of glycogen degradation with an incidence of 1:100,000 (Dagli et al
2010; Kishnani et al
2010; Laforêt et al
2012). GSDIII is caused by mutations in the
AGL gene and the subsequent deficiency of the glycogen debranching enzyme (GDE; EC no. 3.2.1.33 and 2.4.1.25, UniProt P35573). GDE contains two catalytic centres that catalyse one of the last steps in the conversion of glycogen to glucose-1-phosphate.
Patients with GSDIII present clinically with hepatomegaly, failure to thrive and fasting intolerance, biochemically associated with ketotic hypoglycaemia. Phenotypically, patients can be further classified into having either GSDIIIa (±85 %), with involvement of the liver, heart and skeletal muscle, or GSDIIIb (±15 %), in which only the liver is affected (Shen et al
1996; Laforêt et al
2012). Dietary management aims to maintain normoglycaemia and prevent hyperketonaemia by dividing sufficient carbohydrate intake throughout the day, and using additional protein as a substrate for gluconeogenesis (as recently reviewed by Derks and Smit
2015). During long-term follow-up the clinical focus shifts to the prevention and management of progressive hepatic, cardiac and myopathic complications (Dagli et al
2009; Sentner et al
2012; Verbeek et al
2015).
Current knowledge on the clinical course and outcome has been based on case reports and small single centre cohort studies, of mainly young patients, on which the current management guidelines are based (Dagli et al
2010; Kishnani et al
2010). The International Study on GSDIII (ISGSDIII) is a descriptive, retrospective, international, multi-centre cohort study on the diagnosis, genotype, management, clinical course and outcome in 175 patients with GSDIII, with follow-up into adulthood in 91 patients.
Methods
The Medical Ethical Committee of the University Medical Centre Groningen, the Netherlands approved the study protocol (ref.no. METc2008.035). Patients were included from 17 metabolic centres in ten countries. Between 2007 and 2011 data on GSDIII patients were collected using a case record form (CRF) for every patient and anonymously archived in a database. The CRF was based on the European Study on GSDI (ESGSDI; Rake et al
2002), and modified for ISGSDIII by two authors (GPAS, CPS) (for the complete CRF, see Supplemental data
1). The CRFs were filled out either by the treating physician or by one single investigator (CPS).
GSDIII patients were included when an enzyme assay and/or AGL molecular analysis had confirmed the diagnosis. GSDIIIa was defined as (a) deficient GDE activity in muscle or (b) clinical and/or biochemical signs of cardiac and/or skeletal muscular involvement. Based on the family history, individual patients could be categorized as proband, symptomatic sibling or neonatally screened patient due to an affected older sibling. To study the relationship between AGL genotypes and GSDIII phenotypes, statistical analyses were only performed in adult patients. Cardiac involvement was defined as the presence of abnormalities corresponding to cardiac hypertrophy in the electrocardiographic and/or echocardiographic investigations. Cardiomyopathy was defined as the presence of cardiac hypertrophy in combination with 1) (severe) exercise intolerance (and/or 2) the use of pharmacological treatment for (symptoms of) heart failure.
AGL mutations were grouped according to the type of mutation, i.e. missense or non-missense
AGL genotypes. Non-missense mutations resulting in either frameshift or splicing modifications were assumed to be pathogenic. Pathogenicity of novel missense mutations was predicted by five methods: Alamut Version 2.2 (©Interactive Biosoftware), PolyPhen-2 (
http://genetics.bwh.harvard.edu/pph2/), SIFT (
http://sift.bii.a-star.edu.sg/), whether the mutation was located in the catalytic site (exon 6, 13-15, 26-27) or an exon encoding the glycogen binding domain (exons 31-34), and the NHLBI Exome Sequencing Project (ESP) Exome Variant Server (
http://evs.gs.washington.edu/EVS/).
Data were processed with IBM® SPSS® Statistics Version 20 (SPSS Inc., Chicago, Il, USA). The results were expressed as median (range) for non-parametric data, and mean (standard deviation) for parametric data. Differences between normally distributed continuous data were analysed using the unpaired two-tailed T-test. Not normally distributed data were analysed using the Mann-Whitney-U or Kruskal-Wallis test. For dichotomous data, the Fisher’s exact test was used. The level of significance was set at p < 0.05.
Discussion
ISGSDIII is a descriptive, retrospective, international, multi-centre cohort study of 175 patients. This study addresses important issues regarding clinical presentation and follow-up into adulthood.
Before discussing the results, some methodological issues need to be addressed. First, as ISGSDIII is a retrospective study, we have predominantly collected cross-sectional rather than longitudinal patient data, and there has been missing data. Unfortunately, IGSDIII did not collect data on fasting tolerance. Second, the ISGSDIII cohort is still relatively young, and follow-up has not extended into adulthood for all patients. This may have caused an underrepresentation of long-term complications. Third, as most participating centres and colleagues are centre of expertise, selection bias towards relatively severely affected patients may have affected the results. Fourth, patients with the extremely rare subtypes GSDIIIc (presumably the result of glucosidase debranching deficiency) and GSDIIId (presumably the result of transferase debranching deficiency) have not been included. Last, despite the use of a CRF, in different centres clinical and laboratory data are not yet recorded in a standardized and quantitative manner (for instance dietary parameters, echocardiographic parameters, quantification of skeletal muscle strength and exercise tolerance). Particularly the availability of dietary management data has been very limited. It additionally needs to be recognized that there may be a difference between prescribed diets and daily practice.
In contrast to the previous reports on GSDIII patients (Dagli et al
2010; Kishnani et al
2010; Laforêt et al
2012), ISGSDIII demonstrates that hypoglycaemia is a presenting symptom in just half of the patients. Therefore, in patients with a traditional clinical (hepatomegaly) and biochemical (elevated transaminase values, hyperlipidaemia) presentation, the diagnosis of GSDIII should not be rejected in the absence of (severe) hypoglycaemia. In addition, the finding of ketotic hypoglycemia after short fasting test is a major argument for GSDIII, as demonstrated recently (Hoogeveen et al
2015). Severe mental retardation and mortality due to metabolic derangement are uncommon in GSDIII patients. ESGSDI has reported high morbidity and mortality because of metabolic derangements with hypoglycaemia (Rake et al
2002). The difference between these studies may be partially explained by the relatively younger cohort of ISGSDIII. More importantly, there is a fundamental difference in metabolic compensation between GSDI patients (alternative lactate accumulates quickly, in the absence of ketones) and GSDIII patients (gluconeogenesis is intact and ketones can gradually be formed) during fasting.
Clear genotype-phenotype correlations are rare in GSDIII. The association between exon 3 mutations and GSDIIIb has been reported previously (Shen et al
1996; Elpeleg
1999; Shen and Chen
2002; Goldstein et al
2010). Interestingly, non-missense
AGL mutations are overrepresented in the ISGSDIII cohort, whereas in most metabolic diseases missense mutations predominate. It can be hypothesized that missense mutations in the large
AGL gene cause only minor reduction of GDE enzyme activity. Moreover, ISGSDIII demonstrates that GSDIIIa patients display a more severe clinical course than GSDIIIb patients. The latter group clinically presents at a later stage and has fewer complications, such as hepatic cirrhosis and HCC. The large ISGSDIII cohort did not identify additional correlations between
AGL genotype and severe complications.
In accordance with previous reports (Vertilus et al
2010) ISGSDIII demonstrates that cardiac hypertrophy is common in GSDIIIa patients, mostly starting in the first decade of life (Fig.
2). Cardiac involvement remains stable over time in the majority of the affected patients, with even a portion of the patients regressing to normal values (data not presented). Hence, functional and clinically relevant hypertrophic cardiomyopathy is rare in GSDIIIa patients. Observations from GSDIIIa patients with severe hypertrophic cardiomyopathy suggest an important role of macronutrient intake (Derks and Smit
2015). To date it is speculative which macronutrient intervention is dominant, because each of the following has been described, i.e. decreased total caloric intake (Sentner et al
2012), increased protein intake (Dagli et al
2009; Sentner et al
2012), increased fat intake (Brambilla et al
2014), ketone bodies (Valayannopoulos et al
2011) and Atkins diet (Mayorandan et al
2014). It is not possible to draw causative conclusions from these single case observations, because increasing one macronutrient (either protein or fat) without affecting the other, inevitably affects the remaining macronutrient (carbohydrates). Based on at least two arguments it can be hypothesized that carbohydrate overtreatment may be an important risk factor for cardiac involvement and/or cardiomyopathy. First, decreased carbohydrate intake was the intervention shared by the above-mentioned reports in which cardiomyopathy resolved after dietary intervention. Secondly, decompensated structural cardiomyopathy is most frequently reported around the time of highest endogenous glucose requirements (i.e. childhood) and the prescription of relatively high amounts of dietary carbohydrate.
ISGSDIII demonstrates that GSDIII patients have (severe) growth retardation in (early) childhood, but eventually reach normal adult height. There is no significant difference in growth between GSDIIIa and GSDIIIb patients, suggesting that the metabolic demands on gluconeogenesis in GSDIII in general are more important than the presence of a muscular GDE deficiency.
ISGSDIII demonstrates a high incidence of bone fractures in paediatric GSDIII patients, suggesting the development of reduced BMD at an early age. The pathophysiology of reduced BMD is unclear, but an association with specific nutritional deficiencies in GSDIII (Folk and Greene
1984; Kishnani et al
1999), and reduced metabolic control in GSDI (Rake et al
2003) are mentioned. Recently, ALAT has been suggested to be a marker for metabolic control in GSDIII (Dagli et al
2010). ISGSDIII demonstrates a negative correlation between osteopenia/osteoporosis and ALAT, supporting the hypothesis that metabolic control affects BMD.
ISGSDIII reports a higher incidence of DM2 in ageing GSDIII patients than in the general population (i.e. 9 %: compared to 6 % in the general population in the western world according to the World Diabetes Foundation). Previous case reports and case studies have described the association between DM2 and GSDIII (Moe et al
1972; Oki et al
2000; Ismail
2009; Sharma
2009; Spengos et al
2009) but the aetiology is largely unknown. In the ISGSDIII cohort, half of the diabetic patients are obese, suggesting that decreased insulin sensitivity might play a role. Second, the constant intake of carbohydrate enriched nutrients to maintain euglycaemia may induce insulin resistance.
Acknowledgments
Acknowledgements to the contributors to ISGSDIII: Germany Prof K Ullrich, Hamburg; Czech Republic Prof J Zéman, Prague; Netherlands Prof D-J Reijngoud, Groningen; Dr E Morava, Nijmegen; Dr E Rubio, Maastricht; Dr G Visser, Utrecht; Dr A Bosch, Amsterdam; United Kingdom Dr M Champion, Dr S Grünewald, Dr L Abulhoul, Dr P Gissen, Dr R Lachmann, Dr Y Rahman, Dr H Mundy London; Dr P Newsome, Birmingham; A. Dalton, Sheffield; Italy Prof GP Comi, Milan; Dr M Di Rocco, Genova; Canada Prof C Polychronakos, Dr AM Sbrocchi, Montreal
The ISGSDIII collaborative researchers received financial support from Junior Scientific Masterclass (Graduate School GUIDE, University of Groningen, the Netherlands), Metakids (the Netherlands), the National Institutes of Health (NIH), National Centre for Research Resources (NCRR; CTSA grant UL1 TR000064; University of Florida), the Johnny Damon Foundation, and the Hope and Fund for GSDIII Research managed through the University of Florida Office of Development.