In addition to these signs that characterize GSDIa, GSDIb patients generally present with neutropenia that appears not to be due to a defect in bone marrow production. Neutrophils and/or monocytes are functionally abnormal, showing decreases in respiratory burst and motility in response to stimuli 3 4 . This dysfunction is responsible for recurrent infections, oral and intestinal mucosal ulcerations and inflammatory intestinal diseases suggestive of Crohn's disease 33 34 35 . This condition has occurred in over 77% of patients with GSDIb by adulthood 33 . The development of diarrhea, persistent abdominal pain, unexplained fever, gastrointestinal bleeding, perianal lesions should lead to further evaluation. Colonoscopy must be performed; should it be negative, endoscopic evaluation of the small bowel has to be considered. 36 . Recently, elevated levels of anti-bacterial flagellin antibodies (anti-CBir1) have been detected in the serum of 17/19 GSDIb patients 37 . As these antibodies have been associated with Crohn disease in the general population, these data are interesting. However, in this study, the antibody did not discriminate patients with and without inflammatory bowel disease and long-term follow-up is necessary to know whether these antibodies can predict the occurrence of intestinal inflammation 37 .

A few of the reported patients (about 10%) did not present with neutropenia and/or neutrophil dysfunction 38 39 40 . Interestingly, one of these patients 38 had a mutation that only partially affected the activity of the transporter 41 . In contrast, neutropenia is very rare in GSDIa patients 42 . Splenomegaly, which is very rarely found in GSDIa patients, was reported (together with hepatomegaly) more frequently in GSDIb patients 2 43 , specially in those receiving treatments with G-CSF.

Fertility is normal in GSDI patients and several pregnancies have been reported in affected women. Close monitoring of these pregnancies is required due to the risk of exacerbating the renal problems, of the enhanced danger of hemorrhages and of the need to provide satisfying metabolic control for the fetus 44 . In GSDIa mothers, most neonates have been delivered by cesarean section 45 . In GSDIb mothers, five successful pregnancies in three patients have been recently reported 46 . There were no major complications related to neutropenia except for oral ulcers and all neonates were delivered vaginally.

Developping a strategy for an optimal management of contraception in GSD I women is crucial. Ethynyloestradiol should be avoided because of a link with hepatic adenomas and is contraindicated in patients with hypertriglyceridemia and hypercholesterolemia. Blockage of ovulation can be achieved using high doses of progestogen alone, administered from the 5^th to the 25^th day of the cycle. Another possibility is based on daily administrations of low doses of progestogen 44 . Mechanical contraception using intra-uterine device is controversial in such patients.

GSDI was first described by von Gierke in 1929. In 1952, Cori and Cori showed that the disease (called GSDIa) was caused by a deficit in G6Pase, an enzyme expressed mainly in the liver and kidney and, to a lesser degree, in the intestine. Subsequently, it was found that some patients are not deficient in G6Pase, even though a number of functional tests demonstrated their inability to degrade G6P in vivo: this condition was called GSDIb. To explain this defect, Arion et al 47 hypothesized that G6P hydrolysis required the participation of several proteins located in the endoplasmic reticulum (ER) membrane: a catalytic unit (G6Pase) capable of hydrolyzing several phosphate esters (G6P, mannose-6-phosphate, carbamylphosphate and pyrophosphate) and a G6P-specific bidirectional translocase (G6PT), which would assure its entry into the lumen of the endoplasmic reticulum, where G6Pase exerts its action. Unlike G6Pase, G6PT is expressed ubiquitously. G6Pase and G6PT were found to be co-dependent as G6Pase activity is required for efficient transport of G6P into the ER lumen 48 .

On the basis of kinetic studies, Arion et al. 47 proposed in 1980 a multicomponent model with specific transmembrane transporter proteins for G6P (T1), phosphate, pyrophosphate and carbamylphosphate (T2) and glucose (T3). Nordlie et al 49 described a patient with GSDIc who had a deficit in the bidirectional translocase (T2) which allows phosphate resulting from G6P hydrolysis to leave the endoplasmic reticulum. The existence of GSD type Ic was discussed when mutations in the gene encoding G6PT were identified in most of these patients 50 . However, no mutation was found in the gene encoding G6PT in the patient originally described by Nordlie 50 51 , thereby suggesting that another protein could be involved in this patient or that the patient was affected by a different disorder. The hypothesis that the gene coding for a microsomal phosphate transporter (NPT4) was mutated in GSDIc patients devoid of mutations in the G6PT gene could not be confirmed 52 . Recently, it has been demonstrated that G6PT is an organophosphate:Pi antiporter transporting G6P into the ER lumen and Pi out. This supports that GSDIb and GSDIc which are deficient in the same G6PT gene represent a single disease 53 .

The existence of GSD type Id, attributed to a deficit in translocase (T3) has never been proven and the hypothesis that the glucose transporter GLUT7 could be involved was ruled out when mutations were identified in the gene encoding G6PT 50 .

The G6Pase deficient in GSDIa was subsequently renamed G6Pase-alpha as another G6P hydrolase named G6Pase-beta (or G6PC3) has been identified. G6Pase-beta is ubiquitously expressed and can form a complex with G6PT in non gluconeogenic organs, which could explain why endogenous glucose is still produced in GSDIa patients 6 54 . However, Wang et al 55 showed that the deletion of the gene encoding the G6Pase-beta in mice does not result in hypoglycemia and that the phenotype of knock-out mice is mild.

The role of G6PT, in addition to that in the glucose-6-phosphatase system, has not yet been fully elucidated. A role in the differentiation of neutrophils has been suggested 56 and studies in mice model have shown that G6PT is also an important immunomodulatory protein 57 . Recent studies provided evidence that neutrophil homeostasis and function is linked to the endogenous glucose production via the G6PT/G6Pase-beta complex. G6PT deficient or G6Pase-beta deficient neutrophils are unable to produce endogenous glucose and would manifest enhanced ER stress and apoptosis, which could contribute to neutrophil dysfunction in GSDIb patients 58 . Cheung et al 59 showed that mice lacking the G6Pase-beta had impaired neutrophil activity and increased susceptibility to bacterial infection. G6Pase-beta deficiency has been recently identified in patients presenting a severe congenital neutropenia syndrome 60 .

In GSDI, the fasting hypoglycemia results from the blockage of the last step of glycogenolysis and gluconeogenesis. However, a few patients show an unexpected tolerance to fasting 61 .

Hyperlactacidemia and glycogen storage are due to excess G6P which cannot be metabolized to glucose. Thus, galactose, fructose and glycerol will not correct hypoglycemia and will contribute to hyperlactacidemia. Hyperlipidemia is a result of both increased synthesis from excess of acetyl-coenzyme A via malonyl-coenzyme A (the first step of fatty acid synthesis), and decreased lipid serum clearance, though mechanisms of both hyperlipidemia and liver steatosis are not completely understood 62 . Accumulation of fats in the liver significantly contributes to the hepatomegaly.

Increased malonyl-coenzyme A inhibits carnitine palmytoyltransferase I, resulting in reduced ketone production and increased dicarboxylic aciduria. Hyperuricemia 1 6 is caused by both decreased renal clearance (lactate competes with uric acid) and increased synthesis (decreased intrahepatic phosphate concentration stimulates the degradation pathway of adenine nucleotides).

The mechanisms underlying the development of HCAs are still not completely elucidated. However, a new classification of HCA and a better knowledge of their relationship with hepatocellular carcinoma was proposed by Zucman Rossi et al 63 . A recent study has reported chromosomal and genetic alterations in HCAs associated with GSDIa: simultaneous gain of 6p and loss of 6q, association of larger HCAs with chromosome 6 aberrations, reduced expression of IGF2R and LATS1 (candidate tumour suppressor genes) at 6q in more than 50% of HCAs associated with GSDIa 64 . The activation of β-catenin has been reported to be an important risk factor for malignant degeneration 63 . To date, the presence of activated β-catenin has been reported in 3 GSDIa HCAs, among which one was associated with a carcinoma.

The role of metabolic control in adenomas formation has also been discussed. In a case-control retrospective study, no significant differences in metabolic balance could be detected between GSDI patients who developed adenomas and those who did not 65 .

Another mechanism could involve serum cytokines. Kim et al 66 have reported that GSDIa mice exhibit a persistent increase in peripheral blood neutrophil counts along with elevated levels of granulocyte colony stimulating factor (G-CSF) and cytokine-induced neutrophil chemoattractant. The authors suggest that there is a progressive low level of immune challenge since there is a long-term damage to the liver in GSDI patients despite dietary controls 66 .

Severe refractory iron deficiency anemia has been reported in GSDIa patients with hepatocellular adenomas, that is alleviated with either adenoma resection or liver transplantation 67 68 . The role of hepcidin, a propeptide that is converted in three mature peptides and which limit both release of iron from macrophages and intestinal iron absorption, has been proposed 68 . High levels of hepcidin mRNA expression were found in the adenomas of anemic GSDIa patients whereas hepcidin mRNA expression was decreased in the unaffected liver 68 . The upregulated production of hepcidin in adenomas would result in the dysregulation of the iron utilization cycle in GSDIa patients 68 . More studies are required to continue to shed some light on the precise mechanisms of anemia in such patients.

Biological hallmarks: fasting tolerance is poor and fasting blood analyses reveal hypoglycemia, hyperlactacidemia, hypertriglyceridemia, hypercholesterolemia and, in many cases, hyperuricemia.

Functional tests show the absence of a glycemic response and an aggravation of hyperlactacidemia after injection of glucagon (1 mg/m² of body surface) in a fasting patient or 2 hours after a meal rich in carbohydrates. Galactose injection (1 g/kg) neither induces hyperglycemia nor corrects the hypoglycemia.

Other biological abnormalities include hypoacetonemia

hypoinsulinemia and hyperglucagonemia. Additionally, marked elevation of serum biotinidase activity has been reported in GSDIa patients 87 .

Study of the G6Pase system in a liver biopsy 88

The biochemical diagnosis of GSDI requires a liver biopsy (ideally fresh, not frozen), sufficiently large to enable the analysis of the different constituents of the G6Pase system. A homogenate is prepared under conditions maintaining or not microsomal membrane integrity. Hydrolytic activity is measured using several substrates: mannose-6-phosphate (to evaluate microsomal membrane integrity), G6P and pyrophosphate. In type Ia, hydrolytic activity is defective, regardless of the substrate used and the status of microsomal membranes. In type Ib, G6P hydrolysis is defective when microsomal membranes are intact 89 .

No biochemical phenotype-clinical phenotype correlations could be established based on the results of residual activity and glycogen storage in the liver.

Histology of the liver shows glycogen and fat-induced hepatocytic distension. Fibrosis may also be present in some patients.

Complete sequencing of the G6PC (GSDIa) and SLC374A (GSDIb) genes allows diagnosis in nearly all patients with evocative clinical and biochemical signs of GSDI, thereby eliminating the need for a liver biopsy.

Treatment aims at preventing hypoglycemia in order to avoid neurological involvement and long-term complications (hepatic, renal, etc.) and to assure normal growth.

Treatment is essentially dietary and consists of frequent meals, continuous nocturnal nasogastric drip feeding, ingestion of slow-absorption carbohydrates (uncooked starch: 96 ), and restricted intakes of both fructose and galactose which can aggravate hyperlactacidemia.

Daily caloric intake must be monitored: insufficient intake does not correct the metabolic disorder (hypoglycemia, hyperlactacidemia and hyperuricemia) and leads to retarded growth, whereas excessive intake increases the glycogen overload, hepatomegaly and hyperlipidemia, and causes obesity. The diet must provide 60-65% of total caloric intake from carbohydrates, 10-15% from proteins and the reminder from fat.

Until 12 months, frequent meals (5 meals per day) and continuous nocturnal feeding via a nasogastric tube, providing 6-8 mg of glucose/kg/min, are recommended. However, continuous nocturnal feeding may be, in some cases replaced by nocturnal meals.

After 1 year, uncooked cornstarch (which may be introduced from 9 to 12 months of age, with progressive increase of doses) can, in some patients, replace the continuous nocturnal feeding at the initial dose of 0.5 g/kg then slowly increased to 1 g/kg every 4 hours. As the child grows older, the cornstarch regimen can be changed to the dose of 1.5-2.0 g/kg every 6 hours.

Another therapeutic regimen may be proposed and discussed for cornstarch which can be started during infancy, and given between meals or before bed so as not to interfere with appetite at meal time (94). Recommendations for dosing are: 1.6 g/kg body weight every 4 hours for infants, 1.7-2.5 g/kg body weight every 6 hours for young children through puberty, and 1.7-2.5 g/kg body weight given before bed time for adults. A novel and modified starch is being tested in GSD patients. To date, it seems that this new starch might allow, in some patients, longer duration of euglycemia and better short-term metabolic control 97 98 . This starch is allowed in several countries (United Kingdom, Netherlands, Germany).

In adults, the cornstarch dose and the intake interval can be increased. However, several adults stop taking cornstarch, even though they are encouraged to continue.

Treatment efficacy is evaluated by monitoring clinical (growth curve, body mass index, importance of hepatomegaly, blood pressure) and biological parameters. The preprandial glycemia must be above 3.5 mmol/L and adjusted to the actual urinary lactate excretion, which must be below 0.06 mmol/mmol creatinine in 12 hours urine samples (night and day). Blood lactate monitoring, when available, may be useful for supplementing glucose monitoring 99 . Hyperlactatemia causes acute clinical deterioration whereas chronic hyperlactatemia has been associated with long-term complications of GSDI. The main use of lactate monitoring is during intercurrent illness, when the rapid development of lactic acidosis is very likely. In such situations, the use of a portable lactate meter seems to be a valuable tool 99 . Triglyceridemia, cholesterolemia, uricemia, blood gazes, proteinuria and complete blood cell count should be measured at each outpatient visit.

Portocaval shunts were recommended in the past but accorded very limited clinical benefit and have been abandoned, since 1984 4 .

Surgery requires special care 4 in these patients at increased risk of hemorrhages and metabolic imbalances (hypoglycemia and hyperlactacidemia): glycemia must be maintained (perfusions of 10% glucose before, during and after the intervention) and solutions containing lactate should be avoided (Ringer's, for example). Corticosteroids may be discussed for a short period even though their use is usually contraindicated., and careful follow-up and correction of hemostasis abnormalities are mandatory.

Therapeutic adjuvants include vitamin supplements (vitamins D and B1, etc.), calcium (considering limited milk intake), iron in case of anemia (after excluding other causes) and allopurinol when hyperuricemia is present. If permanent microalbuminuria is present, treatment with angiotensin-converting enzyme (ACE) inhibitor is started with the aim of preventing renal complications 24 , and additional blood pressure lowering drugs may be added if blood pressure remains elevated. Should the patient become pregnant, ACE inhibitors must be stopped at once. Hyperlipemia only responds partially to dietary treatment, but triglyceride lowering drugs are not indicated if the level remains below 10 mmol/L. Cholesterol lowering drugs are not indicated in young patients because of the low risk of atherogenicity.

For GSD type Ib , granulocyte colony-stimulating factor (G-CSF) is able to correct the neutropenia, reduces the severity of bacterial infections and attenuates inflammatory intestinal disease 33 100 . Chronic G-CSF therapy may consist in two to three weekly injections, the average dose per injection being about 5 μg/kg body weight. G-CSF therapy must be carefully monitored and untoward effects may develop such as splenomegaly, thrombocytopenia, renal carcinoma 101 . Should pegylated G-CSF be used, other side effects must be known such as the occurrence of Sweet syndrome in one patient 102 , respiratory distress and sudden death in another patient 101 . Careful monitoring of the patient's spleen size, total blood cell counts and bone density is recommended in GSD Ib patients receiving G-CSF 35 100 .

Colitis is often treated with success using a combination of G-CSF and 5-aminosalicylic acid derivatives 36 100 . However, in a few cases, this treatment fails and other therapeutic approaches must be discussed. Corticosteroids are generally avoided in such situations, owing to steroid-induced glycogenolysis and the possibility of lactic acidosis and hyperlipidemia., Immunosuppressive drugs such as methotrexate, azathioprine and 6-mercaptopurine carry the risk of excessive immunosuppression and worsening neutropenia in GSD Ib patients. Adalimumab, a fully humanized monoclonal anti-TNF has been reported to be successful in one GS Ib patient with inflammatory bowel disease who was refractory to usual medical treatment 36 . Fecal alpha-1 antitrypsin must be monitored for evaluation of inflammatory bowel disease in GSDIb 38 .

Even though the risk of malignant transformation of adenomas into hepatocellular carcinomas appeared to be low 2 16 17 , ie no malignant transformation had been reported in the European retrospective study, some recent reports suggested that the older the patients get, the more important the risk of transformation becomes 103 . The management of adenomas remains empirical; it may be expectant or surgical. Clinically, should abdominal pains occur and require major painkillers to be controlled, malignant transformation must be suspected, even though these manifestations are not specific. Biological markers (serum α fetoprotein and ACE) are not predictive of malignant transformation. Liver CT scan and MRI may be useful in such cases, even though further evaluations are needed. Surgical resection of hepatic adenomas is feasible. Previous reports have noted frequent haemorragic problems 104 , but such complications may be avoided, provided meticulous metabolic control is obtained before surgery (personal data).

If dietary control fails or hepatic adenomas undergo malignant transformation, treatment of complications consists of liver transplantation 105 . Liver grafting corrects the hypoglycemia and the other biochemical anomalies but the correction of neutropenia is not constant 106 107 . It has not been proven that it can prevent renal involvement, which may even be worsened by the immunosuppressive therapy. However, there is limited experience of liver transplantation for GSDI given the rarity of the disease. Chronic allograft rejection, post-transfusion hepatitis C infection, renal failure, gouty arthritis, and portal vein thrombosis requiring re-transplantation have been reported 67 . The current literature shows that satisfactory medium-term outcomes can be achieved in GSD I patients (review in 67 ). Similar data have been reported regarding living donor liver transplantation in such patients 108 . As pointed out by Davis and Weinstein (2008), the risks and benefits of liver transplantation should be carefully evaluated and the latter should only be performed when there risk for hepatocellular carcinoma or liver dysfunction is high 109 . Liver cell transplantation could provide a less invasive approach in the future 110 . It has been reported in a GSDIb patient with marked improvement up to 250 days following transplantation; however, long-term results are still unknown and the use of immunosuppressive agents is mandatory 111 .

Bone marrow transplantation has been reported in one GSDIb patient who had life-threatening complications related to neutropenia and thrombocytopenia. It resulted in improved metabolic control and reduction of inflammatory bowel disease-related symptoms 112 .

Kidney transplantation, performed in case of severe renal failure, does not correct hypoglycemia 103 . Should grafting be indicated, combined liver-kidney graft has been discussed when renal function is already compromised and successfully performed in a few cases 113 114 .