Fructose-1,6-bisphosphatase deficiency with confirmed molecular diagnosis. An important cause of hypoglycemia in children

An extremely rare case of hypoglycemia with a novel mutation and review of the literature: fructose-1,6 bisphosphatase deficiency in an adult man

Hypoglycemia is an uncommon clinical problem among non-diabetic patients. It requires systematic evaluation to determine the etiology. It may be related to critical illness, hepatic insufficiency, renal insufficiency, cardiac insufficiency, drugs, alcohol, cortisol insufficiency, growth hormone insufficiency, insulinoma, gastric bypass surgery, and paraneoplastic (insulin-like growth factor-2-related) immune-mediated or inherited metabolic disorders. We aimed to summarize the literature and present a case who suffered from hypoglycemia throughout his life and was diagnosed with fructose-1, 6 bisphosphatase deficiency in adulthood to attract attention to the rare causes of hypoglycemia in adulthood.

Immunodeficiency with susceptibility to lymphoma with complex genotype affecting energy metabolism (FBP1, ACAD9) and vesicle trafficking (RAB27A)

Introduction

Inborn errors of immunity (IEI) are characterized by a dysfunction of the immune system leading to increased susceptibility to infections, impaired immune regulation and cancer. We present a unique consanguineous family with a history of Hodgkin lymphoma, impaired EBV control and a late onset hemophagocytic lymphohistiocytosis (HLH).

Methods and results

Overall, family members presented with variable impairment of NK cell and cytotoxic T cell degranulation and cytotoxicity. Exome sequencing identified homozygous variants in RAB27A, FBP1 (Fructose-1,6-bisphosphatase 1) and ACAD9 (Acyl-CoA dehydrogenase family member 9). Variants in RAB27A lead to Griscelli syndrome type 2, hypopigmentation and HLH predisposition.

Discussion

Lymphoma is frequently seen in patients with hypomorphic mutations of genes predisposing to HLH. We hypothesize that the variants in FBP1 and ACAD9 might aggravate the clinical and immune phenotype, influence serial killing and lytic granule polarization by CD8 T cells. Understanding of the interplay between the multiple variants identified by whole exome sequencing (WES) is essential for correct interpretation of the immune phenotype and important for critical treatment decisions.

Fructose-1,6-bisphosphatase is a nonenzymatic safety valve that curtails AKT activation to prevent insulin hyperresponsiveness

Insulin inhibits gluconeogenesis and stimulates glucose conversion to glycogen and lipids. How these activities are coordinated to prevent hypoglycemia and hepatosteatosis is unclear. Fructose-1,6-bisphosphatase (FBP1) is rate controlling for gluconeogenesis. However, inborn human FBP1 deficiency does not cause hypoglycemia unless accompanied by fasting or starvation, which also trigger paradoxical hepatomegaly, hepatosteatosis, and hyperlipidemia. Hepatocyte FBP1-ablated mice exhibit identical fasting-conditional pathologies along with AKT hyperactivation, whose inhibition reversed hepatomegaly, hepatosteatosis, and hyperlipidemia but not hypoglycemia. Surprisingly, fasting-mediated AKT hyperactivation is insulin dependent. Independently of its catalytic activity, FBP1 prevents insulin hyperresponsiveness by forming a stable complex with AKT, PP2A-C, and aldolase B (ALDOB), which specifically accelerates AKT dephosphorylation. Enhanced by fasting and weakened by elevated insulin, FBP1:PP2A-C:ALDOB:AKT complex formation, which is disrupted by human FBP1 deficiency mutations or a C-terminal FBP1 truncation, prevents insulin-triggered liver pathologies and maintains lipid and glucose homeostasis. Conversely, an FBP1-derived complex disrupting peptide reverses diet-induced insulin resistance.