Glycosuria and hyperglycemia in the neonatal period as the first clinical sign of Fanconi-Bickel syndrome

Case Report: Fanconi-Bickel Syndrome in a Chinese Girl With Diabetes and Severe Hypokalemia

Fanconi-Bickel syndrome (FBS) is a rare autosomal recessive carbohydrate metabolism disorder. The main symptoms of FBS are hepatomegaly, nephropathy, postprandial hyperglycemia, fasting hypoglycemia, and growth retardation. Hypokalemia is a rare clinical feature in patients with FBS. In this study, we present a neonate suffering from FBS. She presented with hypokalemia, dysglycaemia, glycosuria, hepatomegaly, abnormality of liver function, and brain MRI. Trio whole-exome sequencing (WES) and Sanger sequencing were performed to identify the causal gene variants. A compound heterozygous mutation (NM_000340.2; p. Trp420*) of SLC2A2 was identified. Here, we report a patient with FBS in a consanguineous family with diabetes, severe hypokalemia, and other typical FBS symptoms. Patients with common clinical features may be difficult to diagnose just by phenotypes in the early stage of life, but WES could be an important tool. We also discuss the use of insulin in patients with FBS and highlight the importance of a continuous glucose monitoring system (CGMS), not only in diagnosis but also to avoid hypoglycemic events.

Molecular mechanisms of β-cell dysfunction and death in monogenic forms of diabetes

Monogenetic forms of diabetes represent 1%-5% of all diabetes cases and are caused by mutations in a single gene. These mutations, that affect genes involved in pancreatic β-cell development, function and survival, or insulin regulation, may be dominant or recessive, inherited or de novo. Most patients with monogenic diabetes are very commonly misdiagnosed as having type 1 or type 2 diabetes. The severity of their symptoms depends on the nature of the mutation, the function of the affected gene and, in some cases, the influence of additional genetic or environmental factors that modulate severity and penetrance. In some patients, diabetes is accompanied by other syndromic features such as deafness, blindness, microcephaly, liver and intestinal defects, among others. The age of diabetes onset may also vary from neonatal until early adulthood manifestations. Since the different mutations result in diverse clinical presentations, patients usually need different treatments that range from just diet and exercise, to the requirement of exogenous insulin or other hypoglycemic drugs, e.g., sulfonylureas or glucagon-like peptide 1 analogs to control their glycemia. As a consequence, awareness and correct diagnosis are crucial for the proper management and treatment of monogenic diabetes patients. In this chapter, we describe mutations causing different monogenic forms of diabetes associated with inadequate pancreas development or impaired β-cell function and survival, and discuss the molecular mechanisms involved in β-cell demise.

Fanconi-Bickel syndrome in an infant with cytomegalovirus infection: A case report and review of the literature

BACKGROUND

Fanconi–Bickel syndrome (FBS) is a rare autosomal recessive disorder caused by mutation of the SLC2A2 gene, which encodes glucose transporter protein 2 (GLUT2).

CASE SUMMARY

We report a 7-mo-old girl with cytomegalovirus infection presenting hepatomegaly, jaundice, liver transaminase elevation, fasting hypoglycemia, hyperglycosuria, proteinuria, hypophosphatemia, rickets, and growth retardation. After prescription of ganciclovir, the levels of bilirubin and alanine aminotransferase decreased to normal, while she still had aggravating hepatomegaly and severe hyperglycosuria. Then, whole exome sequencing was conducted and revealed a homozygous c.416delC mutation in exon 4 of SLC2A2 inherited from her parents, which was predicted to change alanine 139 to valine (p.A139Vfs*3), indicating a diagnosis of FBS. During the follow-up, the entire laboratory test returned to normal with extra supplement of vitamin D and corn starch. Her weight increased to normal range at 3 years old without hepatomegaly. However, she still had short stature. Although there was heterogeneity between phenotype and genotype, Chinese children had typical clinical manifestations. No hot spot mutation or association between severity and mutations was found, but nonsense and missense mutations were more common. Data of long-term follow-up were rare, leading to insufficient assessment of the prognosis in Chinese children.

CONCLUSION

FBS is a rare genetic metabolic disease causing impaired glucose liver homeostasis and proximal renal tubular dysfunction. Results of urine and blood testing suggesting abnormal glucose metabolism could be the clues for FBS in neonates and infants. Genetic sequencing is indispensable for diagnosis. Since the diversity of disease severity, early identification and long-term follow-up could help improve patients’ quality of life and decrease mortality.

Functional and structural analysis of rare SLC2A2 variants associated with Fanconi-Bickel syndrome and metabolic traits

Deleterious variants in SLC2A2 cause Fanconi-Bickel Syndrome (FBS), a glycogen storage disorder, whereas less common variants in SLC2A2 associate with numerous metabolic diseases. Phenotypic heterogeneity in FBS has been observed, but its causes remain unknown. Our goal was to functionally characterize rare SLC2A2 variants found in FBS and metabolic disease-associated variants to understand the impact of these variants on GLUT2 activity and expression and establish genotype-phenotype correlations. Complementary RNA-injected Xenopus laevis oocytes were used to study mutant transporter activity and membrane expression. GLUT2 homology models were constructed for mutation analysis using GLUT1, GLUT3, and XylE as templates. Seventeen FBS variants were characterized. Only c.457_462delCTTATA (p.Leu153_Ile154del) exhibited residual glucose uptake. Functional characterization revealed that only half of the variants were expressed on the plasma membrane. Most less common variants (except c.593 C>A (p.Thr198Lys) and c.1087 G>T (p.Ala363Ser)) exhibited similar GLUT2 transport activity as the wild type. Structural analysis of GLUT2 revealed that variants affect substrate-binding, steric hindrance, or overall transporter structure. The mutant transporter that is associated with a milder FBS phenotype, p.Leu153_Ile154del, retained transport activity. These results improve our overall understanding of the underlying causes of FBS and impact of GLUT2 function on various clinical phenotypes ranging from rare to common disease.