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FBP1
fructose-bisphosphatase 1
FBP1 is located on the long (q) arm of chromosome 9, at band 9q22.32. Arm ratio per GRCh38 - banding schematic.
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Overview
FBP1 encodes fructose-bisphosphatase 1, a hepatic enzyme that catalyses a rate-limiting step in gluconeogenesis. This metabolic pathway becomes essential during periods of fasting, illness, or reduced dietary carbohydrate intake, when the body must synthesise glucose rather than obtain it directly from food. The enzyme converts fructose-1,6-bisphosphate to fructose-6-phosphate, effectively reversing one of the committed steps of glycolysis.
Pathogenic variants in FBP1 cause fructose-1,6-bisphosphatase deficiency, a rare autosomal recessive metabolic disorder. Affected individuals typically present in infancy or early childhood with episodes of hypoglycaemia triggered by fasting or illness. The condition is included on multiple NHS genomic medicine panels due to its diagnostic and management implications.
What the gene does
Fructose-bisphosphatase 1 functions as the primary hepatic enzyme responsible for the irreversible hydrolysis of fructose-1,6-bisphosphate to fructose-6-phosphate and inorganic phosphate. This reaction represents one of four bypass steps unique to gluconeogenesis, distinguishing this anabolic pathway from the catabolic process of glycolysis. The enzyme's activity is tightly regulated by metabolic signals, including allosteric inhibition by AMP and fructose-2,6-bisphosphate, which coordinate glucose production with cellular energy status.
The protein operates primarily in liver and kidney cortex, tissues specialised for glucose synthesis. During fasting states, when hepatic glycogen stores become depleted, FBP1 activity becomes critical for maintaining euglycaemia. The enzyme works in concert with other gluconeogenic enzymes to synthesise glucose from substrates including lactate, glycerol, and amino acids. Without functional FBP1, individuals cannot adequately produce glucose through this pathway, rendering them vulnerable to metabolic decompensation during periods when dietary glucose intake is insufficient.
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Chromosome location
FBP1 is located on the long arm of chromosome 9 at position 22.32 (9q22.32). The gene spans approximately 31 kilobases of genomic DNA and comprises seven exons. This chromosomal region has been well characterised through clinical cytogenetic and molecular genetic studies of metabolic disorders.
Protein structure
Domain architecture has not been experimentally characterised in detail for this protein.
Key variants
Pathogenic variants in FBP1 follow an autosomal recessive inheritance pattern, meaning that affected individuals carry alterations in both gene copies. The variant spectrum includes missense changes, nonsense mutations, and small deletions that disrupt enzyme function through various mechanisms including protein misfolding, reduced catalytic efficiency, or premature truncation. Carrier frequency varies across populations, with some variants showing founder effects in specific ethnic groups.
Sample of pathogenic variants
10 pathogenic / likely-pathogenic variants from ClinVar, ranked by review status (expert-panel-reviewed first). This is a sample; recurrent founder variants in a specific population may not appear here - see the full ClinVar listing via the link above.
| Variant (HGVS) | Protein change | Classification | Evidence | Associated condition |
|---|---|---|---|---|
c.392del | p.Val131fs | Pathogenic | ★★☆☆ | Fructose-biphosphatase deficiency |
c.426+1G>A | - | Pathogenic | ★★☆☆ | Fructose-biphosphatase deficiency |
c.472C>T | p.Arg158Trp | Pathogenic | ★★☆☆ | Fructose-biphosphatase deficiency |
c.611_614del | p.Lys204fs | Pathogenic/Likely pathogenic | ★★☆☆ | Fructose-biphosphatase deficiency |
c.616_619del | p.Lys206fs | Pathogenic/Likely pathogenic | ★★☆☆ | Fructose-biphosphatase deficiency |
c.704del | p.Pro235fs | Pathogenic/Likely pathogenic | ★★☆☆ | Fructose-biphosphatase deficiency |
c.778G>A | p.Gly260Arg | Pathogenic/Likely pathogenic | ★★☆☆ | Fructose-biphosphatase deficiency |
c.841G>A | p.Glu281Lys | Pathogenic/Likely pathogenic | ★★☆☆ | Fructose-biphosphatase deficiency |
c.841G>T | p.Glu281Ter | Pathogenic | ★★☆☆ | Fructose-biphosphatase deficiency |
c.960delinsGG | p.Ser321fs | Pathogenic | ★★☆☆ | Fructose-biphosphatase deficiency |
Evidence stars indicate ClinVar review status. Individual variant interpretation should always be performed by a qualified clinical laboratory - many variants remain classified as Variants of Uncertain Significance (VUS) pending more research.
Associated conditions
Biallelic pathogenic variants in FBP1 cause fructose-1,6-bisphosphatase deficiency, a rare inherited disorder of gluconeogenesis. Clinical presentation typically occurs during infancy or early childhood, often triggered by intercurrent illness, prolonged fasting, or fever. Affected individuals experience episodes of severe hypoglycaemia accompanied by lactic acidosis, ketosis, and hepatomegaly. Between episodes, individuals are typically asymptomatic when maintaining regular carbohydrate intake. Early diagnosis through metabolic investigation enables implementation of dietary management strategies that can prevent life-threatening hypoglycaemic episodes.
Inheritance pattern
Conditions caused by pathogenic FBP1 variants typically follow autosomal recessive inheritance.
When both parents are carriers, each child has a 25% chance of being affected, 50% of being a carrier, and 25% of being unaffected.
UK clinical status
FBP1 appears on multiple NHS Genomic Medicine Service panels reflecting its established role in inherited metabolic disease. The gene holds green (high evidence) status on the DDG2P panel, the Glycogen storage disease panel (R274), the Ketotic hypoglycaemia panel, the Likely inborn error of metabolism panel (R98), and the Undiagnosed metabolic disorders panel. This inclusion supports genomic testing for individuals presenting with unexplained hypoglycaemia, metabolic acidosis, or recurrent metabolic decompensation, particularly when episodes correlate with reduced carbohydrate intake or illness.
Sources: NHS GMS PanelApp · Genomics England PanelApp · NHS National Genomic Test Directory
Frequently asked questions
What inheritance pattern does FBP1 follow?
FBP1-related conditions follow an autosomal recessive inheritance pattern. This means an individual must inherit a pathogenic variant from both parents to develop fructose-1,6-bisphosphatase deficiency. Carriers, who have one variant copy, typically do not experience symptoms.
Why does FBP1 deficiency cause hypoglycaemia during fasting?
FBP1 deficiency impairs the body's ability to produce glucose through gluconeogenesis when dietary glucose is unavailable. During fasting or illness, the body normally synthesises glucose from alternative sources, but without functional fructose-bisphosphatase 1, this critical pathway cannot operate efficiently, resulting in dangerously low blood sugar levels.
Is FBP1 included on UK clinical genomic testing panels?
Yes, FBP1 holds green (high evidence) status on multiple NHS Genomic Medicine Service panels including those for glycogen storage disease, ketotic hypoglycaemia, and likely inborn errors of metabolism. This supports its use in diagnostic genomic testing for individuals with compatible clinical presentations.