Genetic Subtypes

Congenital hyperinsulinism is genetically heterogeneous disorder with mutations in a number of different genes reported. Hyperinsulinism may occur in isolation or can present as part of a syndrome. The phenotype of the patient is dependent on the genetic aetiology and in some cases the specific mutation within a gene can also influence the phenotype.

Currently a genetic diagnosis is possible for 40-50% of individuals with congenital hyperinsulinism, suggesting that mutations in novel genes remain to be discovered rapid genetic diagnosis is clinically important for patients. For example the mode of inheritance of a KATP channel mutation(s) can provide information regarding the histological subtype which will inform on the need for 18F-DOPA-PET scanning. A genetic diagnosis is also important for patients with medically managed hyperinsulinism. For example the identification of a GLUD1 or HADH mutation will inform the clinician of the protein sensitive nature of hypoglycaemia allowing for the manipulation of diet (protein restriction) as a useful, sometimes mandatory, adjunct to medical therapy. Furthermore, the presence of a HNF4A mutation would identify patients who are at a risk of developing maturity onset diabetes of the young (MODY) which can be successfully managed with low dose sulphonylureas. Finally, a genetic diagnosis will also allow for an accurate recurrence risk for siblings and future offspring.

Below is a brief overview of the most common genetic aetiologies for congenital hyperinsulinism for which our laboratory provide testing.

K-ATP Channel – ABCC8 and KCNJ11

Inactivating mutations in the KCNJ11 or ABCC8 genes which encode the pancreatic beta cell ATP-sensitive potassium (K-ATP) channel are the most common known cause of congenital hyperinulinism. Individuals with this genetic subtype often show a poor response to medical therapy and may require a partial or near total pancreatectomy. Diffuse disease results from the inheritance of a dominant or two recessively acting mutations. In contrast focal lesions result from paternal uniparental disomy (UPD) of chromosome 11p15.5–11p15.1 within a single pancreatic beta cell. The UPD unmasks the paternally inherited KATP channel mutation at 11p15.1 and causes altered expression of imprinted genes that include the maternally expressed tumor suppressor genes, H19 and CDKN1C, and the paternally expressed growth factor IGF2, at 11p15.5. This disruption in the expression of key cell cycle genes results in clonal expansion of the single cell and dysregulated insulin secretion from the resulting focal lesion. A genetic diagnosis of focal hyperinsulinism can be confirmed following surgery, by analysis of microsatellite makers on chromosome 11 to investigate loss of heterozygosity for the maternal allele within the focal lesion.

Gene Information

Location: Chromosome 11p15.1 (hg38: 17,385,859 – 17,476,845)
Protein name: Inwardly rectifying potassium channel (Kir6.2), Sulphonylurea Receptor 1 (SUR1)
Number of coding exons: 1 (KCNJ11), 39 (ABCC8)
Prevalence of mutations in Exeter Cohort: 37% of referrals

Glutamate Dehydrogenase – GLUD1

Heterozygous activating mutations in GLUD1, which encodes the mitochondrial enzyme glutamate dehydrogenase (GDH), cause protein-sensitive hyperinsulinism. In the majority of cases the mutations arise spontaneously so there is often no family history of hyperinsulinism. Individuals usually present with a milder form of HH that is often diagnosed outside of the neonatal period and shows good response to diazoxide therapy. In some patients dietary protein restriction may also be required. A consistent feature is the presence of hyperammonaemia with plasma ammonium levels being persistently raised to two to three times the upper limit of normal. This feature of the disease has resulted in this subtype of hyperinsulinism being referred to as Hyperinsulinism/Hyperammonaemia (HA/HA) syndrome. There is also an increased risk of epilepsy in some patients.

Gene Information

Location: Chromosome 10q23.2 (hg38: chr10: 87,050,486 – 87,094,866)
Protein name: Glutamate Dehydrogenase (GDH)
Number of coding exons: 13
Prevalence of mutations in Exeter Cohort: 2.5% of referrals

 

Glucokinase – GCK

Patients with heterozygous activating GCK mutations often have a dominant family history of hypoglycaemia. The absence of a family history of HH should however not preclude testing as de novo mutations have also been reported in some individuals. In those families with multiple affected individuals the severity of symptoms will often vary between family members. The age at presentation ranges from birth to adulthood and in some cases is asymptomatic. Whilst individuals are often diazoxide responsive a few cases who required surgery have been described.

Gene Information

Location: Chromosome 7p13 (hg38: chr7:44,144,275 – 44,189,423)
Protein name: Glucokinase
Number of coding exons: 10
Prevalence of mutations in Exeter Cohort:  0.8% of referrals

 

Hepatocyte Nuclear Factor 4 Alpha – HNF4A

Patients with heterozygous inactivating HNF4A mutations are often born macrosomic and approximately 10% of individuals are diagnosed with hyperinsulinism within the first week of life. The clinical severity ranges from mild transient hypoglycemia that does not require pharmacological treatment to persistent HH treated with diazoxide for up to 8 years. As HNF4A mutations cause maturity onset diabetes of the young (MODY) patients will be at increased risk of developing diabetes in later life and will often have a family history of diabetes. For further information on HNF4A-MODY see out diabetes genes website.

Gene Information

Location: Chromosome 20q13.12 (hg38: chr20:44,355,801-44,429,671)
Protein name: Hepatocyte Nuclear Factor 4 alpha
Number of coding exons: 10
Prevalence of mutations in Exeter Cohort:  1.7% of referrals

 

Hydroxyacyl-coenzyme A dehydrogenase – HADH

Recessively inherited inactivating mutations in HADH cause leucine-induce HH which responds well to diazoxide. The clinical presentation is heterogeneous, with age at presentation ranging from birth to late infancy. In some patients increased plasma hydroxybutyrylcarnitine and urinary 3-hydroxyglutarate levels are observed.

Gene Information

Location: Chromosome 4q25 (hg38: chr4:107,989,784 – 108,035,175)
Protein name: Hydroxyacyl-CoA dehydrogenase
Number of coding exons: 8
Prevalence of mutations in Exeter Cohort: 2.6% of referrals

 

Monocarboxylate Transporter 1 (MCT1) – SLC16A1

Heterozygous activating mutations in SLC16A1 encoding monocarboxylate transporter 1 (MCT-1), cause exercise-induced hyperinsulinism (EIHI). For these patients treatment is not usually necessary as hypoglycaemic episodes may be prevented by avoiding strenuous exercise.

Gene Information

Location: Chromosome 1p13.2 (hg38: chr1:112,911,847-112,956,353)
Protein name: Hepatocyte Nuclear Factor 4 alpha
Number of coding exons: 5 (+1 non-coding exon)
Prevalence of mutations in Exeter Cohort: 0% of referrals

 

 

Syndromes featuring hyperinsulinaemic hypoglycaemia

Beckwith-Wiedemann Syndrome – Chromosome 11p15.5

Beckwith-Wiedemann Syndrome (BWS) is a severe overgrowth disorder characterized by macroglossia, abdominal wall defects, hemihypertrophy, macrosomia, hypoglycaemia and increased risk of tumors. A number of different genetic mechanisms can lead to BWS, all of which result in abnormalities in methylation at one of two imprinting centers (ICR1 and ICR2) on chromosome 11p15.5. In approximately 20-30% of cases BWS results from paternal uniparental disomy (UPD) across the 11p15.5 region leading to an imbalance in imprinting and dysregulation of genes that are important for cell cycle regulation. As UPD is a sporadic event which may occur during embryogenesis there is often variability in the tissues which are affected. This can explain the differences in phenotype observed between individuals with BWS. Testing for BWS is not a routine test, so please contact the laboratory directly to discuss individual cases.

TRMT10A

A recessively inherited loss of function mutation has been identified in the TRMT10A gene, which encodes a methyltransferase involved in the post-transcriptional modification of RNA, has been reported in three siblings from a single family with microcephaly, intellectual disability, short stature, delayed puberty, seizures and hyperinsulinaemic hypoglycaemia diagnosed outside of infancy (1). Post-prandial hyperglycaemia was also reported in one of the siblings.

Gene Information

Location: Chromosome 4q23 (hg38: chr4:99,546,709-99,564,017)
Protein name: tRNA methyltransferase 10A
Number of coding exons: 13

Hypoinsulinaemic Hypoketotic Hypoglycaemia – AKT2

Our laboratory can provide screening of the AKT2 gene. AKT2 encodes a serine/threonine protein kinase which is acts to mediate the physiological effects of insulin downstream to the insulin receptor. Activating AKT2 mutations lead to the autonomous activation of the downstream insulin signaling pathway resulting in hypoinsulinemic hypoketotic hypo-fatty-acidemic hypoglycemia. Individuals are also reported to have hemi-hypertophy.

Genetic testing for AKT2 is not offered as a routine test by the laboratory so please contact us directly if you would like to discuss the possibility of testing for an individual patient.

Gene Information

Location: Chromosome 19q13.2 (hg38: chr19:40,230,317-40,285,395)
Protein name: v-akt murine thymoma viral oncogene homolog 2
Number of coding exons: 13

 

References

  1. Gillis et al. TRMT10A dysfunction is associated with abnormalities in glucose homeostasis, short stature and microcephaly. J Med Genet. 2014 Sep;51(9):581-6.