Congenital hyperinsulinism is a disorder affecting the pancreatic beta cell, where insulin is inappropriately secreted during hypoglycaemia. It often appears within the first few weeks of life, and can have a severe impact on a neonate’s health and development. It is a genetically heterogeneous disorder, with 10 genes reported to cause isolated hyperinsulinism, and 28 known causes of syndromic hyperinsulinism. This thesis aimed to gain novel insights into the clinical and genetic basis of congenital hyperinsulinism, using the large cohort of individuals referred for genetic testing in Exeter. The first section of this thesis introduces the clinical, molecular, and genetic basis of congenital hyperinsulinism, along with how genetic testing and clinical management are carried out. The following section describes how the cohorts included in this thesis were recruited. In Chapter 1, I used clinical features commonly observed in the early stages of hyperinsulinism, including birth weight, response to treatment, and biochemical measurements, in order to identify the likelihood of individuals having a mutation in either ABCC8 or KCNJ11, the two most common causes of monogenic hyperinsulinism. I identified that an increased birth weight and failure to respond to diazoxide, the first line treatment for hyperinsulinism, are highly predictive of a mutation in one of these two potassium channel genes. These features can be used to predict an individual’s likelihood of carrying one of these mutations, which may have an impact on their medical management. In Chapter 2, I identified eleven cases of coinciding Down syndrome and congenital hyperinsulinism, and performed a review of the clinical features of these individuals in order to identify the links between these two disorders. I identified that the prevalence of Down syndrome in the Exeter hyperinsulinism cohort is around four times higher than that found in the general population. From the review of clinical details, I found that non-genetic risk factors for hyperinsulinism were present in the majority of cases: these risk factors included complications of gastric surgery, chemotherapy treatment for acute lymphoblastic leukaemia, prematurity, and intra-uterine growth restriction. This indicated that the link between Down syndrome and hyperinsulinism was most likely based in these non-genetic risk factors. In Chapter 3, I identified tandem duplications in the vicinity of the gene KDM6A in three patients. Point mutations and deletions in KDM6A are known to cause Kabuki syndrome and syndromic hyperinsulinism but there have been no reports of duplications causing disease. I used genetic and epigenetic analyses, including publically available data, in order to predict the likelihood that these duplications were leading to a loss of function, and therefore to disease. In one case, a frameshift leading to a premature stop codon was detected. In two cases, I performed analysis of DNA methylation data and compared it to known cases of Kabuki syndrome, showing that one duplication was likely to be pathogenic, while the other did not have evidence to support that conclusion. In Chapter 4, I identified individuals with clinical features suggestive of a disorder of DNA methylation, and performed whole genome sequencing in order to identify novel causes of syndromic hyperinsulinism. By this method, I identified two individuals with a protein-truncating variant in the gene MAGEL2, known to cause Schaaf-Yang syndrome. I identified that previous reports of the endocrine features of this disorder have indicated a predisposition towards hypoglycaemia, though this is caused by growth hormone deficiency in some cases. Ultimately, I believe these two cases of hyperinsulinism, in addition to one previously reported in the literature, provide sufficient evidence to include MAGEL2 in genetic testing panels for hyperinsulinism. In summary, this thesis has resulted in a better understanding of the clinical, molecular, and genetic basis of congenital hyperinsulinism. This understanding is vital, as it can inform medical management, identify new genes involved in insulin secretion pathways, and inform on the development of new treatments for this disorder. The work presented in this thesis expands our understanding of the most common cause of congenital hyperinsulinism, as well as identifying novel causes of syndromic hyperinsulinism, and helps us to determine avenues which should be followed in further studies of this disorder. It will also inform the development of guidelines for the management of monogenic hyperinsulinism, and inform on the genes that should be included in routine sequencing panels for this disorder.Research England