p57 Expression in Normal Islet Cells and in Hyperinsulinism of Infancy

Most cases of hyperinsulinism of infancy (HI) are caused by mutations in either the sulfonylurea receptor-1 (SUR1) or the inward rectifying K channel Kir6.2, two subunits of the -cell ATP-sensitive K channel (KATP channel). Histologically, HI can be divided into two major subtypes. The diffuse form is recessively inherited and involves all -cells within the pancreas. Focal HI consists of adenomatous hyperplasia within a limited region of the pancreas, and it is caused by somatic loss of heterozygosity (LOH), including maternal Ch11p15-ter in a -cell precursor carrying a germline mutation in the paternal allele of SUR1 or Kir6.2. Several imprinted genes are located within this chromosomal region, some of which, including p57 and IGF-II, have been associated with the regulation of cell proliferation. Using double immunostaining, we examined p57 expression in different islet cell types, in control pancreases from different developmental stages (n 15), and in pancreases from patients with both diffuse (n 4) and focal HI (n 9). Using immunofluorescence and computerized image analysis, we quantified IGF-II expression in -cells from patients with focal HI (n 8). Within the pancreas, p57 was specifically localized to the endocrine portion. -Cells demonstrated the highest frequency of expression (34.9 2.7%) compared with 1–3% in other cell types. The fraction of -cells expressing p57 did not vary significantly during development. -Cells within the focal lesions did not express p57, whereas IGF-II staining inside focal lesions was mildly increased compared with unaffected surrounding tissue. In conclusion, we demonstrate that p57 is expressed and is paternally imprinted in human pancreatic -cells. Loss of expression in focal HI is caused by LOH and is associated with increased proliferation and increased IGF-II expression. Manipulation of p57 expression in -cells may provide a mechanism by which proliferation can be modulated, and thus this gene is a potential therapeutic target for reversing the -cell failure observed in diabetes. Diabetes 50:2763–2769, 2001 Hyperinsulinism of infancy (HI) is a rare genetic disorder with a prevalence in outbred populations of 1/50,000 live births (1,2). An incidence as high as 1/2,500 has been reported in inbred populations (2,3). The molecular basis of the disease was recently elucidated, and most cases are caused by mutations in either the sulfonylurea receptor-1 (SUR1) gene ABCC8 or the inward rectifying K channel Kir6.2 gene KCNJ11, the two subunits of the -cell ATP-sensitive K channel (KATP channel) (4–7). A minority of patients have glucokinase or glutamate dehydrogenase-1 mutations, whereas in 40–50% of the patients, the genetic cause of the disease is still not known (5,8–10). The clinical presentation of HI can be variable, ranging from mild disease to severe, life-threatening hypoglycemia that, if not adequately treated, causes irreversible neurological damage (11,12). The histological appearance of the pancreases from affected children is heterogeneous and can be subdivided into two major forms: diffuse HI and focal HI (13–15). The diffuse form is more common and bears some histological characteristics of nesidioblastosis, a normal phenomenon observed in the fetus and newborn that includes poorly defined islets, small clusters of endocrine cells scattered throughout the exocrine tissue, and a high frequency of endocrine cells interposed between ductular cells (15–17). Focal HI can generally be recognized as a discrete region of adenomatous hyperplasia, often too small to be identified macroscopically. Histologically, the lesion is comprised of nodules of endocrine and exocrine elements. The -cells are pleomorphic, some having giant nuclei and abundant cytoplasm (13). The rest of the pancreas has normal endocrine architecture for age, with -cells containing small nuclei and shrunken cytoplasm (18). We have previously reported increased frequencies of proliferating -cells in pancreases from HI patients and in pancreases in early stages of human development. Focal HI presented the highest proliferation frequency compared with diffuse HI and control subjects (19). The mechanisms regulating the rate of -cell proliferation are not known; however, the genetic alteration in focal HI may provide an insight into the control of -cell turnover. Focal HI is caused by the somatic loss of part of the short arm of maternal chromosome 11 in a -cell precursor of a patient carrying a mutant SUR-1 gene on the paternal allele (20,21). In all cases, it is the paternal allele that carries the mutation and the maternal allele that is somatically lost, suggesting that the gene(s) responsible From the Department of Endocrinology and Metabolism, Hebrew University, Hadassah Medical Center, Jerusalem, Israel; the Department of Pathology, Hebrew University, Hadassah Medical Center, Jerusalem, Israel; the Division of Endocrinology/Diabetes, Children’s Hospital of Philadelphia, Philadelphia, Pennsylvania; and the London Centre for Paediatric Endocrinology and Metabolism, Great Ormond Street Hospital for Children National Health Service Trust and the Institute of Child Health, University College London, U.K. Address correspondence and reprint requests to Benjamin Glaser, Director, Endocrinology and Metabolism Service, Hebrew University, Hadassah Medical Center, POB 12000, 91120 Jerusalem, Israel. E-mail: [email protected]. Received for publication 11 May 2001 and accepted in revised form 17 September 2001. Cdk, cyclin-dependent kinase; HI, hyperinsulinism of infancy; IOD, integrated optical density; KATP, ATP-sensitive K channel; LOH, loss of heterozygosity; SUR1, sulfonylurea receptor-1.