Lipase gene fusion: a new route to chronic pancreatitis

Chronic pancreatitis is characterized by longterm progressive inflammation of the pancreatic gland, leading to permanent deterioration of its structure and to development of malabsorption, debilitating pain and diabetes mellitus. The annual incidence of the disease has been estimated to 5-10 per 100,000 persons in Western countries. The persistent pancreatic inflammation also confers a high risk for pancreas cancer, one of the major causes of cancer-related death in both men and women. The most common explanation for chronic pancreatitis is long-term alcohol abuse, accounting for around 70% of all cases. If the disease presents as an autosomal dominant condition in a family, the term hereditary pancreatitis (HP) is often used. In 1996, a dominant loss-of-function mutation in the cationic trypsinogen gene PRSS1 was identified as the underlying cause of HP [1]. In the two decades following this first demonstration of a genetic predisposition to chronic pancreatitis, variants in several other genes (e.g. SPINK1, CTRC, CPA1) [2] have been identified to increase the likelihood of disease not only in HP, but also in idiopathic (unknown cause) or even alcoholic pancreatitis. Apparently, the additive effects of environmental and inherited factors shape the ”complex disease risk” of each individual for developing chronic pancreatitis. Most pancreatitis genes discovered so far either directly encode components of the protease/antiprotease system of the exocrine pancreas or are likely to perturb this system indirectly. We have now implicated a new pathway in chronic pancreatitis by finding that a lipase-encoding gene is a significant risk factor for the disease. In a recent paper published in Nature Genetics, we reported that an allele of the carboxyl-ester lipase gene (CEL), carried by 0.5-1.0% of individuals in the general population, is more than fivefold overrepresented in subjects with idiopathic chronic pancreatitis [3]. A very intriguing twist is the way that this risk allele has arisen: it appears to stem from a nonallelic homologous recombination event between CEL and its neighboring pseudogene CELP. The result is a hybrid gene, denoted CEL-HYB, in which the proximal part stems from CEL and the distal part from CELP (Figure 1, top). CEL is a remarkable gene due to its very polymorphic nature, and this is not the first time it has been linked to pancreatic disease. In 2006, we discovered that rare CEL mutations can cause an autosomal dominant syndrome of maturity-onset diabetes of the young (MODY) and exocrine pancreatic dysfunction, denoted MODY8 [4]. The hallmarks are exocrine insufficiency detectable in childhood, onset of diabetes in the third or fourth decade of life and development of pancreatic cysts after the age of forty [4, 5]. Although CEL is implicated in both MODY8 and chronic pancreatitis, the molecular mechanisms involved seem not to be identical. The last of the eleven CEL exons contains a variable number of tandem repeat (VNTR) region. In MODY8, a single base pair deletion has occurred in the first repeated segment, leading to a different reading frame of the C-terminus. The resulting “junk” protein (CEL-MUT, Figure 1, bottom) has altered biochemical and cellular properties, is taken up by endocytosis upon secretion, and may have a toxic effect on pancreatic cells [6]. In subjects carrying the newly discovered CEL-HYB allele, the long VNTR is exchanged Editorial