Neurofibromatosis type 1: II. Answers from animal models

Neurofibromatosis type 1 (NF1), or von Recklinghausen’s disease, is a complex disease resulting from mutations in the Nf1 gene. Moreover, the product of the NF1 gene neurofibromin also plays a very important role(s) in various aspects of the normal development and physiology of the organism. Although NF1 has been recognized for centuries, it is only relatively recently that there has been progress in the identification, isolation, and characterization of the NF1 gene and its protein product, neurofibromin (Seizinger et al., 1987; Fountain et al., 1989; Wallace et al., 1990; Marchuck et al., 1991). Nevertheless, progress in understanding the molecular regulation(s) and in identifying the molecular and cellular targets underlying the NF1 disease has lagged. There is much to be learned about the regulation(s) of the Nf1 gene expression and the function(s) and molecular interactions of neurofibromin. The role of neurofibromin in producing the multiple symptoms of the disease in the various cell types remains an enigma. Furthermore, there is still some controversy regarding the primary cellular target that in NF1 leads to neurofibroma and other NF1-associated symptoms and signs. This is the second part of a two-part review on NF1. The first part includes a general overview of the NF1 disease, the NF1 gene, and the genetic and cellular basis of the NF1 disease, with reference to controversial issues that hamper the full understanding of the disease and consequently hinder the efforts to find a cure (Lakkis and Tennekoon, 2000). In this part, the focus is on the animal models of NF1 disease, and we review both in vivo studies on these animals and in vitro studies using cells derived from the Nf12/2 mice. Although these studies have helped us to address some unanswered questions and enhance our knowledge about the molecular and cellular basis of NF1, additional questions have arisen that require further studies. Genetic manipulations in animals, especially in the mouse, have provided very good models to study and improve our understanding of human genetic diseases. The disruption of the Nf1 gene in the mouse is no exception. The targeted disruption of the Nf1 gene in the mouse (Jacks et al., 1994; Brannan et al., 1994) has not only provided new insights into the role of this gene in NF1 pathogenesis but has also revealed new and unexpected roles for NF1 gene in normal growth and development. Although the heterozygous mice appeared, in general, to be normal and did not display the classical symptoms of NF1 disease, Jacks et al. (1994) observed that, as these mice aged, they demonstrated a greater predisposition to a variety of tumor types, including lymphoma, lung adenocarcinoma, hepatoma, and fibrosarcoma. Furthermore, some of these mice developed tumors that are characteristically seen in human NF1, such as neurofibrosarcoma, pheochromocytoma, and myeloid leukemia. Cytological examination of tumor material showed that the wild-type Nf1 allele was lost in approximately half of the tumors, supporting the view that loss of heterozygosity (LOH) is required for tumorigenesis, thus making these mice a potential model for human disease. On the other hand, the homozygous mutant mice display a variety of developmental abnormalities. The most critical is a developmental cardiac defect that causes major circulatory dysfunction, leading to midgestation lethality (between days 12.5 and 14 of gestation). The mutant mice display generalized edema with systemic vascular congestion and signs of hemorrhage, all indicative of cardiac failure. Anatomical and histological analyses of the mutant hearts reveal many abnormalities, including double-outlet right ventricle (DORV) and ventricular septal defect (VSD) in addition to a dramatic enlargement of the endocardial cushions in the outflow tract in the atrioventricular region. The enlarged endocardial cushions most likely represent the critical abnormality, causing blockage of the inflow of circulating blood to the heart and causing the ultimate lethality. This endocardial cushion abnormality is caused by Ras up-regulation because of the absence of normal Nf1 gene product (Lakkis and Epstein, 1998), suggesting that this developmental process is regulated by