Analysis of genomic imprinting of Gs alpha gene

Somitic disruption of GNAS in chick embryos mimics progressive osseous heteroplasia

Progressive osseous heteroplasia (POH) is a rare developmental disorder of heterotopic ossification (HO) caused by heterozygous inactivating germline mutations in the paternal allele of the GNAS gene. Interestingly, POH lesions have a bewildering mosaic distribution. Using clinical, radiographic, and photographic documentation, we found that most of the 12 individuals studied had a lesional bias toward one side or the other, even showing exclusive sidedness. Most strikingly, all had a dermomyotomal distribution of HO lesions. We hypothesized that somatic mutations in a progenitor cell of somitic origin may act on a background of germline haploinsufficiency to cause loss of heterozygosity at the GNAS locus and lead to the unilateral distribution of POH lesions. Taking advantage of the chick system, we examined our hypothesis by mimicking loss of heterozygosity of GNAS expression using dominant-negative GNAS that was introduced into a subset of chick somites, the progenitors that give rise to dermis and muscle. We observed rapid ectopic cartilage and bone induction at the axial and lateral positions in a unilateral distribution corresponding to the injected somites, which suggests that blocking GNAS activity in a targeted population of progenitor cells can lead to mosaic ectopic ossification reminiscent of that seen in POH.

Genetic and hereditary aspects of childhood obesity

Genetic factors are involved in the regulation of body weight and in determining individual responses to environmental factors such as diet and exercise. The identification and characterization of monogenic obesity syndromes have led to an improved understanding of the precise nature of the inherited component of severe obesity and has had undoubted medical benefits, whilst helping to dispel the notion that obesity represents an individual defect in behaviour with no biological basis. For individuals at highest risk of the complications of severe obesity, such findings provide a starting point for providing more rational mechanism-based therapies, as has successfully been achieved for one disorder, congenital leptin deficiency.

Monogenic obesity in humans

Until relatively recently, the small number of identifiable inherited human diseases associated with marked obesity were complex, pleiotropic developmental disorders, the molecular basis for which were entirely obscure. The molecular basis for many of these complex syndromes, such as Bardet Beidl syndrome, has been revealed, providing novel insights into processes essential for human hypothalamic function and energy balance. In addition to these discoveries, which were the fruits of positional cloning, the molecular constituents of the signaling pathways responsible for the control of mammalian energy homeostasis have been identified, largely through the study of natural or artificial mutations in mice. We discuss the increasing number of human disorders that result from genetic disruption of the leptin-melanocortin pathways that have been identified. Practical implications of these findings for genetic counseling, prognostication, and even therapy have already emerged.

Minireview: human obesity-lessons from monogenic disorders

Genetic influences on the determination of human fat mass are profound and powerful, a statement that does not conflict with the obvious influence of environmental factors that drive recent changes in the prevalence of obesity. The assertion of the importance of genetic factors has, until recently, largely been based on twin and adoption studies. However, in the last 6 yr, a number of human genes have been identified in which major missense or nonsense mutations are sufficient in themselves to result in severe early-onset obesity, usually associated with disruption of normal appetite control mechanisms. Progress in the identification of more common, subtler genetic variants that influence fat mass in larger numbers of people has been slower, but discernible. Human genetics will continue to make an invaluable contribution to the study of human obesity by identifying critical molecular components of the human energy balance regulatory systems, pointing the way toward more targeted and effective therapies and assisting the prediction of individual responses to environmental manipulations.