The extreme obesity of the obese (ob/ob) mouse is attributable to mutations in the gene encoding leptin, an adipocyte-specific secreted protein which has profound effects on appetite and energy expenditure. We know of no equivalent evidence regarding leptin's role in the control of fat mass in humans. We have examined two severely obese children who are members of the same highly consanguineous pedigree. Their serum leptin levels were very low despite their markedly elevated fat mass and, in both, a homozygous frame-shift mutation involving the deletion of a single guanine nucleotide in codon 133 of the gene for leptin was found. The severe obesity found in these congenitally leptin-deficient subjects provides the first genetic evidence that leptin is an important regulator of energy balance in humans.

Thiazolidinediones are a new class of antidiabetic agent that improve insulin sensitivity and reduce plasma glucose and blood pressure in subjects with type 2 diabetes. Although these agents can bind and activate an orphan nuclear receptor, peroxisome proliferator-activated receptor gamma (PPARgamma), there is no direct evidence to conclusively implicate this receptor in the regulation of mammalian glucose homeostasis. Here we report two different heterozygous mutations in the ligand-binding domain of PPARgamma in three subjects with severe insulin resistance. In the PPARgamma crystal structure, the mutations destabilize helix 12 which mediates transactivation. Consistent with this, both receptor mutants are markedly transcriptionally impaired and, moreover, are able to inhibit the action of coexpressed wild-type PPARgamma in a dominant negative manner. In addition to insulin resistance, all three subjects developed type 2 diabetes mellitus and hypertension at an unusually early age. Our findings represent the first germline loss-of-function mutations in PPARgamma and provide compelling genetic evidence that this receptor is important in the control of insulin sensitivity, glucose homeostasis and blood pressure in man.

Inherited defects in signaling pathways downstream of the insulin receptor have long been suggested to contribute to human Type 2 diabetes mellitus. Here we describe a mutation in the gene encoding the protein kinase AKT2/PKBβ in a family that shows autosomal dominant inheritance of severe insulin resistance and diabetes mellitus. Expression of the mutant kinase in cultured cells disrupted insulin signaling to metabolic end-points and inhibited the function of coexpressed, wild type AKT. These findings demonstrate the central importance of AKT signaling to insulin sensitivity in humans.Most forms of diabetes are likely to be polygenic in origin, although a number of monogenic forms are being recognised (1, 2). Although rare, these monogenic examples offer insights into the function of the affected gene in humans as well as offering important clues to understanding more common forms.We have been screening genomic DNA from 104 unrelated subjects with severe insulin resistance for mutations in genes that are implicated in insulin signalling. We identified a † To whom correspondence should be addressed. E-mail: sorahill@hgmp.mrc.ac.uk. * These authors contributed equally to this work. Europe PMC Funders Group Europe PMC Funders Author ManuscriptsEurope PMC Funders Author Manuscripts missense mutation in the serine/threonine kinase gene AKT2 in one Caucasian proband.AKT2 (also known as PKBβ) is highly expressed in insulin sensitive tissues and is activated in response to growth factors and related stimuli (3, 4) a process that requires its phosphorylation by the phosphoinositide-3 phosphate-dependent kinase activities designated PDK1 and PDK2 (3). The proband, (iii)/1 (Fig. 1D), is a non-obese 34 year old female who developed diabetes mellitus at 30 years of age. The proband, her non-obese mother, (ii)/2, maternal grandmother, (i)/2, and a maternal uncle, (ii)/3, were all heterozygous for a G to A substitution predicted to result in an R to H substitution at amino acid 274 (Fig. 1 A, B). All were markedly hyperinsulinemic (Table S1) and the mother and maternal grandmother developed diabetes mellitus in their late 30′s. Three other first-degree relatives available for study were all clinically normal with normal fasting glucose and insulin and were homozygous for the wild-type AKT2 sequence ( Fig. 1D and Table S1). This mutation was not found in genomic DNA of 1500 Caucasian control subjects from the UK.R274 forms part of an RD sequence motif within the catalytic loop of the AKT2 kinase domain that is invariant in AKT isoforms in all species, and is also highly conserved within the protein kinase family (Fig. 1C) (5). The RD motif includes the invariant D residue (D275 of AKT2) that performs an essential catalytic function in all protein kinases.R274 is positioned in the core of the catalytic domain, forming critical hydrogen bonds with the phosphate moiety of phosphoT309 in the activation segment permitting correct positioning the substrate peptide relative to the catalytic base and adenosine triphosphate (A...