Obesity associated with a mutation in a genetic regulator of adipocyte differentiation

BACKGROUND

There is increasing evidence of genetic factors leading to obesity, but the exact genes involved have not been defined. Peroxisome-proliferator-activated receptor gamma2 (PPARgamma2) is a transcription factor that has a key role in adipocyte differentiation, and therefore mutations of the gene for this factor might predispose people to obesity.

METHODS

We studied 358 unrelated German subjects, including 121 obese subjects (defined as those with a body-mass index [the weight in kilograms divided by the square of the height in meters] of more than 29). We evaluated these subjects for mutations in the gene for PPARgamma2 at or near a site of serine phosphorylation at position 114 that negatively regulates the transcriptional activity of the protein, using a polymerase-chain-reaction-based assay coupled with specific endonuclease digestion. The activity of the mutation identified was analyzed by retroviral transfection and overexpression in murine fibroblasts.

RESULTS

Four of the 121 obese subjects had a missense mutation in the gene for PPARgamma2 that resulted in the conversion of proline to glutamine at position 115, as compared with none of the 237 subjects of normal weight (P=0.01). All the subjects with the mutant allele were markedly obese, with body-mass-index values ranging from 37.9 to 47.3, as compared with a mean of 33.6 in the other obese subjects. Overexpression of the mutant gene in murine fibroblasts led to the production of a protein in which the phosphorylation of serine at position 114 was defective, as well as to accelerated differentiation of the cells into adipocytes and greater cellular accumulation of triglyceride than with the wild-type PPARgamma2. These effects were similar to those of an in vitro mutation created directly at the Ser114 phosphorylation site.

CONCLUSIONS

A Pro115Gln mutation in PPARgamma2 accelerates the differentiation of adipocytes and may cause obesity.

An association between NIDDM and a GAA trinucleotide repeat polymorphism in the X25/frataxin (Friedreich's ataxia) gene

Friedreich's ataxia is the most common hereditary ataxia and is frequently associated with disturbances of glucose metabolism. This autosomal recessive disease is caused by the decreased expression of a mitochondrial protein, frataxin, encoded by the X25 gene. Homozygous expansion of a GAA repeat in the first intron of X25 inhibits frataxin expression and is associated with clinical disease. We evaluated whether heterozygous expansions of the triplet repeat in the frataxin gene X25 may be associated with NIDDM in two genetically distinct populations--one in Germany (n = 358) and the other in the U.S. (n = 292)--using a polymerase chain reaction-based assay. Intermediate expansions (10-36 repeats), which are longer than normal but not sufficient for the appearance of the ataxia phenotype, were found in 24.7 and 27.3% of these two NIDDM cohorts compared with 7.6 and 6.3% of the matched control subjects (both P < 0.001). The odds ratios were 3.36 (95% CI 1.72-6.55) for the German group and 4.01 (2.08-7.74) for the U.S. group. Therefore, we conclude that the X25/frataxin GAA repeat polymorphism is associated with NIDDM in a frequency higher than any other mutation heretofore described. Further studies are needed to elucidate the possible role of frataxin in the pathogenesis of NIDDM.

Deficiency of phosphofructo-1-kinase/muscle subtype in humans impairs insulin secretion and causes insulin resistance

Non-insulin-dependent diabetes mellitus (NIDDM) is caused by peripheral insulin resistance and impaired beta cell function. Phosphofructo-1-kinase (PFK1) is a rate-limiting enzyme in glycolysis, and its muscle subtype (PFK1-M) deficiency leads to the autosomal recessively inherited glycogenosis type VII Tarui's disease. It was evaluated whether PFK1-M deficiency leads to alterations in insulin action or secretion in humans. A core family of four members was evaluated for PFK1-M deficiency by DNA and enzyme-activity analyses. All members underwent oral and intravenous glucose tolerance tests (oGTT and ivGTT) and an insulin-sensitivity test (IST) using octreotide. Enzyme activity determinations in red blood cells showed that the father (46 yr, body mass index [BMI] 22. 4 kg/m2) and older son (19 yr, BMI 17.8 kg/m2) had a homozygous, while the mother (47 yr, BMI 28.4 kg/m2) and younger son (13 yr, BMI 16.5 kg/m2) had a heterozygous PFK1-M deficiency. DNA analyses revealed an exon 5 missense mutation causing missplicing of one allele in all four family members, and an exon 22 frameshift mutation of the other allele of the two homozygously affected individuals. The father showed impaired glucose tolerance, and the mother showed NIDDM. By ivGTT, both parents and the older son had decreased first-phase insulin secretion and a diminished glucose disappearance rate. The IST showed marked insulin resistance in both parents and the older, homozygous son, and moderate resistance in the younger son. PFK1-M deficiency causes impaired insulin secretion in response to glucose, demonstrating its participation in islet glucose metabolism, and peripheral insulin resistance. These combined metabolic sequelae of PFK-1 deficiency identify it as a candidate gene predisposing to NIDDM.

Circulating tumor necrosis factor alpha is elevated in male but not in female patients with type II diabetes mellitus

The cytokine tumor necrosis factor alpha (TNF alpha) was proposed to mediate obesity related insulin resistance upon production in fat cells and to participate in tissue remodelling leading to vascular complications upon being released by macrophages. To assess its putative role in diabetes we determined plasma levels of TNF alpha in 105 adult humans. Male nondiabetic subjects had significantly lower TNF alpha levels than female controls (4.4 +/- 0.3, n = 17 vs. 6.6 +/- 1.0 pg/ml, n = 13; p = 0.049). Men with NIDDM had elevated TNF alpha (6.7 +/- 0.6 pg/ml, n = 34) compared to nondiabetic subjects (4.4 +/- 0.3 pg/ml, n = 17; p = 0.012). Such a difference was not apparent in women. Levels of TNF alpha were correlated with serum triglyceride levels in male controls (r2 = 0.64; p = 0.007) but not in NIDDM. Neither body mass index nor glycosylated hemoglobin correlated with TNF alpha in any of the groups. The presence of retinopathy (p = 0.046) but not of neuropathy or nephropathy or macroangiopathy was associated with significantly elevated plasma TNF alpha. We conclude that plasma levels of TNF alpha are sex-dependent and that increased TNF alpha occurs in male but not female NIDDM and may participate in the development of diabetic complications.

Muscle phosphofructokinase deficiency in two generations

Phosphofructokinase (PFK) is the key regulatory enzyme of glycolysis. Patients lacking the muscular isoform of PFK typically present with myopathy and compensated hemolysis (glycogenosis type VII or Tarui's disease). Since 1965 about 30 cases of muscular PFK deficiency have been reported. In most cases family history suggests a recessive inherited trait. We describe a family of Ashkenazi Jewish origin with two members in subsequent generations suffering from muscular PFK deficiency. The propositus, a 19-year-old male patient presented with weakness, myalgias and exercise intolerance since early infancy. His father also had early fatigue on exercise with myalgias; the mother and a 12-year-old brother were asymptomatic. Muscle biopsy of both the propositus and his father showed increased glycogen storage and absent histochemical stain for PFK. Biochemical studies of muscle revealed a markedly decreased PFK activity and DNA analysis of the muscle PFK gene revealed compound heterozygosity in both cases. This is the first description of proven muscle PFK deficiency (glycogenosis type VII) in two subsequent generations.

Restricted geographical extension of the association of a glucagon receptor gene mutation (Gly40Ser) with non-insulin-dependent diabetes mellitus

A mutation within the glucagon receptor gene (GRG) at codon 40 (Gly40Ser) was reported to be associated with non-insulin-dependent diabetes mellitus (NIDDM) in France and Sardinia. Since the frequency of the mutation, about 5% in the French and 8% in the Sardinian group was higher than that of any of the candidate gene mutations described so far, it appeared to be relevant to determine the prevalence within different populations. While a single mutation has not been found recently either in a Japanese, a Finnish or a Dutch cohort, but only in a British study in 2.2% of NIDDM patients, in this study we investigated the association of this mutation with NIDDM in 104 German patients by polymerase chain reaction-restriction fragment length polymorphism analysis (PCR-RFLP). None of the German NIDDM patients had a Gly40Ser mutation. The results indicate that the mutation plays little role in susceptibility to NIDDM in this German region, and also indicate the genetic heterogeneity in NIDDM, and further emphasize the importance of studies in different ethnic groups.

Widespread expression of a new putative hormone extrusion pump: NG-TRA transporter protein in human and rat tissue

In 1991 Becker and co-workers isolated a new ATP-dependent transporter from a neuroblastomaXglioma-hybrid-cell line, called NG-TRA. This protein was proposed to represent a hormone extrusion pump. To gain further insight into its significance we studied the expression of NG-TRA in human and rat tissues and compared the nucleotide sequences derived from both species using reverse transcription PCR, subcloning and nucleotide sequencing. A 97% sequence homology was found between the original and the human subcloned fragments, derived from thyroid and colon carcinoma tissue, while the rat subcloned fragments, derived from brain cortex and heart muscle, showed a 25 amino acid deletion at the 3'end compared with both the original and human subclones. Expression of NG-TRA at the mRNA level was found in all tissues studied except blood and fat tissue, and additionally rat lung tissue. We conclude that NG-TRA is a widely expressed protein consistent with a general function in diverse species.

The ethics of research into the cause(s) of homosexuality

This paper gives a brief overview on the current state of the art of biomedical research on homosexuality. It concludes that so far the cause(s) of homosexuality is (are) unknown and that biomedical research has failed to provide evidence for a possible causation of homosexuality. We do think, however, that homosexuality is not merely a social construction and the quest for its cause is intelligible. It is less clear, however, whether research into the cause(s) of homosexuality should be done at all. We explore the different arguments brought forward in favor of doing this research and reject all of them. Furthermore, we argue that research into the causes of homosexuality is at the present time unethical and should not be undertaken. Research into the causes of homosexuality assumes more often than not that homosexuality is one or another form of mental illness or undesirable deviance from the heterosexual norm, and should be cured. These views will be criticized as heterosexist.