Dominant and recessive inheritance of morbid obesity associated with melanocortin 4 receptor deficiency

Abstract

Over 20 severely obese subjects in 11 independent kindreds have been reported to have pathogenic heterozygous mutations in the gene encoding the melanocortin 4 receptor (MC4R), making this the most common known monogenic cause of human obesity. To date, the detailed clinical phenotype of this dominantly inherited disorder has not been defined, and no homozygous subjects have been described. We determined the nucleotide sequence of the entire coding region of the MC4R gene in 243 subjects with severe, early-onset obesity. A novel two-base pair GT insertion in codon 279 was found in two unrelated subjects, and four novel missense mutations, N62S, R165Q, V253I, C271Y, and one mutation (T112M) reported previously were found in five subjects. N62S was found in homozygous form in five children with severe obesity from a consanguineous pedigree. All four heterozygous carriers were nonobese. Several features of the phenotype, e.g. hyperphagia, tendency toward tall stature, hyperinsulinemia, and preserved reproductive function, closely resemble those reported previously in Mc4r knock-out mice. In addition, a marked increase in bone mineral density was seen in all affected subjects. In transient transfection assays, the N62S mutant receptor showed a responsiveness to alphaMSH that was intermediate between the wild-type receptor and mutant receptors carrying nonsense and missense mutations associated with dominantly inherited obesity. Thus MC4R mutations result in a syndrome of hyperphagic obesity in humans that can present with either dominant or recessive patterns of inheritance.

Figure 1

Sequence variants detected in MC4R. The positions of the sequence variants described in Table 1 are indicated on a model of the MC4R protein structure. Amino acids are indicated as circles in single-letter code. Amino acids affected by mutations are shaded, and specific mutations are then indicated in a shaded oval. Shaded ovals outlined in black indicate mutations where phenotypic studies on patients have been performed. Codon number 1 refers to the initiator methionine.

Figure 2

Inheritance of mutations in MC4R. Cosegregation of mutational status with BMI is shown for the five families in the mutational screening study for whom family data were available. Also shown is the family tree for the family with the CTCT deletion reported previously by us (9). Filled symbols represent subjects with severe obesity. The first line of the symbols corresponds to the identification number. The second line shows the genotype: N, normal allele; M, mutant allele. The third and fourth lines show current age of subject (years) and BMI standard deviation score, respectively.

Figure 3

Signaling properties of mutant MC4Rs, N62S, GTins, CTCTΔ, and C271Y. HEK293 cells were transiently transfected with either empty pcDNA3 vector or the same vector expressing wild-type, CTCTΔ, GTins, N62S, and C271Y-mutant MC4Rs. The response to increasing concentrations of ligand (αMSH) was assessed by cotransfection with a cAMP-responsive reporter plasmid as described previously (18) and by measuring luciferase activity in a luminometer (see Methods). Data obtained were normalized for transfection efficiency by cotransfection of an internal control plasmid, pRL-CMV, which constitutively expresses Renilla luciferase. Data is expressed as a fold induction of luciferase activity. Each point represents the mean (± SE) of at least three independent experiments performed in quadruplicate.