Deletion of the MC4R Gene in a 9-Year-Old Obese Boy

Abstract

Background:

The most common monogenic form of obesity is caused by mutations in the melanocortin 4 receptor (MC4R) gene. More than 150 mutations have been reported in the MC4R gene, the majority being point mutations. Most individuals with MC4R gene mutations have early-onset obesity, hyperphagia, and increased longitudinal growth.

Methods:

A 9-year-old Caucasian boy was referred to genetics for obesity, food-seeking behavior, and developmental delay. History and physical exam were not consistent with Prader Willi syndrome, but revealed several minor anomalies. Owing to significant obesity and hyperphagia, a Prader Willi syndrome methylation test and a microarray were requested.

Results:

Methlylation testing for Prader Willi syndrome was normal. Microarray analysis revealed two changes: (1) A 2.6-Mb deletion at chromosome 18q21.31 was identified and contained several OMIM genes, including the MC4R gene, and (2) an 0.87-Mb duplication at chromosome region 16p13.3 was found and contained one gene. Parental samples revealed that the boy's father had the same deletion and duplication. This case appears to be the first with a deletion of 18q21.31 encompassing the MC4R gene presenting with features of hyperphagia and obesity.

Conclusions:

Haploinsufficiency of the MC4R gene either through whole gene deletion or nonsense or missense mutations is associated with a significant risk of obesity. The case emphasizes both the role of the MC4R gene in obesity as well as the importance of looking for chromosomal microdeletions/duplications as a cause of obesity in children with minor anomalies or developmental delay.

Introduction

The melanocortin 4 receptor (MC4R) is a seven-transmembrane G-protein-coupled receptor whose ligand, alpha-melanocyte-stimulating hormone, is a post-translational derivative of pro-opiomelanocortin.¹ Mutations in the MC4R gene are the most common monogenic form of obesity, with more than 150 distinct mutations reported to date.^2,3 Early-onset obesity, hyperphagia, and increased longitudinal growth are some of the features of individuals that have mutations in the MC4R gene.⁴

Methods

Early Case History

The proband, a male, was the second child born to an unrelated mother and father who were 27 and 29 years of age, respectively. Pregnancy was uneventful. The mother was on a salbutamol inhaler for asthma during the pregnancy, but was not on any other medications. There was no history of hypertension (HTN) or gestational diabetes and no exposure to cigarettes, alcohol, or other street drugs. He was delivered at 38.5 weeks by a scheduled caesarean section owing to a history of previous sections. Birth weight was 3.85 kg (8 lbs 10 oz) and he was 50 cm in length. He was discharged from the hospital at 3 days of age. He bottle fed well in the newborn period, and there was no history of failure to thrive or significant hypotonia. There were developmental concerns that were first recognized at 1 year of age. Ambulation was attained at 22 months of life. Speech was delayed with single words attained at 1.5 years and two-word combinations at age 3 years. He was socially interactive and did not have autistic features. In school, he was in a modified program, and at the age of 8, he was reading at a 5-year-old level. Formal psychoeducational testing was not done.

The boy was referred to the Medical Genetics Clinic at the age of 8 years because of a combination of features that include obesity, excessive food-seeking behavior, and developmental issues. The mother recalled that, after 2 years of age, he started to gain a significant amount of weight. After obtaining recorded weight and length, we confirmed that, at 19 months, his weight was at the 75th percentile and length was at the 50th percentile; by age 34 months, weight was 2.5 standard deviations (SDs) above the mean, and at 4 years and 10 months, weight was 5 SDs above the mean and height was between the 75th–95th percentile. At the time of assessment in the Medical Genetics Clinic, his mother reported that food-seeking behaviors had been occurring for several years. For example, on multiple occasions, empty cartons of chicken nuggets, cereal, and cookies were found under his bed. His mother pointed out that her son would take the chicken nugget carton out of the freezer and eat the chicken nuggets uncooked. Food records indicated that, for the most part, his parents were encouraging healthy eating and providing healthy food, such as fruits and vegetables. However, he consumed excessively more food than he was provided at meals; for example, he would have five or six apples or six or seven yogurts at one sitting. His behavior also included taking food from his siblings' plates. For a period of time, this was also an issue at school; although, after disciplinary action, this issue resolved. The mother also reported that he had a tendency to pick at his skin.

At the time of his first assessment at the Medical Genetics Clinic at 8 years of age, his head circumference was 55 cm (98th percentile), his height was 136 cm (90th percentile), and his weight was 56.6 kg (greater than +8 SDs). His BMI was 30.7 kg/m². He had slightly downslanting palpebral fissures. He had mild bitemporal narrowing. His ears were normal in position and placement. He had thick earlobes. He had a normal philtrum and a bulbous tip to his nose. His palate and uvula were normal. He did have a mild buffalo hump. Examination of the chest revealed prominent veins on the chest and arm, more noticeable on the right. He had significant gynecomastia (Fig. 1A,B).

Figure 1.

(A) Proband at 9 years of age. (B) Hand of proband at 9 years of age demonstrating hypoplasia of the middle phalanyx of the fifth finger.

He had mildly increased pigment in the axilla. Examination of the extremities revealed broad hands. The total right hand measured 14 cm (50th percentile), with D3 measuring 5.8 cm (50th percentile). His right fifth finger appeared to be disproportionately small. His left fifth finger appeared to have hypoplasia of the middle phalanx. He had broad, flat feet. He had normal tone and reflexes.

Family History

The boy's father was 177 cm, 80.4 kg, and his BMI was 25 kg/m², considered to be normal weight. The father had six siblings, five brothers, and one sister, including an identical twin (zygosity was not confirmed). Three siblings had a reported history of attention deficit hyperactivity disorder (ADHD) and two had a reported history of seizures. Many of these individuals' children were also reported as having ADHD. The father's identical twin brother reportedly put on a significant amount (e.g., 45 kg) of weight after quitting smoking and his weight was reported to be around 122 kg. Shortly after the proband's first assessment, the proband's father died of a cardiac arrest at the age of 39 years. Consent to review a copy of the autopsy was obtained from the family. The autopsy reported severe occlusive coronary artery disease and myocardial fibrosis. The cause of death was felt to be acute coronary insufficiency resulting from coronary arthrosclerosis. He was a smoker and had high blood pressure. The paternal parents are still living.

The proband's mother was in relatively good health other than a history of asthma. She was overweight (101 kg). She reported a prepregnancy weight of 60 kg and was not overweight as a child. She has one brother who is in good health. The maternal father is deceased. He had a history of diabetes, thyroid problems, HTN, and seizures. He came from a large sibship of 11 individuals; many of these individuals were reported to be obese and tall. Two of the siblings died in their fifties, reportedly from acute myocardial infarctions. The maternal mother is living, and there is no history of obesity in the maternal mother's family.

The proband has a full brother who is almost 12 years of age and reportedly of normal weight. This child has had some difficulties in school with math and reading. The proband also has a half-sister on his mother's side who is almost 15. She does very well in school and is of normal weight.

Follow-up Visit to Medical Genetics Clinic

The boy returned to the Medial Genetics Clinic for a follow-up visit 16 months later. During the clinic visit, his mother reported that his health had been fairly good. However, he continued to gain weight over the time period between clinic visits. At the second clinic visit, his weight was 82.6 kg (+7 SDs), an increase of 26 kg. His BMI was 38 kg/m². The boy's head circumference was 56 cm (+2 SDs) and his height was 146 cm (90th–95th percentile). Food-seeking behaviors continued to be quite significant, although he was not taking food from the plates of other children at school or at home. However, he continued to eat excessive amounts; for example, he would wake before anyone else in the household to seek out and consume all types of food in excess (e.g., 12 servings of ice cream or seven apples). If he was caught in the middle of eating more than he should and was asked to stop eating, he became upset. In the past, his mother had tried locking cupboards and the fridge, but she felt that this had not been helpful and did not seem to have an impact on the boy's weight.

At school, the boy was reported to be easy going and very helpful. However, he had a short attention span. He was unable to sit through the curriculum, but could spend hours playing electronic games, playing with puzzles, and building with Lego blocks. He was not involved in formal activities. Skin-picking behavior continued.

Cytogenetic Studies

The proband did not have the typical story of failure to thrive, although he did have other behaviors in keeping with Prader Willi syndrome, including excessive food-seeking and skin-picking behaviors as well as developmental problems. Investigations were ordered to rule out Prader Willi syndrome. In addition, a microarray analysis was ordered to rule out other microdeletion syndromes that have overlapping features with Prader Willi syndrome. Testing for Prader Willi syndrome (methylation testing and deletion analysis of 15q11 by microarray) was normal. However, microarray analysis indicated that the boy had a microdeletion at chromosome region 18q21.31. This deletion was 2.6 Mb (Fig. 2A,B) and contained 15 RefSeq genes, including OMIM genes MC4R, MALT1, RAX, and LMAN1. He was also found to have an 0.87-Mb duplication at chromosome region 16p13.3, which partially overlaps one RefSeq gene, RBFOX1, and includes only exon 1 of this gene. In order to further investigate the significance of the findings, parental samples were requested. The parental samples revealed that the boy's father also had the same deletion and duplication.

Figure 2.

(A) Genomic microarray analysis shows a 2.9-Mb deletion of chromosome region 18q21.31. (B) This region contains approximately 15 RefSeq genes, including MC4R (UCSC Genome Browser, hg18).

Discussion

This patient was found to have a deletion at 18q21.31 and duplication at 16p13.3. The significance of the 16p13.3 duplication is uncertain. The duplication at chromosome region 16p13.3 contained exon 1 of RBFOX1 (A2BP1), and it is not known whether expression of this gene has been altered. However, given the fact that the deletion at 18q21.31 contains the MC4R gene, it is likely that this deletion is a significant factor in the patient's features. The deletion at chromosome region 18q21.31 contained 15 genes including OMIM genes MC4R, MALT1, RAX, and LMAN1.

In 1998, the importance of the MC4R gene in the regulation of human body weight became apparent when mutations in the gene were first described as a cause of human obesity. Two groups independently reported heterozygous frameshift mutations in the human MC4R gene that were found to be associated with severe early-onset obesity^4,5; however, the precise mechanism responsible for this association could not be identified. It was proposed that the mutations found in the MC4R gene resulted in a loss of function, ultimately resulting in obesity owing to haploinsufficiency of the MC4R gene.

In order to better understand the mechanism by which MC4R mutations resulted in obesity, Cody and colleagues¹ studied a group of individuals with chromosome 18q deletions, some of whom had deletions that included the MC4R gene and some whose deletions did not include the gene. Individuals whose deletion included the MC4R gene were not obese, in comparison to the individuals whose deletion did not include the MC4R gene. Therefore, Cody and colleagues concluded that it was not haploinsufficiency that was the mechanism by which the MC4R gene mutations caused the obesity phenotype, but it could be a dominant negative effect. Cody and colleagues did point out that the other genes deleted on 18q could override the ability of the MC4R gene to affect growth. Indeed, the deletions in this study were all greater than 20 Mb and contained at least 70 additional genes. Since that time, multiple different missense and nonsense mutations in MC4R have been reported in severe early-onset childhood obesity,^6–9 and MC4R mutations now represent the most common known monogenic cause of human obesity.^2,3 However, the precise mechanism as to how MC4R mutations result in obesity remains unclear.

The current case is a proband with an extremely obese phenotype that has a 2.6-Mb deletion of 18q21.31 that includes the MC4R gene. This case indicates that haploinsufficiency of the MC4R gene is a significant factor in the proband's obese phenotype and thereby provides more evidence that mutations causing haploinsufficiency of this gene is a cause of obesity. In addition, the case emphasizes the importance of looking for microdeletions or -duplications as a cause of obesity in children with learning problems or minor anomalies.

Our case has raised a number of questions regarding the MC4R gene that we cannot answer at this point. Unfortunately, our patient's father, who also carried the 18q21.31 deletion, died of a cardiac arrest at the age of 39. It is not clear whether the deletion of the MC4R gene was a causative factor in his death. He did not have a reported history of food-seeking behavior or significant obesity as a child or adult, despite having the deletion involving the MC4R gene, indicating that although the proband's features support haploinsufficiency of the MC4R gene as a mechanism for obesity, other genetic and environmental factors influence the phenotype. A study published by Stutzmann and colleagues in 2008¹⁰ reported a penetrance of 63.5% in heterozygous MC4R mutation carriers and 94.6% in homozygous and compound heterozygous carriers. In addition, in reviewing the family history, there appeared to be a significant number of individuals with reported ADHD. Although ADHD is common, it does appear that individuals with MC4R gene mutations are at increased risk of ADHD.¹¹ Further family studies are needed to confirm whether the 18q21.31 deletion encompassing the MC4R gene is contributing to the ADHD in family members.

Footnotes

Acknowledgments

The authors thank the family for their participation and contribution to this clinical report. Parental consent was obtained for publication of this clinical report and photographs and has been included in the files submitted for review.

Author Disclosure Statement

No competing financial interests exist.

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