A Clinical Guide to Pediatric Weight Management and Obesity, 1st Edition
Specific Genetic Causes of Obesity
More than 600 genes, gene markers, and chromosomal regions have been associated with human obesity (1).
Mechanisms of inheritance include the following:
- Single gene mutations
- Autosomal and recessive inheritance
- Candidate genes with obesity as an associated feature
- Linkages with obesity-related phenotypes
The following factors have all been associated with genetic markers (1):
- Body weight
- Body mass index (BMI)
- Fat distribution
- Body composition
- Phenotypes related to energy expenditure
- Waist circumference
- Metabolic syndrome
- Energy and macronutrient intake
- Age at adiposity rebound (1)
The association of obesity with a wide array of genes is not surprising because obesity is intimately associated with energy regulation, a critical factor in survival. Also not surprising is the interaction of genetic susceptibility with the environment. This susceptibility manifests itself in populations who transition from energy-scarce to energy-dense environments either geographically or through successive generations as lifestyles change. The following sections describe a number of genetic syndromes and mutations associated with obesity and give a picture of some of the associated anomalies, which, if seen, should trigger genetic evaluation of the obese child.
Genetic Syndromes and Mutations
Melanocortin Receptor 4 Mutation
Single gene mutations are a rare cause of obesity. The melanocortin receptor 4 mutation is the most common of these. Leptin stimulates neurons in the arcuate nucleus of the hypothalamus, which express α-melanocyte-stimulating hormone (MSH), and agouti-related peptide, which binds to the MC4R receptor to produce a decrease in food intake (2). Heterozygous missense mutations of MC4R have been found in severely obese children (3).
In a study of obese adults, the prevalence of MC4R mutations was similar in patients who developed obesity in childhood (2.83%) and in those becoming obese in adulthood (2.35%) (4). In a family study with patients who had the onset of obesity before age 10 years, 5.8% had mutations of the MC4R gene. Both homozygous and heterozygous inheritance gave rise to early onset obesity and a pattern of codominance, with the homozygous individuals more obese than the heterozygous individuals. Mutation of MC4R is also associated with severe hyperinsulinemia, which precedes hyperphagia and obesity. Unexpectedly, 32% of the individuals in this study, who were heterozygous, were not obese (5).
Prader-Willi syndrome (PWS) affects about 1 per 10,000 to 1 per 15,000 births. Seventy-three percent of cases result from a deletion of 15q11-q13 from the paternally derived chromosome, involving approximately 4 MB of DNA. The exact gene that causes this syndrome is unknown. Twenty-five percent of cases result from uniparental disomy, which involves inheriting two maternal alleles for chromosome 15 and is associated with advanced maternal age. Rarely, the paternally derived chromosome having maternal DNA methylation can cause Prader-Willi syndrome (4).
Decreased fetal movement and/or abnormal fetal position at delivery can be a prenatal manifestation of the hypotonia associated with PWS. In the neonate, hypogonadism may be present and can include a hypoplastic scrotum and bilateral or unilateral cryptorchidism. The newborn with PWS can also have poor suck and feeding problems and may present as an infant with failure to thrive. Hyperphagia and food-seeking behavior can become evident between 1 and 6 years of age and are characteristic of this syndrome. The hypothalamic abnormality in PWS results in lack of satiety, and this combined with decreased caloric requirement due to hypotonia, decreased lean body mass, and decreased activity results in obesity (6). Ghrelin, an enteric hunger-producing hormone, is elevated in patients with PWS independent of their BMI (7). Phenotypic features of PWS include a narrow bifrontal facial diameter, almond-shaped palpebral fissure, narrow nasal bridge, and microacria (small
hands and feet), with 50% of children reported to have hypopigmentation for the family skin tone. Short stature with growth hormone deficiency is also in the spectrum of findings. Manifestations of hypogonadism include small genitalia, incomplete or delayed pubertal development, and, not uncommonly, infertility. Delayed motor development is always present, with delay of early milestones, language and cognitive delay, and learning disabilities as part of the spectrum. Behavior difficulties are not unusual; temper tantrums, stubborn behavior, obsessive-compulsive tendencies, and difficulty with change are the most common (8). Five percent to 10% of patients may have significant mental illness, including psychosis, bipolar disorder, and obsessive-compulsive disorder. Some special characteristics of this syndrome include having a high threshold for pain, thick viscous saliva, skin picking, high threshold for vomiting, and unusual skill with puzzles (8).
Classical diagnostic criteria for PWS are listed in Table 14.1 (9). Recently, diagnostic criteria have been expanded to lead to a higher likelihood of diagnosis and earlier intervention. In a review of criteria for diagnosis, children between 2 and 6 years of age with global developmental delay and hypotonia and a history of being a “floppy baby” and having poor feeding with a weak suck would meet criteria for diagnostic testing. By 6 and 12 years of age, children with developmental delay, hyperphagia, and obesity with a similar history of hypotonia in infancy would be candidates for testing. By 13 years and older, evaluation of patients with a similar history, obesity, hyperphagia, and hypogonadotropic hypogonadism would include testing for PWS (6). Genetic testing is performed by DNA methylation test (6). Table 14.2lists features sufficient to prompt DNA testing.
Control of the nutritional environment is crucial in the treatment of PWS. Restricting caloric intake and access to food is the only reliable therapeutic strategy to prevent or limit weight gain. Children with PWS are characterized by a constant drive to find and eat food; they may sneak and hide food, ask others for food, and eat beyond satiety. Parents, family members, and caregivers must be sensitive to these characteristics and work to create a nutritional and activity environment appropriate for these children. Supporting parents and caregivers, educating school regarding portion sizes and access to food, and planning for long-term care are all important aspects of ongoing care.
SA is a girl, 3 years and 8 months old, whose mother and aunt bring her to your office for a respiratory illness. As you begin to evaluate her, you note that her weight is 21.2 kg, which is greater than the 95th percentile, and her height is 91.3 cm, which
is less than the 5th percentile, giving her a BMI of 25.4. Her mother tells you that SA was a premature baby with a birth weight of 3 lb 4 oz at 35 weeks. The mother's pregnancy was complicated by gestational diabetes and premature rupture of membranes. The family history is positive for obesity, hypertension, diabetes, thyroid disease, and cardiovascular disease. SA had a complicated neonatal course, which involved respiratory distress syndrome. Mom also noted that a gastrostomy tube had to be placed for poor feeding and hypotonia, but now she is growing well and is “hungry all the time.” Over the next few years, SA had repeated episodes of respiratory illness. On physical examination you note that SA has thick saliva, central obesity, and small-appearing hands and feet. You discuss the possibility of a genetic etiology for SA's obesity and developmental delay with the mother and order a methylation study for PWS.
TABLE 14.1. Prader-Willi syndrome—diagnostic criteria
Three Weeks Later
Three weeks later when SA returns to your office, she is feeling much better. You have asked her mother and father to come in to discuss the results of testing. SA
does have PWS. You explain the results and describe the syndrome to the parents. You also arrange to have the parents tested for uniparental disomy. They have questions that revolve around SA's development and her eating behavior. You explain about hypothalamic control of eating and the fact that SA, by virtue of having PWS, needs environmental control of access to food to manage her weight. SA's parents are upset but somewhat relieved to find a reason for SA's developmental delay and her food-seeking behavior. You arrange to see the family again in 1 month.
TABLE 14.2. Features sufficient to prompt DNAtesting for Prader-Willi syndrome
One month later, SA returns, having lost 3 lb. Her parents say they have spoken to other relatives and the daycare she attends and have asked everyone to limit her access to food. They have also eliminated sugar-containing beverages, as you have requested. You urge the parents to keep SA active and ask them to use her stroller as little as possible, encouraging walking and supervised free play. You also ask them to contact SA's school about early developmental programs. You arrange to follow up with her monthly.
Other Single Gene Mutations
Other single gene mutations are listed in Table 14.3.
Albright Hereditary Osteodystrophy
Albright hereditary osteodystrophy is a sex-influenced autosomal dominant syndrome with a female-to-male ratio of 2:1.
TABLE 14.3. Single gene mutations
One of the earliest identifiable features of the syndrome is early onset of hypocalcemia, with 80% of affected children having hypocalcemia in the first or second year of life. Other features in addition to obesity and hypocalcemia include the following:
- Short stature
- Ectopic calcifications
- Mental retardation
Short stature is characterized by generalized shortening of the extremities and cone-shaped, absent, or prematurely closed epiphysis (14).
Autosomal Recessive Disorders
Bardet-Biedl syndrome is a heterogeneous autosomal recessive disorder associated with five separate genetic loci: 11q13 (BBS1), 16q21 (BBS2), 3p12-13 (BBS3), 15q22.22-q23 (BBS4), and 2q31 (BBS5).
Characteristics of this syndrome include the following:
- Mild mental retardation
- Pigmentary retinopathy with loss of visual acuity and dark adaptation
- Renal dysfunction
Symptoms are variable among subgroups: 90% of adults in all groups are above the 90th percentile for weight, children younger than 10 years may be below the 90th percentile in some subtypes, and obligate heterozygotes are reported to have increased incidence of obesity, diabetes, hypertension, and renal disease. Pigmentary retinopathy in this syndrome is associated with abnormal electroretinograms in infancy, degeneration of retinal photoreceptors affecting cones and rods, and progressive loss of visual acuity and dark-adapted sensitivity. Visual acuity decreases to the 20/100 range by about 11 to 12 years, and 93% of patients older than 30 years are legally blind. There is universal infertility in males with hypogonadism, poorly developed secondary sexual characteristics, and primary testicular pathology. Hypothalamic-pituitary and ovarian dysfunction is present in females; secondary sexual characteristics are normal, but there is an increased rate of structural genitourinary abnormalities. Ninety percent of patients have renal involvement, which is characterized by heterogeneous parenchymal lesions, distal tubular dysfunction, and glomerular disease (15).
Other Autosomal Recessive Disorders
Other autosomal recessive disorders are listed in Table 14.4.
Obesity has also been associated with X-linked disorders (Table 14.5).
TABLE 14.4. Autosomal recessive disorders
TABLE 14.5. X-linked disorders
- Perusse L, Rankinen T, Zuberi A, Chagnon YC, Wesinagel AJ, Argyropoulos G, Waltsm B, Snyder EE, Bouchard C. The human obesity gene map; the 2004 update. Obes Res.2005;13(3):381–490.
- Lubrano-Berthelier C, Le Stunff C, Bougneres P, Vaisse C. A homozygous null mutation delineates the role of the melanocortin 4 receptor in humans. J Clin Endocrinol Metab.2004;89(5):2028–2032.
- Dubern B, Clement K, Pelloux V, Froguel P, Girardet JP, Guy-Grand B, Tounian P. Mutational analysis of melanocortin-4 receptor, agouti-related protein and alpha melanocyte stimulating hormone genes in severely obese children. J Pediatr.2001;139(2):204–209.
- Lubrano-Berthelier C, Dubern B, Lacorte JM, Picard F, Shapiro A, Zang S, Bertais S, Hercberg S, Basdevant A, Clement K, Vaisse C. Melanocortin 4 receptor mutations in a large cohort of severely obese adults; prevalence, functional classification, genotype-phenotype relationship and lack of association with binge eating. J Clin Endocrinol Metab.2006;91(5):1811–1818.
- Farooqi IS, Keogh JM, Yeo GS, Lank EJ, Cheetham T, O'Rahilly S Clinical spectrum of obesity and mutations in the melanocortin 4 receptor gene. N Engl J Med.2003;348(12):1085–1095.
- Gunay-Aygun M, Schwartz S, Heeger S, O'Riordan MA, Cassiday SB. The changing purpose of Prader-Willi syndrome (clinical diagnostic criteria and proposed revised criteria). Pediatrics.2001;108(5):e92.
- Cummings DE, Clement K, Parnell JQ, Vaisse C, Foster KE, Frayo RS, Schwartz MW, Basdevant A, Weigle DS. Elevated plasma ghrelin levels in Prader-Willi syndrome. Nat Med.2002;8(7):643–644.
- Cassidy SB. Prader-Willi syndrome. J Med Genet.1997;34(11):917–923.
- Holm VA, Cassidy SB, Butler MG, Hanchett JM, Greenswag LR, Whitman BY, Greenberg F. Prader Willi syndrome: consensus diagnostic criteria. Pediatrics.1993;91(2):398–402.
- Clement K, Vaisse C, Lahlou N, Cabrol S, Pelloux V, Cassuto D, Gourmelen M, Dina C, Chambaz J, Lacorte JM, Basdevant A, Bougneres P, Lebouc Y, Froguel P, Guy-Grand B. A mutation in the human leptin receptor gene causes obesity and pituitary dysfunction.Nature.1998;392(6674):398–401.
- O'Rahilly S, Gray H, Humphreys PJ, Krook A, Polonsky KS, White A, Gibson S, Taylor K, Carr, C. Brief report: impaired processing of prohormones associated with abnormalities of glucose homeostasis and adrenal function. N Engl J Med.1995;333(21):1386–1390.
- Rimoin DL, Phillips JA. Genetic disorders of the pituitary gland. In: Rimoin DL, Connor JM, Pyeritz RE, eds. Principles and practice of medical genetics, Vol. I, 3rd ed. New York: Churchill Livingstone; 1997:1331–1364.
- Schinzel A, Illig R, Prader A. The ulnar-mammary syndrome: an autosomal dominant pleiotropic gene. Clin Genet.1987;32:160–168. Erratum: Clin Genet. 1987;32:425.
- Ong KK, Amin R, Dunger DB. Pseudohypoparathyroidism—another monogenic obesity syndrome. Clin Endocrinol (Oxf).2000;52(3):389–391.
- Green JS, Parfrey PS, Harnett JD, Farid NR, Cramer BC, Johnson G, Heath O, McManamon PJ, O'Leary E, Pryse-Phillips W, The cardinal manifestations of Bardet-Biedl syndrome, a form of Laurence-Moon-Biedl syndrome. N Engl J Med.1989;321(15):1002–1009.
- Marshall JD, Ludman MD, Shea SE, Salisbury SR, Willi SM, LaRoche RG, Nishina PM. Genealogy, natural history, and phenotype of Alstrom syndrome in a large Acadian kindred and three additional families. Am J Med Genet.1997;73:150–161.
- Kolehmainen J, Wilkinson R, Lehesjoki, AE, Chandler K, Kivitie-Kallio S, Clayton-Smith J, Traskelin AL, Waris L, Saarinen A, Khan J, Gross-Tsur V, Traboulsi EI, Warburg M, Fryns JP, Norio R, Black GC, Manson FD. Delineation of Cohen syndrome following a large-scale genotype-phenotype screen. Am J Hum Genet.2004;75(1):122–127.
- Steinmuller R, Steinberger D, Muller U. MEHMO (mental retardation, epileptic seizures, hypogonadism and -genitalism, microcephaly, obesity), a novel syndrome: assignment of disease locus to Xp21.1-p22.13. Eur J Hum Genet.1998;6(3):201–206.
- Wilson M, Mulley J, Gedeon A, Robinson H, Turner, G. New X-linked syndrome of mental retardation, gynecomastia, and obesity is linked to DXS255. Am J Med Genet.1991;40(4):406–413.