Introduction

The risk associated with childhood as well as adolescent obesity is parental obesity. If both the parents are obese, the risk involved will get elevated. The inheritance of obesity does not seem to be falling under Mendelian inheritance. Obesity is considered to be the result of the combination of deletions, SNPs and gene mutations. The gene for obesity contributes a little to its phenotype while genetic variations together determine the body mass and maintain the balance between nutrition and physical activity.

Monogenic obesity

In this type of obesity condition, the phenotype is said to be directly and strongly related to the genotype. If the variation in the gene is responsible for the high risk of the disease in the lifetime, then it is considered as major gene.

It is observed that the main reason behind monogenic obesity is associated with a leptin-melanocortin pathway. Food intake and energy steadiness are maintained by this pathway and a disturbance in this pathway will result in severe obesity. The collection of afferent signals from the gut, fat and pancreatic beta cells are concentrated in the brain. These signal inputs are responsible for controlling energy expenditure, food intake, neuroendocrine status and energy partitioning.

Leptin is the hormone that is derived from adipocyte similar in quantity to the body fat. In Leptin melanocortin pathway, leptin crosses the blood brain barrier and triggers the neurons present in the hypothalamus to generate peptides. These peptides are known to decrease feeding and enhance the energy expenditure. Leptin is also found to be involved in blocking the neurons of hypothalamus producing peptides, which enhance feeding and reduce energy expenditure. The receptor gene mutations might be one of the reasons for severe obesity.

Congenital leptin deficiency

The deficiency in leptin is found to be related to hyperphagia and enhanced energy uptake. In a specific study on a nine-year old boy having a congenital leptin deficiency, a recombinant human leptin was given as an injection for about a year, which led to the obesity reversal and loss of fat mass.

Mutation in the leptin receptor

Another phenotype similar to the phenotype having a leptin deficiency and with enhanced plasma leptin levels was identified. The emergence of these two phenotypes was due to the homozygous mutation in the receptor of leptin. The reason for severe morbid obesity was due to the mutations in leptin receptor.

Melanocortin-4 receptor deficiency

The mutation in the melanocortin-4 receptor protein (MC4R) which is a component of leptin-melanocortin pathway was detected to be related to obesity. The common monogenic obesity disorder was represented by MC4R deficiency. The MC4R deficiency is correlated with high incidence of severe and earlier onset of the condition in obese individuals. Apart from the increase in mass of fat, the individuals deficient in MC4R was found to have higher lean mass and elevation in mineral density of bone. The obesity was accounted due to homozygous and heterozygous mutations in MC4R.

Syndromic Obesity

This condition includes certain disorders that follow Mendelian inheritance.

Prader-Willi syndrome

This syndrome that frequently occurs is an autosomal disorder constituting obesity, muscular hypotonia, hyperphagia, short stature, hypogonadotropic hypogonadism and mental retardation. This condition results when a deletion is inherited paternally at 15q11.2-q12 region of the chromosome or due to maternal uniparental disomy.

Single minded homologue1 (SIM1)

The deletion of the SIM1 gene has led to hyperphagia accompanied by early obesity onset due to intake of large amounts of food (Prader-Willi phenotype).

WAGR syndrome

Heterozygous adjoining gene deletions involving WT1 and PAX6 genes at 11p13 region of the chromosome will lead to WAGR syndrome. This syndrome is characterized by Wilms tumor, genitourinary anomalies, obesity, mental retardation syndrome and aniridia.

Pseudoparathyroidism type 1A or PHP1A

This syndrome results due to a mutation in GNAS1 that is transmitted maternally and it encodes for alpha subunit of Gs protein. The mutated Gs protein expression that controls energy balance in hypothalamic circuit will cause abnormalities in intake of food by the patients.

Bardet-Biedl syndrome (BBS)

In these patients, early onset of obesity results in appearance of additional features like rod-cone dystrophy, earning disability, polydactyly, renal abnormalities and hypogonadism in males.

Albright’s hereditary osteodystrophy

This is an autosomal dominant gene associated disorder caused by mutations in the GNAS1, resulting in the appearance of features like brachydactyly, obesity, ectopic ossifications and short adult stature.

Fragile X syndrome

This is an X-linked gene disorder due to disturbance in the FMR1 gene transcription. It leads to large ears, macrochildism, mental retardation, obesity, prominent jaw, macrocephaly etc.

Borjeson-Forssman-Lehmann syndrome

A mutation in the plant homeodomain zinc finger like gene called PHF6 is found to be responsible for this syndrome leading to epilepsy, mental retardation, obesity, and hypogonadism.

Alstrom syndrome

This is an autosomal recessive disease constituting the symptoms like childhood obesity with hyperinsulinaemia, neurosensory deficits, hepatic dysfunction, male hypogonadism, short stature, dilated cardiomyopathy and chronic hyperglycemia.

Polygenic Obesity

This condition appears when genes in the person are influenced by the environment, which is known to enhance the consumption of energy than the expenditure of energy. Poygenic obesity is the result of genetic, environmental and behavioral factors showing impact on the physical activity and diet of the person. The various alleles present at different loci regulating the phenotype quantitatively as well as qualitatively are grouped as polygenic variants. These polygenic variants play vital role in controlling the body weight.

The increase in childhood obesity recently has proved that environmental influence on body weight is higher than the obesity syndrome or single obesity gene. But, genetic factors might also show impact on the environmental factors.

Reference:

Maria Puiu, Adela Chirita Emandi and Smaranda Arghirescu (2013) Genetics and Obesity, Genetic Disorders, prof. Maria Puiu (Ed.), ISBN: 978-953-51-0886-3, InTech, DOI: 10.5772/52403. Available from http://www.intechopen.com/books/genetic-disorders/genetics-and-obesity.

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