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Leptin Deficiency

Leptin deficiency

Leptin deficiency is a condition that causes severe obesity in early childhood. The individuals affected are of normal weight at birth, however, are constantly hungry and gain weight quickly. The extreme hunger leads to chronic eating (hyperphagia) and obesity. Similarly, the children tend to develop abnormal eating behaviors, for example, eating in secret hoarding food and fighting other children over food. People affected with leptin deficiency also have hypogonadotropic hypogonadism, a condition caused by reduced production of sexual development hormones. The individuals affected experience delayed puberty or do not go through puberty. Similarly, they may also be infertile.

Other names for leptin deficiency include:
congenital deficiency of the leptin receptor
leptin receptor-related monogenic obesity
obesity due to leptin receptor gene deficiency
obesity, morbid, due to leptin receptor deficiency
obesity, morbid, nonsyndromic 2

Leptin deficiency is a rare cause of obesity and the prevalence is unknown. The deficiency is caused by mutations in the LEPR gene which is responsible for providing instructions for making a protein called leptin receptor. The leptin receptor regulates body weight and is found on the surface of cells of tissues of the body and many organs. It is also found on a part of the brain called hypothalamus. The hypothalamus is responsible for controlling hunger, thirst and other functions including, body temperature, moods and sleep. It further regulates the release of many hormones that have different functions throughout the body. The leptin hormone is activated by a hormone called leptin that attaches to the receptor. Normally, the fat cells of the body release leptin in proportion to their size. The fat cells produce leptin as they become larger. The rise in leptin is an indication that the fat stores are increasing.

The binding of leptin and its receptors triggers a series of chemical signals in the hypothalamus that affect hunger and help produce a feeling of fullness (satiety). The LEPR gene mutations that cause leptin receptor deficiency prevents the receptor from responding to leptin and causes an increase in hunger and weight gain.
Role of leptin

The discovery of leptin has advanced our understanding of the metabolic importance of adipose tissue significantly. In addition, the understanding of leptin has also shown that both deficiency of leptin and excess leptin are associated with severe immunological, endocrine and metabolic consequences. Leptin plays a prominent role in humans to mediate the neuroendocrine adaptation (interaction between the nervous system and endocrine system) to a lack of energy in the body. People believe that weight gain and weight loss is all about willpower and the number of calories that you eat. However, research shows that the hormone leptin plays a significant role in maintaining a healthy weight. Leptin resistance causes the body to not respond to the hormone and is believed to lead to fat gain. Leptin hormone is produced by the fat cells of the body and is often referred to as the satiety or starvation hormone. The primary target of leptin is in the hypothalamus of the brain. The hormone is supposed to tell the brain that when you have enough fat stored in the body and that you do not need to eat more food and burn calories at a normal rate [1].

Leptin also has several other functions related to brain function, immunity and fertility. However, the main role of leptin is to regulate energy in the long-term, the number of calories you eat and how much fat you store in the body. Over the years, the leptin system has evolved to keep humans from overeating and starving, both of which make it difficult to survive in the natural environment. Leptin keeps you from starving, but leptin deficiency will prevent it from working efficiently. Leptin is a hormone produced by the fat cells in the body. The main role of leptin is to regulate the storage of fat and the calories you eat and burn.
What impact does leptin have on the brain?

Leptin is produced by the fat cells of the body, therefore, the more fat the body carries, the more leptin is produced. The bloodstream carries leptin to the brain where it sends a signal to the hypothalamus. The fat cells tell the brain how much body fat they carry using leptin. If the levels of leptin are high, leptin tells the brain that there is enough storage of fat. Whereas, in case of low levels of fat stores, leptin tells the brain that you need to eat. The amount of fat in the body increases when you eat, this leads to an increase in the levels of leptin. Similarly, you eat less and burn more. However, when you do not eat, the amount of fat in your body increases, which leads to a decrease in the levels of leptin. At this point, you eat more and burn less calories. This system is known as a negative feedback loop which is similar to the control mechanism of different physiological functions, for example, blood pressure, body temperature and breathing. The main function of the hormone is to send a signal to the brain telling it how much fat is stored in the fat cells of the body.
Leptin resistance

People who are obese have a lot of fat in their fat cells. Moreover, since the fat cells produce fat depending on the amount of fat in the body, people who are obese also have very high levels of leptin [2]. Given how leptin is supposed to work, people who are obese should try and limit their intake of food. The brain should know that there is enough energy stored in the body. However, at times, the leptin signaling may not work. Despite leptin being present, the brain does not see it. This particular condition is known as leptin resistance. It is considered to be one of the main biological contributors of obesity. When the brain does not receive leptin signal, it thinks that the body is starving, although it has more than enough energy stored. This causes the brain to change its behavior to regain body fat. It causes you to eat more because your brain thinks you must eat to prevent starvation. Similarly, there is a decrease in energy expenditure in an effort to conserve energy. The brain decreases the energy levels, burning fewer calories at rest. Therefore, eating more and less exercising is not the underlying cause of weight gain. However, it is a possible consequence of leptin resistance. Willing to overcome the leptin-driven starvation signal in people who struggle with leptin resistance is next to impossible.

Leptin receptor deficiency obesity

Leptin receptor deficiency, also referred to as LEPR deficiency obesity is the part of a group of disorders known as rare genetic disorders obesity. The leptin hormone tells the body when we have enough fat stored and we do not need to eat more, so that we can burn calories at a usual rate [4]. The LEPR body provides the body with instructions to make a specific protein called the leptin receptor. The receptor is important for helping regulate body weight. When there is a change in the LEPR gene, the receptor cannot function properly, and leptin does not attach to the receptor [5]. When the leptin does not attach to the receptor, the brain does not receive signals that the body is full. The body can start to think that it is starving without the correct signal.

Clinical features of LEPR deficiency

The main signs and symptoms of leptin deficiency include:
Insatiable hunger
Early-onset obesity
Hypogonadotropic hypogonadism
Like many rare disorders of obesity, LEPR deficiency begins at a very young age. Babies are born with normal weight, however, start to show signs of constant and insatiable hunger, which is followed by severe, early-onset rapid weight gain. The abnormal eating and food seeking behaviors develop within the first few weeks of life. Children will often be seen fighting over food, secretly hiding and hoarding food. Aggressive behavior is noted when food is denied. In addition to this, some children also develop type 2 diabetes in early adulthood [6].
Is leptin deficiency inherited?

Leptin deficiency is an autosomal recessive disorder. This means that someone with leptin deficiency will have two copies of the genetic variant that causes the disorder. You can only inherit an autosomal recessive disorder if both your parents are carriers for the disorder. In simpler words, they both carry one copy of the genetic variant that causes the disorder. However, in very rare cases, a carrier of leptin deficiency may develop some of the disorder’s symptoms. The diagnosis of leptin deficiency can be confirmed with genetic testing.
Leptin controls a significant variety of physiological processes in humans. If leptin activity in insufficient or if there is an overabundance of the hormone it can be associated with various different pathologies. The discovery of leptin initially led scientists to believe that obesity in humans could be related to leptin deficiency. However, congenital leptin deficiency rarely causes obesity and the high level of fat in the body is associated with increased leptin levels [3]. Leptin deficiency also causes hypogonadism, hyperphagia and obesity. On the contrary, obese humans express high levels of leptin but do not respond to leptin-mediated signals because of leptin resistance. Similarly, short-term fasting also causes a decrease in the circulating leptin levels. In addition, the levels of leptin drop abruptly in individuals who follow a low-calorie diet. This results in an increase in appetite and rapid weight gain once the low-calorie diet is eliminated. Furthermore, it is also associated with significant disturbances in the immunological, endocrine and metabolic system, including disturbed immune function, dyslipidemia, insulin resistance and glucose intolerance.
Summing it all up!

Leptin deficiency is a condition that causes severe obesity a few weeks after birth. It causes individuals to feel constantly hungry and causes them to eat excessively, consequently leading to obesity. People with leptin deficiency also have hypogonadotropic hypogonadism.

References
1.Allison MB, Myers Jr MG. Connecting leptin signaling to biological function. The Journal of endocrinology. 2014 Oct;223(1):T25.
2.Considine RV, Sinha MK, Heiman ML, Kriauciunas A, Stephens TW, Nyce MR, Ohannesian JP, Marco CC, McKee LJ, Bauer TL, Caro JF. Serum immunoreactive-leptin concentrations in normal-weight and obese humans. New England Journal of Medicine. 1996 Feb 1;334(5):292-5.
3.Farooqi IS. Monogenic human obesity. InObesity and Metabolism 2008 (Vol. 36, pp. 1-11). Karger Publishers.
4.Dubern B, Clement K. Leptin and leptin receptor-related monogenic obesity. Biochimie. 2012 Oct 1;94(10):2111-5.
5.Zhou Y, Rui L. Leptin signaling and leptin resistance. Frontiers of medicine. 2013 Jun 1;7(2):207-22.
6.Soliman AT, Yasin M, Kassem A. Leptin in pediatrics: a hormone from adipocyte that wheels several functions in children. Indian journal of endocrinology and metabolism. 2012 Dec;16(Suppl 3):S577.