Genetic Basis Of Endocrine System

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02 Nov 2017

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Subject Molecular genetics

Submitted to Dr. Tayyaba Sultana

Submitted by Ayesha ilyas

Roll no 7408

Session 2012-2014

Semester 2nd Msc Zoology

GC UNIVERSITY FAISALABAD

Genetic basis of endocrine system

Definition:-

The glands and parts of glands that produce endocrine secretions, help to incorporate and control physically metabolic activity, and consist of especially the pituitary, thyroid, parathyroid, adrenals, islets of Langerhans, ovaries, and testes.

Introduction:-

Endocrine system effected by genes:-

Portions of our endocrine system affect by genes. These are units of hereditary information passed from parent to baby. Genes have the information for the production of proteins, which are some of the vital components of the body. Genes are contained in chromosomes. The normal number of chromosomes is 46.Sometimes extra, missing, altered, or damaged chromosomes can result in diseases or conditions that affect hormone production or function.

For example:-

The 23rd pair is the sex chromosome pair. The mother and father each supply a sex chromosome to the child. Girls have two X chromosomes while boys have one X from mother and one Y from father chromosome. Sometimes, a chromosome or piece of a chromosome may be absent. In Turner syndrome, only one normal X chromosome is present and this can cause poor growth. In another example, a child with Prader-Willi syndrome (PWS) may be missing all or part of chromosome 15, which also affect development, metabolism, and puberty.

Genetic variation affecting endocrine system:-

There are many examples; in which if we examine the genetic variation affecting endocrine system has successfully led to the identification of the diverse effects of single hormone. As we know a single hormones will draw diverse responses in different target tissues; the accurate response depends upon the developmental programming of the responding cells. Since each step from the production of the hormones, its control release from the cell, its transport in the body fluids, its interaction with the target organ, to its final degradation and excretion must be gene controlled. Some endocrine variation are the result of single-gene mutations but most are due to several factors or polygenes. organized genetic approaches to the problem of endocrine variation are lacking but the many differences between genetically different groups of mice give explanation for the statement that endocrine variations are under genetic control. The genetic makeup of a strain can influence the way a particular homeostatic system is assembled.

VARIATIONS IN STRAINS

Pituitary

Pituitary dwarfism in the mouse may be caused by genes, dw and df. In both cases reduced growth is associated with sterility and absence of X-zone of the adrenal cortex; the developmental effects of both disorders are consistent with deficiencies of growth hormones and prolactin.

The hypothesis shows fewer elderly changes, but cholesterol clefts, and chromo phobic or basophilic tumors are found in some animals. The gland has a wet weight of about 1 to 2 mg and a dry weight of about 0.30 to 0.85 mg. The pituitary is always heavier in the female than in the male and there are also strain differences in the gland weights.

The hormone substance of the pituitary gland has been estimated by using a variety of pituitary-ovary-uterine interaction. Uterine weight shows a correlation with gonadotropin content. The weak ovulatory response in prepuberal animals shows the low gonadotropin content of obese female pituitaries.

Thyroid

Thyroid hormone activity is relatively reduced by 16 months of age. Strains C57BL/6J and C57BR/cdJ and their hybrids (B6BRF1) have a significantly higher yield rate and lower I131 uptake than strains BALB/cJ and A/J and their CAF1 hybrids.

Thyroid activity as measured by secretion rates is different in females and males. The average thyroid secretion rates, expressed as micrograms of L-thyroxin per 100 g of body weight per day indicate that strains BALB/cJ and A/J and the CAF1 hybrid have a significantly lower secretion rate than strains C57BL/6J and C57BR/cdJ and the B6BRF1 hybrid. Strain C57BL/6J has the highest secretion rate, and the B6BRF1 hybrid has a rate intermediate between the parental strains. These results suggest that thyroid activity is under polygenic control and that different genetic factors may control the discharge of thyrotropic hormone and thyroid hormone. Thyroid-secretion rates can also be measured using thyroid weight. Although values may be higher, perhaps because of the older ages of the animals, the order and magnitude of differences of secretion rate using this latter method generally agree with the estimates using I131.

Adrenal

Aging changes in the adrenal cortex include an increase in the number of sub capsular cells, rearrangement of fasciculata cells, degeneration of the X-zone, and brown fat or ceroid pigment in the juxtamedullary area. In strain A the adrenals of old animals have amyloid deposits in the cortex. In strain NH the adrenals develop cortical hypertrophy as a result of early spontaneous gonadal atrophy. Adrenals are always smaller and dark red in males in difference to the light pinkish large adrenals of females, and there are strain differences in average weights of the glands.

There are few reports concerning the adrenal medulla in mice, but some tumors such as pheochromocytomas have been observed. Microscopic medullary tumors have been found in the adrenals of old D2CEF1 mice and in offspring of these F1 animals backcrossed to strain CE.

4) Testis:-

The Tfm mutation prevents the recognition of testosterone by target cells. The resulting phenotypic effects include rudimentary testes, female genitalia and mammary gland development in genotypic males carrying Tfm on the X-chromosomes.

5) Insulin diabetes:-

Occurs in mice homozygous for the db2j gene, although the expression of the disease depend on particular combination of modifier genes present in different strain; on the C57BL/Ksj background, the diabetes is severe and marked hyperglycaemia and hypertrophy of the islets.

VARIATIONS DUE TO SINGLE GENES

Many point mutations cause secondary endocrine changes, but few mutations appear to cause primary endocrine defects. Some examples of mutations with endocrine effects are given.

Snell's dwarf (dw/dw):-

Discovery:-

The classic case of primary endocrine defects caused by a single-gene mutation is Snell's dwarf, discovered in 1929.

Elftman and Wegelius ( 1959) confirmed previous findings and provided cytological evidence that there is also a deficiency of thyrotropic basophiles in the anterior lobe of the pituitary of dwarf mice.

The concentration of protein-bound iodine in the serum of dwarf mice is about 1.3 g/100 ml compared to 7.3 g/100 ml in serum of normal siblings. The thyroid glands of untreated dwarf mice do not accumulate radioactive iodine, but injection of thyrotropin significantly increases the iodine uptake in the glands. This confirms the idea that the defective thyroids and myxedema are secondary and suggests that the primary defect may be lack of thyrotropic hormone in the pituitary.

A combination of thyroxine and growth hormone exerts a pronounced growth-promoting effect and causes an increase in body length and weight. Growth hormone alone gives a lesser effect, and thyroxine alone the least effect.

Secondary effects of the dw gene have been observed in other organs including the thymus, adrenal cortex, and gonads. Both sexes are sterile but some males become fertile and increase in size when given daily injections of pituitary from a normal mouse the same age as the dwarf.

Dwarf mice have prolonged hypoglycemia after fasting and are extremely sensitive to insulin, reacting to a dose only 3 per cent of that needed to cause shock in normally sensitive mice. When dwarfs are given growth hormone, adrenocorticotropic hormone, and cortisone in combination, these compounds act as anti-insulin agents preventing convulsions and allowing the mice to withstand a dose four times greater than that causing shock in untreated dwarf mice.

Ames dwarf (df/df)

Another mutant, Ames dwarf, phenotypically similar to Snell's dwarf, has been discovered in an irradiated stock. The body weight of Ames dwarf mice averages 12 g at 2 months. Treatment with growth hormone increases weight in males to 22 to 24 g and allows them to sire young, whereas females reach 18 to 21 g and remain infertile. Growth hormone alone appears to be more effective in Ames dwarf mice than in Snell's dwarf mice.

Obese (ob/ob):-

In this recessive mutant, the islets of Langerhans are greatly enlarged, the β-cells being hypertrophied and very active. Obese mice have attained a maximum weight of 128 g but most weigh between 80 and 100 g and have lifespans of about 14 to 16 months. There is increased glucose 6-phosphatase activity in the β-cells, and studies of distribution of enzymes in islet tissue suggest the existence of an active hexose monophosphate shunt. Obese mice have high blood sugar levels (150 to 400 mg/100 ml of blood) and are insulin resistant. Obese mice of both sexes are infertile but under long-term diet restriction some males will mate. They become insensitive to insulin, the islet cells become normal, and lifespan is greatly increased.

Obese male pituitaries appeared to have a slightly higher gonadotropin content than obese female pituitaries. Small does of estrogen produce constant estrus in obese females.

Endocrinopathies similar to that in the obese mice, not caused by mutations, are found in other types of mice. An obese strain has been developed by selection of obese animals from a mixed colony. Although there were some obese animals in the first ten generations, they appeared regularly after the 12th. This syndrome is not due to a single gene but appears to have a polygenic basis. NZO (obese strain) females and males are fertile, attain a weight of 50 to 60 g, and have a blood glucose level over 200 mg/100 ml after 12 months. They have a greater oxygen consumption and carbon dioxide output per gram of body weight than normal (NZC strain), are resistant to insulin, and have hypertrophied islets of Langerhans comprised of enlarged β-cells and few α-cells.

Grey-lethal (gl/gl)

A major defect in grey-lethal mice appears to be lack of secondary bone absorption but it is suggested that the primary defect involves the parathyroid gland. When parathyroids from grey-lethal mice are grown in tissue culture or in intracerebral grafts, they can initiate osteoclastic resorption of normal bone as well as grey-lethal bone. Grüneberg (1963) concluded that parathormone is inactivated or destroyed too rapidly in grey-lethal mice.

Polydypsic mutation

The adrenals of mice of strain DE/J become enlarged and opalescent after 12 to 14 months of age. Their wet weight increases from 6 mg in females and 3 mg in males up to 25 mg. large areas of the cortex are replaced by a cellular homogeneous material. Coincident with these adrenal changes, water intake is increased from an average of 4 to 8 ml/day to 30 to 70 ml/day and excretion of urine is increased from 2 to 5 ml/day to 25 to 65 ml/day. Tolerance to water deprivation is greatly impair in these animals. Specific gravity of the urine measures 1.055 (normal mice, 1.36 to 1.78) and total solids 1.6 per cent (normal mice, 18 to 20 per cent); tests of the urine for sugar; acetone, albumin, and heme were negative. The adrenals, liver, and testes have heavy deposits of a cellular material.

Summary:-

First i explain the variation in strains in pituitary,adrenl,thorid, testis and in Insulin diabetes then the effect of five single-gene mutations on the endocrine system, Snell's dwarf, Ames dwarf, obese, grey-lethal, and a polydypsic mutation; the dwarfs have defects in the pituitary, the obese in the islets of Langerhans, grey-lethal probably in the parathyroid, and the polydypsic mutant probably in the adrenal. There are huge variations in responses to endocrine alteration among genetically different groups of mice. It is evident that the differences between inbred strains indicate that the responses to hormones, rate of secretion, and tissue sensitivity, i.e., endocrine variations, are quantitative genetic characters affected by multiple factors or polygenes

Inbred strains of mice, with genetically fixed characteristics, are perhaps the best research animals available for study of the genetics of endocrine variation and endocrine relationships.

Refrences:-

http://www.informatics.jax.org/greenbook/chapters/chapter20.shtml

http://diabetes.diabetesjournals.org/content/37/9/1163.abstract



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