G protein Mutations Causing Disease LiYang

GPCRs are known for their physiological functions. Induction of these diverse biological functions results from the activation of a collection of heterotrimeric G- proteins, which consist of αsubunits and closely associated β-γ subunits.

G-proteins, which consist of αsubunits and closely associated β-γ subunits. The α subunit is responsible for GTP and GDP binding and for GTP hydrolysis, whereas the βand γ subunits are associated in a tightly linked β- γcomplex.

Following a brief overview of G protein– coupled signal transduction, we review the growing body of evidence that mutations in genes encoding GPCRs and G proteins are an important cause of human disease.

Sorting of the diseases according the mechanism Inactive or absent Gs (αsubunit) Constitutively active Gs (αsubunit) Temperature-sensitive Gαs Constitutively active Gi α2

gsp mutations of Gαs G protein α subunit in its GTP- bound form, highlighting amino acids changed by point mutations that cause human endocrine diseases. Mutational replacements of red residues impair GTP hydrolysis; these sites are mutated in growth hormone secreting tumors of the pituitary. Replacement of either cyan residue produces an inactive G protein αsubunit, causing pseudohypoparathyroidism. Bound nucleotide is light green.

1.Inactive or absent Gs (a subunit) Pseudohypoparathyroidism-- type I (PHP-I), is an inherited human disease caused by mutational inactivation of the α subunit of Gstype I (PHP-I), Multiple endocrine abnormalities in cAMP regulated organs Occurs when bad gene inherited from mother Pseudopseudohypoparathyroidism--clinically less severe syndrome, same as mutation in one Gs Occurs when bad gene inherited from father Both conditions--as protein levels about half of normal

Pseudohypoparathyroidism Patients with this disease, which manifests in a short stature, round face, one or more foreshortened metacarpal or metatarsal bones (particularly the fourth metacarpal), obesity, and subcutaneous calcifications. Albright's hereditary osteodystrophy

Pseudohypoparathyroidism One inactive copy of Gs α Resistant to PTH （甲状旁腺 激素）, TSH （促甲状腺激 素）, ACTH （促肾上腺皮质 激素） (and others) Complex tissue-specific genetic imprinting: syndrome worse if inherit bad copy from mom than from dad 。

2. Constitutively active Gs (a subunit) Tumors Mutations in αs --block GTPase activity, cause constitutive activity as candidate oncogene (termed gsp) Activating mutations found in 40% of growth hormone secreting pituitary adenomas; found in other endocrine tumors including pituitary, thyroid McCune Albright Syndrome Somatic mutation in αs in early embryonic development Patients mosaic for constitutively active Gs (as)

McCune-Albright Syndrome. Somatic mutation of Gs alpha early in development Effects of activating MSH and gonadotrophin receptors evident

3.Temperature-sensitive αs --"Testotoxicosis" Testotoxicosis ——the gain-of-function disorder Symptoms: Males have general features of precocious puberty and hypoparathyroidism with a mutant as that is inactive at 37°C and constitutively active at testicular temperature

Both patients were found to contain a single amino acid substitution in one of the Gαisoforms. The alternation in amino acid sequence caused two effects on the mutant G protein. precocious puberty--indicating premature testicular activation (normally testosterone production is stimulated by LH, a GPCR coupled to cAMP formation) hypoparathyroidism--impaired responses to PTH （甲状旁腺激 素）, TSH （促甲状腺激素） causing PTH and thyroid abnormalities

At temperatures below normal body temperature, the mutant G protein remained in the active state, even in the absence of a bound ligand. In contrast, at normal body temperatures, the mutant G protein was inactive, both in the presence and absence of bound ligand, the testes, which are housed outside of the body’s core, have a lower temperature than the body’s visceral organs (33 ℃ versus 37 ℃ ).

precocious puberty （青春期早熟） Normally, the endocrine cells of the testes initiate testosterone （睾丸激素） production at the time of puberty in response to the pituitary hormone LH, which begins to be produced at that time. The circulating LH binds to LH receptors on the surface of the testicular cells, inducing the synthesis of cAMP and subsequent production of the male sex hormone, the testicular cells of the patients bearing the G protein mutation were stimulated to synthesize cAMP in the absence of the LH ligand, leading to premature synthesis of testosterone and precocious puberty.

Hypoparathyroidism （甲状旁腺功能减退） In contrast, the mutation in this same Gα subunit in the cells of the parathyroid glands,which function at a temperature of 37 ℃, caused the G protein to remain inactive. As a result, the cells of the parathyroid gland could not respond to stimuli that would normally cause them to secrete parathyroid hormone, leading to the condition of hypoparathyroidism. The fact that most of the bodily organs functioned in a normal manner in these patients suggests that this particular Gα isoform is not essential in the activities of most other cells.

Testotoxicosis Conclusion: Syndrome due to a temperature-sensitive Gs alpha Protein inactive at 37° but constitutively active at the lower temperature of the testes at32°C 37° 33° normal α-s-A366S mutation Gs α cAMP Produced in cyc- cells normal mutation 37° 33° Western Blot for Gs alpha

4 Constitutively active Gi α2 Tumors Mutations in ai2 --block GTPase activity, cause constitutive activity αi2 candidate oncogene (termed gip) Activating mutations found in >30% of adrenal, ovarian tumors

compare ovarian tumor

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