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From the Clinical Research Centers |
, and Gi2 Genes in Isolated Familial Acromegaly1
Endocrine Genetics Unit (LIM-25) and Hormone and Molecular Genetics Laboratory, Endocrinology Section, Department of Medicine, University of Sao Paulo School of Medicine (B.H.J., V.S.L., R.R.B., N.A., S.P.A.T.), Sao Paulo SP 01246-903, Brazil; Metabolic Disease Branch, National Institute of Diabetes and Digestive and Kidney Disease, National Institutes of Health (S.K.A., S.J.M.), Bethesda, Maryland 20892; and Division of Endocrinology, Departments of Medicine and Pediatrics, The Johns Hopkins University School of Medicine (R.S., M.A.L.), Baltimore, Maryland 21287
Address all correspondence and requests for reprints to: Dr. Beatriz H. Jorge, Endocrine Genetics Unit, University of Sao Paulo School of Medicine, Avenue Dr. Arnaldo, 455 5th Floor, Sao Paulo SP 01246-903, Brazil. E-mail: beatrizjorge{at}hotmail.com
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Abstract |
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(GNAS1), and Gi2 (GNAI2)], as
well as the GHRH receptor (GHRH-R) gene. Genomic DNA was used to
amplify exons 2–10 of MEN1, followed by dideoxy
fingerprinting mutation analysis and direct sequencing. The GHRH-R gene
was analyzed via direct sequencing of PCR-amplified fragments
representing the coding exons and intron-exon junctions. To exclude
mutation at hot spot areas of GNAS1 and
GNAI2, exons 8 and 9 of GNAS1 and exons 5
and 6 of GNAI2 were amplified and screened for mutation
via denaturing gradient gel electrophoresis. No mutations were detected
in any of the four genes. The present data extend prior reports of the
absence of mutation in MEN1, GHRH-R, and
GNAS1 and describe the first family with isolated
acromegaly in which germline mutation in GNAI2 has been
searched. |
Introduction |
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TopAbstractIntroductionCase ReportsMaterials and MethodsResultsDiscussionReferences |
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The molecular basis of pituitary adenoma formation is still unknown. It probably derives from monoclonal proliferation of pituitary cells. Multiple molecular events may occur during tumor development, particularly the activation of oncogenes and inactivation of tumor suppressor genes (8).
Sporadic pituitary tumors (9) and some sporadic somatotropinomas have been associated with loss of heterozygosity on chromosome 11 (LOH on 11q13) (10), the locus of the MEN1 gene. Deletions on chromosome 11q13 have been reported in somatotropinomas from three families with isolated acromegaly (2, 4). Further, Gadelha et al. (11) reported linkage of isolated familial somatotropinomas to chromosome 11q13. Considering the possible linkage of familial acromegaly with the MEN1 region, it seems reasonable to search for mutation in this gene in familial somatotropinoma.
Activating mutations in G protein-coupled receptors genes, such as LH receptor, have been described as causing neoplasia (12). As GHRH receptor (GHRH-R) is involved in somatotroph proliferation and GH secretion, one may predict that activating mutation in its gene might lead to GH overproduction and acromegaly. Thus, a germline activating mutation of the GHRH-R gene is another possible cause of isolated familial acromegaly.
The heterotrimeric G proteins, composed of -, ß-, and
-subunits, are linked to cell surface receptors, resulting in the
generation of an intracellular second messenger. Mutations of the
-subunit genes of Gs (GNAS1) and Gi2
(GNAI2) are associated with human neoplasms
(13). Somatic mutations of GNAS1 are found in
GH-secreting pituitary adenomas and in somatotropinomas from patients
with McCune-Albright syndrome (13, 14). Point mutations in
the -subunit of the Gi2 protein gene have also
been detected in 3 of 22 nonfunctioning pituitary adenomas and in 1 of
32 corticotroph adenomas (15, 16), but to date have not
been sought in somatotropinomas.
The purpose of the present study was thus to characterize a newly identified Brazilian kindred with isolated acromegaly and to search for germline mutation in MEN1, GHRH-R, GNAS1, and GNAI2 in this family.
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Case Reports |
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TopAbstractIntroductionCase ReportsMaterials and MethodsResultsDiscussionReferences |
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A 24-yr-old woman was referred for clinical evaluation in 1982 because of headaches, galactorrhea, menstrual irregularities, and progressive enlargement of hands and feet. Physical examination revealed evident acromegalic facial and acral features, with a height of 169 cm. GH levels were 34 µg/L (normal value, <5 µg/L) at baseline and 64 µg/L after oral administration of 75 g glucose. She had elevated basal PRL levels (111 µg/L; normal values, 2.5–14.5 µg/L). Insulin-like growth factor I (IGF-I) could not be measured before surgery. Pneumoencephalography showed a sellar volume of 2120 mm3 (compatible with macroadenoma). Transsphenoidal adenomectomy was performed. Pathology report confirmed the diagnosis of pituitary adenoma, but immunohistochemical staining was not performed. After surgery, the PRL level was 9.0 µg/L, but elevated levels of GH and IGF-I led to the administration of radiotherapy.
Case 2
In 1986 the index case’s 29-yr-old brother complained of a 10-yr history of progressive enlargement of hands, feet, and mandible. Physical exam revealed clear acromegalic features with a height of 179 cm. Basal GH level was 45.9 µg/L and rose to 66.5 µg/L after oral glucose administration. The PRL level was 7.9 µg/L. Pituitary computed tomography and magnetic resonance imaging scans documented a macroadenoma (2.3 x 1.1 x 1.5 cm). Transsphenoidal adenomectomy was performed, and pituitary irradiation was given because of high postoperative serum GH and IGF-I levels.
In both patients repeated values of basal intact PTH, total serum calcium, alkaline phosphatase, fasting glucose and insulin, and calcium-stimulated gastrin levels were normal. Exams in 1999 confirmed the diagnosis of isolated acromegaly not related to other endocrine syndromes.
Their father was likely to be the gene carrier for this condition. He had acromegalic features, confirmed by family pictures and died of a nonrelated cause at the age of 40 yr without endocrine evaluation. Five other first degree family members were screened. Blood samples were drawn after an overnight fast, and the levels of GH, IGF-I, PRL, total serum calcium, alkaline phosphatase, intact PTH, gastrin, and fasting glucose and insulin were measured, with no abnormality detected. This family is not related to another Brazilian kindred with somatotropinoma (4).
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Materials and Methods |
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TopAbstractIntroductionCase ReportsMaterials and MethodsResultsDiscussionReferences |
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The diagnostic criteria used for isolated familial acromegaly were two or more members with GH-secreting pituitary adenoma without extrapituitary endocrine tumor. This study was approved by the ethics committees of the University of Sao Paulo School of Medicine and University Hospital. All subjects gave written informed consent to participate.
DNA isolation
Genomic DNA was isolated from peripheral blood leukocytes using DNA Quick-Prep Mini Kit (Lifecodes Corp., Stamford, CT) according to the manufacturer’s protocol. Tumor samples for DNA analysis were not available.
MEN1 gene analysis
Exons 2–10 of MEN1 comprising the coding region and
flanking exon-intron boundaries were amplified individually or in
groups from genomic DNA using published primers
(http://www.niddk.nih.gov). PCR conditions were previously reported
(17). The dideoxy fingerprinting (ddF) method
(18) was used for mutation screening. The primers used for
the ddF reactions were end labeled with -33P
using the T4 Polynucleotide Kinase Kit (Promega Corp.,
Madison, WI). ddF reactions were electrophoresed on a nondenaturing gel
[0.75 x MDE (FMC Bioproducts, Rockland, ME) in 0.5 x TBE]
using a sequencing apparatus as previously described
(17).
Purified PCR products (Wizard PCR Preps DNA Purification System, Promega Corp., Madison, WI) were also subjected to direct sequencing (AmpliCycle Sequencing Kit, Perkin-Elmer Corp., Branchburg, NJ) with the same primers used for the ddF reactions. The sequencing reactions were run on a 6% denaturing polyacrylamide gel and autoradiographed overnight.
GHRH receptor gene analysis
The entire coding region and the exon-intron boundaries of GHRH-R were analyzed from peripheral genomic DNA via direct sequencing of PCR-amplified fragments. Each of the 13 exons and intron-exons boundaries was amplified individually, with the exception of exons 2 and 3, which were amplified together because they are divided by a short intron. PCR conditions and primer sequences were previously published (19). PCR products were separated through agarose gel and isolated in low melting agarose. They were subjected to direct sequencing using the Thermo Sequenase Cycle Sequencing Kit (Amersham Pharmacia Biotech, Cleveland, OH).
G protein gene analysis
Selected fragments, including exons 8 and 9 of GNAS1 and exons 5 and 6 of GNAI2, were amplified by PCR and screened for mutation via denaturing gradient gel electrophoresis (DGGE) as previously reported (20). A 40-nucleotide G-C-rich sequence was attached to the 5'-end of the downstream primer to enhance the sensitivity of DGGE screening (21).
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Results |
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TopAbstractIntroductionCase ReportsMaterials and MethodsResultsDiscussionReferences |
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MEN1 mutation was screened with the primer sets covering the entire coding region and splice junctions. The ddF method and direct sequencing of MEN1 did not reveal any mutation in the two affected subjects. A common silent MEN1 polymorphism in codon 418 with a sequence change from GAC to GAT, both coding for aspartic acid, was found in both siblings (18).
The entire coding sequence and the intron-exon boundaries of GHRH-R were subjected to direct sequencing, and no changes were detected.
Mutations at the hot spot regions in exons 8 and 9 of GNAS1 and in exons 5 and 6 of GNAI2 were not detected when screened by DGGE in genomic DNA from the two affected patients.
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Discussion |
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TopAbstractIntroductionCase ReportsMaterials and MethodsResultsDiscussionReferences |
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We also searched for germline mutation in other candidate genes, such as GHRH-R. To date, only inactivating mutations in GHRH-R have been reported in patients with familial isolated GH deficiency (19). Activating mutation in this gene would theoretically be able to cause somatotroph cells proliferation and GH hypersecretion, which might lead to acromegaly. Our present results extend the literature reports that a germline activating mutation in GHRH-R has not yet been found as a cause of familial acromegaly (4) and is not present in sporadic somatotropinomas (22A ).
Naturally occurring (13) and experimentally induced
(23) activating mutations of the -subunit of
Gs protein that lead to constitutive stimulation
of adenylyl cyclase are associated with excessive secretion of GH as
well as somatotroph proliferation. Mutations at hot spot regions of
GNAS1, such as codons 201 and 227, were reported in human
GH-secreting pituitary tumors (13). Considering these
findings, we decided to search for germline mutation at hot spots areas
of GNAS1, because tumor samples were not available, and no
mutation was found. Somatic mutation at GNAS1 has been
sought by Ackermann et al. (5) in a family with
isolated acromegaly, and direct sequencing of exons 7–13 of
GNAS1 detected no mutation. Taken together, these data
suggest that GNAS1 is not involved in the pathogenesis of
these familial somatotropinoma cases.
Gi2 protein inhibits adenylyl cyclase activity
and has transforming activity in some cells in its mutated form
(24). Amino acid homology alignment between the
-subunits of proteins Gi2 and
Gs revealed conservation of identical amino acids
between codons 179 and 205 of GNAI2 and codons 201 and 227
of GNAS1. Mutations in those homologous regions of
GNAI2 have been described in ovarian, adrenal, and pituitary
tumors (13, 15, 16). As Gi protein
binds to specific membrane receptor such as somatostatin, it seems
reasonable that a mutation in the GNAI2 could lead to excess
secretion of GH. This idea was considered by searching for germline
mutation at the homologous regions between s
and i2-subunits of the G protein-coupled
receptor. Our results show the absence of mutation in those
regions, suggesting that hot spot areas of GNAI2 may not be
involved in the tumorigenesis of familial acromegaly. Although we have
screened for GNAS1 and GNAI2 mutations using
DGGE, this method has been documented to present sensitivity and
specificity as high as 96% for detecting point mutations
(25).
In conclusion, we extend prior reports of the absence of mutation in MEN1, GHRH-R, and GNAS1 in families with isolated acromegaly and report for the first time the absence of mutation at hot spot areas of GNAI2 in one family with this syndrome. Further studies of GNAI2 in this entity should strengthen our conclusions.
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Footnotes |
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2 A CNPq (300346/82–4) research investigator.
Received June 9, 2000.
Revised October 4, 2000.
Accepted October 17, 2000.
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  A. Beckers and A. F Daly The clinical, pathological, and genetic features of familial isolated pituitary adenomas Eur. J. Endocrinol., October 1, 2007; 157(4): 371 - 382. [Abstract] [Full Text] [PDF]
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