Characterization of a de novo 43-bp deletion of the Gs alpha gene (GNAS1) in Albright hereditary osteodystrophy

Pulmonary Adenofibroma: Clinicopathological and Genetic Analysis of 7 Cases With Literature Review

Introduction

Pulmonary adenofibroma (PAF), characterized by biphasic differentiation composed of gland-like space lined by respiratory epithelium and stromal spindle cells, is a rare benign tumor of the lung. PAF was reported infrequently and inconsistently with diagnostic criteria and withstood higher risk of misdiagnosis as solitary fibrous tumors (SFTs) due to their morphological resemblance. In this study, we report seven cases of PAF with gene sequencing results and summarize the data of previous literature.

Materials and Methods

Seven cases of PAF with surgically resection samples were collected from Pathology department of West China Hospital, Sichuan University between 2009 to 2020. Immunohistochemical studies were performed in all cases and 3 cases underwent a 425-gene panel next-generation sequencing (NGS).

Results

Five female and two male patients were included in this study, with an average age of 51 years. All the patients were asymptomatic, and the lesion was identified on routine chest radiography. The tumor size measured by computed tomography (CT) ranged from 0.5 to 2.7 cm. Gland-like structures were mostly positive for glandular epithelium markers. The spindle cells in stroma expressed Desmin, SMA, ER and PR in 3 of 7 cases. No well-recognized molecular abnormalities can be identified by NGS in the 3 cases. To date, all the patients are alive, with no evidence of recurrence and metastasis.

Conclusion

PAF is a unique benign pulmonary tumor with low incidence. Biphasic morphology, IHC stains along with molecular detection is of great significance to make a clear diagnosis.

Progressive osseous heteroplasia: diagnosis, treatment, and prognosis

Progressive osseous heteroplasia (POH) is an ultrarare genetic condition of progressive ectopic ossification. Most cases of POH are caused by heterozygous inactivating mutations of GNAS, the gene encoding the alpha subunit of the G-stimulatory protein of adenylyl cyclase. POH is part of a spectrum of related genetic disorders, including Albright hereditary osteodystrophy, pseudohypoparathyroidism, and primary osteoma cutis, that share common features of superficial ossification and association with inactivating mutations of GNAS. The genetics, diagnostic criteria, supporting clinical features, current management, and prognosis of POH are reviewed here, and emerging therapeutic strategies are discussed.

New mutation type in pseudohypoparathyroidism type Ia

CONTEXT

The GNAS gene encodes the alpha-subunit of the stimulatory G proteins, which play a crucial role in intracellular signal transduction of peptide and neurotransmitter receptors. Heterozygous inactivating maternally inherited mutations of GNAS (including translation initiation mutations, amino acid substitutions, nonsense mutations, splice site mutations and small insertions or deletions) lead to a phenotype in which Albright hereditary osteodystrophy is associated with pseudohypoparathyroidism type Ia.

OBJECTIVE

We sought to identify the molecular defect in a patient who was thought to have PHP-Ia.

METHODS AND RESULTS

The GNAS gene of a 5-year-old boy with brachydactily, mental retardation, pseudohypoparathyroidism and congenital hypothyroidism was investigated. We found a heterozygous inversion of exon 2 and part of intron 1 of de novo origin. Molecular studies of cDNA from blood RNA demonstrated that both the normal and the mutant variants were stable and that new splice-sites were generated.

CONCLUSION

This report demonstrates the first evidence for an inversion at the GNAS gene responsible of pseudohypoparathyroidism type Ia.

Diagnostic and mutational spectrum of progressive osseous heteroplasia (POH) and other forms of GNAS-based heterotopic ossification

Progressive osseous heteroplasia (POH) is a rare, disabling disease of heterotopic ossification (HO) that progresses from skin and subcutaneous tissues into deep skeletal muscle. POH occurs in the absence of multiple developmental features of Albright hereditary osteodystrophy (AHO) or hormone resistance, clinical manifestations that are also associated with GNAS inactivation. However, occasional patients with AHO and pseudohypoparathyroidism 1a/c (PHP1a/c; AHO features plus hormone resistance) have also been described who have progressive HO. This study was undertaken to define the diagnostic and mutational spectrum of POH and progressive disorders of HO, and to distinguish them from related disorders in which HO remains confined to the skin and subcutaneous tissues. We reviewed the charts of 111 individuals who had cutaneous and subcutaneous ossification. All patients were assessed for eight characteristics: age of onset of HO, presence and location of HO, depth of HO, type of HO, progression of HO, features of AHO, PTH resistance, and GNAS mutation analysis. We found, based on clinical criteria, that POH and progressive HO syndromes are at the severe end of a phenotypic spectrum of GNAS-inactivating conditions associated with extra-skeletal ossification. While most individuals with superficial or progressive ossification had mutations in GNAS, there were no specific genotype-phenotype correlations that distinguished the more progressive forms of HO (e.g., POH) from the non-progressive forms (osteoma cutis, AHO, and PHP1a/c).

Cognitive impairment is prevalent in pseudohypoparathyroidism type Ia, but not in pseudopseudohypoparathyroidism: possible cerebral imprinting of Gsalpha

OBJECTIVE

Pseudohypoparathyroidism type Ia (PHP-Ia) is a hereditary disorder characterized by resistance to multiple hormones that work via cAMP such as PTH and TSH, accompanied by typical skeletal features including short stature and brachydactyly, termed Albright hereditary osteodystrophy (AHO). In affected kindreds, some members may have AHO but not hormone resistance; they are termed as pseudopseudohypoparathyroidism (PPHP). The molecular basis for the disorder is heterozygous inactivating mutation of the Gsalpha gene. In affected families, subjects with both PHP-Ia and PPHP have the same Gsalpha mutations. The skeletal features common to PPHP and PHP-Ia are presumably caused by tissue-specific Gsalpha haploinsufficiency. Other features that distinguish between PPHP and PHP-Ia, such as the multihormone resistance, are presumably caused by tissue-specific paternal imprinting of Gsalpha. This suggests that major differences in phenotype between PHP-Ia and PPHP point to specific tissues with Gsalpha imprinting. One such major difference may be cognitive function in PHP-Ia and PPHP.

DESIGN

Description of a large family with PHP-Ia and PPHP.

PATIENTS

Eleven affected subjects with PHP-Ia or PPHP in one family.

MEASUREMENTS

Cognitive impairment (CI) was defined by a history of developmental delay, learning disability and the Wechsler intelligence scale.

RESULTS

CI occurred only in the five PHP-Ia but not in the six PPHP subjects. Hypothyroidism which occurred in all PHP-Ia subjects was apparently not the cause of CI as it was mild, and was treated promptly. Analysis of additional Israeli cases, and the published cases from the literature, all with documented Gsalpha mutations, revealed that CI is prevalent in PHP-Ia [60 of 77 subjects (79%)] but not in PPHP [3 of 30 subjects (10%)] (P < 1 x 10(-6)).

CONCLUSION

We suggest that Gsalpha is imprinted in the brain.

Analysis of GNAS1 and overlapping transcripts identifies the parental origin of mutations in patients with sporadic Albright hereditary osteodystrophy and reveals a model system in which to observe the effects of splicing mutations on translated and untranslated messenger RNA

Albright hereditary osteodystrophy (AHO) is caused by heterozygous deactivating GNAS1 mutations. There is a parent-of-origin effect. Maternally derived mutations are usually associated with resistance to parathyroid hormone termed "pseudohypoparathyroidism type Ia." Paternally derived mutations are associated with AHO but usually normal hormone responsiveness, known as "pseudo-pseudohypoparathyroidism." These observations can be explained by tissue-specific GNAS1 imprinting. Regulation of the genomic region that encompasses GNAS1 is complex. At least three upstream exons that splice to exon 2 of GNAS1 and that are imprinted have been reported. NESP55 is exclusively maternally expressed, whereas exon 1A and XL alphas are exclusively paternally expressed. We set out to identify the parental origin of GNAS1 mutations in patients with AHO by searching for their mutation in the overlapping transcripts. This information would be of value in patients with sporadic disease, for predicting their endocrine phenotype and planning follow-up. In doing so, we identified mutations that resulted in nonsense-mediated decay of the mutant Gs alpha transcript but that were detectable in NESP55 messenger RNA (mRNA), probably because they lie within its 3' untranslated region. Analysis of the NESP55 transcripts revealed the creation of a novel splice site in one patient and an unusual intronic mutation that caused retention of the intron in a further patient, neither of which could be detected by analysis of the Gs alpha complementary DNA. This cluster of overlapping transcripts represents a useful model system in which to analyze the effects that mutant sequence has on mRNA-in particular, splicing-and the mechanisms of nonsense-mediated mRNA decay.

Endocrine manifestations of stimulatory G protein alpha-subunit mutations and the role of genomic imprinting

The heterotrimeric G protein G(s) couples hormone receptors (as well as other receptors) to the effector enzyme adenylyl cyclase and is therefore required for hormone-stimulated intracellular cAMP generation. Receptors activate G(s) by promoting exchange of GTP for GDP on the G(s) alpha-subunit (G(s)alpha) while an intrinsic GTPase activity of G(s)alpha that hydrolyzes bound GTP to GDP leads to deactivation. Mutations of specific G(s)alpha residues (Arg(201) or Gln(227)) that are critical for the GTPase reaction lead to constitutive activation of G(s)-coupled signaling pathways, and such somatic mutations are found in endocrine tumors, fibrous dysplasia of bone, and the McCune-Albright syndrome. Conversely, heterozygous loss-of-function mutations may lead to Albright hereditary osteodystrophy (AHO), a disease characterized by short stature, obesity, brachydactyly, sc ossifications, and mental deficits. Similar mutations are also associated with progressive osseous heteroplasia. Interestingly, paternal transmission of GNAS1 mutations leads to the AHO phenotype alone (pseudopseudohypoparathyroidism), while maternal transmission leads to AHO plus resistance to several hormones (e.g., PTH, TSH) that activate G(s) in their target tissues (pseudohypoparathyroidism type IA). Studies in G(s)alpha knockout mice demonstrate that G(s)alpha is imprinted in a tissue-specific manner, being expressed primarily from the maternal allele in some tissues (e.g., renal proximal tubule, the major site of renal PTH action), while being biallelically expressed in most other tissues. Disrupting mutations in the maternal allele lead to loss of G(s)alpha expression in proximal tubules and therefore loss of PTH action in the kidney, while mutations in the paternal allele have little effect on G(s)alpha expression or PTH action. G(s)alpha has recently been shown to be also imprinted in human pituitary glands. The G(s)alpha gene GNAS1 (as well as its murine ortholog Gnas) has at least four alternative promoters and first exons, leading to the production of alternative gene products including G(s)alpha, XLalphas (a novel G(s)alpha isoform that is expressed only from the paternal allele), and NESP55 (a chromogranin-like protein that is expressed only from the maternal allele). A fourth alternative promoter and first exon (exon 1A) located approximately 2.5 kb upstream of the G(s)alpha promoter is normally methylated on the maternal allele and transcriptionally active on the paternal allele. In patients with isolated renal resistance to PTH (pseudohypoparathyroidism type IB), the exon 1A promoter region has a paternal-specific imprinting pattern on both alleles (unmethylated, transcriptionally active), suggesting that this region is critical for the tissue-specific imprinting of G(s)alpha. The GNAS1 imprinting defect in pseudohypoparathyroidism type IB is predicted to decrease G(s)alpha expression in renal proximal tubules. Studies in G(s)alpha knockout mice also demonstrate that this gene is critical in the regulation of lipid and glucose metabolism.

STK25 is a candidate gene for pseudopseudohypoparathyroidism

We determined the chromosomal location of the mouse gene Stk25, encoding a member of the Ste20/PAK family of serine/threonine kinases, by interspecific backcross analysis. We mapped Stk25 to the central region of mouse chromosome 1 linked to Chrng (formerly Acrg) and En1. This central region of mouse chromosome 1 shares a region of homology with the long arm of human chromosome 2, suggesting that the human homologue of Stk25 would also map to 2q. We proved this prediction of syntenic homology correct by mapping human STK25 to 2q37. Deletion of the 2q37 region has been implicated in the expression of pseudopseudohypoparathyroidism (PPHP), a disease which shares features of the Albright hereditary osteodystrophy (AHO) phenotype. To investigate a pathogenetic relationship between STK25 and PPHP, we carried out fluorescence in situ hybridization (FISH) using an STK25 gene probe and chromosomes from PPHP patients characterized as having small deletions near the distal end of 2q. PPHP patient DNA showed no hybridization to STK25 genomic DNA, indicating that STK25 is contained within the deleted chromosomal region. This finding, in conjunction with previous studies demonstrating the role of Ste20/PAK kinases in heterotrimeric G protein signaling, suggests that STK25 is a positional candidate gene for PPHP.

Mutational analysis of GNAS1 in patients with pseudohypoparathyroidism: identification of two novel mutations

Pseudohypoparathyroidism (PHP) refers to two major variants that generally coexist in the same family, PHP type Ia (PHP Ia), in which both PTH resistance and a constellation of physical features, termed Albright's hereditary osteodystrophy (AHO), are present, and pseudopseudohypoparathyroidism (PPHP), in which AHO occurs without PTH resistance. Most patients with PHP Ia show a partial deficiency (50%) of Gs activity, due to loss of function mutations in Gsalpha gene (GNAS1). The present study reports clinical, biochemical, and molecular data of 8 unrelated families with PHP Ia and PPHP. The 13 exons of GNAS1 were screened for mutations by PCR and direct sequencing of the amplified products. We detected heterozygous mutations in the affected members of the 4 families in which PHP Ia was present. In 2 families 2 previously reported deletions in exons 5 and 7 were found, whereas in the other 2 families, 2 novel frameshift deletions were identified in exons 1 and 11, causing a premature stop codon in the mutant allele. No mutation was detected in the families in which PPHP was the only clinical manifestation. In conclusion, we report the first mutational analysis of Italian patients with PHP Ia and PPHP, and we describe two novel deletions in GNAS1. Furthermore, we confirm that these mutations cannot be detected in families with isolated PPHP, suggesting that these forms of AHO are genetically distinct from PHP Ia.

G protein defects in signal transduction

G proteins couple receptors for many hormones to effectors that regulate second messenger metabolism. Several endocrine disorders have been shown to be caused by either loss- or gain-of-function mutations in G proteins or G protein-coupled receptors. In pseudohypoparathyroidism type Ia (PHP Ia), there are generalized hormone resistance (parathyroid hormone [PTH], thyroid-stimulating hormone, gonadotropins) and associated abnormal physical features, Albright hereditary osteodystrophy. Subjects with PHP Ib are normal in appearance and show renal resistance to PTH. In McCune-Albright syndrome (MAS), subjects show autonomous endocrine hyperfunction associated with fibrous dysplasia of bone and skin hyperpigmentation. Germline loss-of-function mutations have been identified in the G(s)-alpha gene in PHP Ia, and recent evidence suggests that the G(s)-alpha gene is paternally imprinted in a tissue-specific manner. Abnormal imprinting of the G(s)-alpha gene may be the cause of PHP Ib. MAS, in contrast, is caused by gain-of-function missense mutations of the G(s)-alpha gene.

Activating and inactivating mutations in the human GNAS1 gene

GNAS1 on chromosome 20 is a complex locus, encoding multiple proteins, of which G(s)alpha, the alpha-subunit of the heterotrimeric stimulatory G protein G(s), is of particular interest clinically. Amino acid substitutions at two specific codons lead to constitutive activation of G(s)alpha. Such gain-of-function mutations are found in a variety of sporadic endocrine tumors and in McCune-Albright syndrome, a sporadic condition characterized by multiple endocrine abnormalities. Heterozygous loss of G(s)alpha function results in the dominantly inherited condition, Albright hereditary osteodystrophy (AHO). Here we present a review of published GNAS1 mutations and report 19 additional mutations, of which 15 are novel. A diverse range of inactivating mutations has been detected, scattered throughout the gene but showing some evidence of clustering. Only one, a recurring 4 bp deletion in exon 7, could be considered common among AHO patients. The parental origin of the mutation apparently determines whether or not the patient shows end-organ resistance to hormones such as parathyroid hormone. G(s)alpha is biallelically expressed in all tissues studied to date and thus there is no direct evidence that this transcript is imprinted. However, the recent identification of other imprinted transcripts encoded by GNAS1 and overlapping G(s)alpha, together with at least one imprinted antisense transcript, raises intriguing questions about how the primary effect of mutations in GNAS1 might be modulated.

RDCI, the vasoactive intestinal peptide receptor: a candidate gene for the features of Albright hereditary osteodystrophy associated with deletion of 2q37

Albright hereditary osteodystrophy (AHO) is an autosomal dominant disorder characterised by the presence of brachymetaphalangism, short stature, obesity, and mental retardation. Variable biochemical changes many represent either pseudohypoparathyroidism (PHP) owing to resistance to parathormone (PTH) or pseudopseudohypoparathyroidism (PPHP) with no hormone resistance. In most cases of AHO, reduced levels of Gs alpha have been found and a number of deactivating mutations in the gene for Gs alpha located on chromosome 20q13 have been described. Recently a number of people with an AHO-like phenotype have been reported in whom a deletion of chromosomal region 2q37 has been found in the absence of biochemical abnormalities or a reduction in Gs alpha activity. We present a further female patient with a cytogenetically visible deletion of 2q37, an AHO-like phenotype, and unusual biochemistry suggesting moderate PTH resistance. The vasoactive intestinal peptide receptor (RDCI) has recently been mapped to 2q37 and we propose that this is a candidate gene, hemizygosity of which affects signal transduction and leads to the AHO-like phenotype found in patients with 2q37 deletions.

Isolated autosomal dominant type E brachydactyly: exclusion of linkage to candidate regions 2q37 and 20q13

Type E brachydactyly is a digital malformation which characteristically causes an asymmetrical shortening of one or more metacarpals or metatarsals or both. Although commonly seen as part of a syndrome, it can be inherited as an autosomal dominant characteristic, the gene acting with variable expressivity, but complete penetrance. As an Albright hereditary osteodystrophy (AHO)-like syndrome including brachydactyly type E and mental retardation may be caused by (micro) deletions at chromosome 2q37, this region together with the AHO locus at chromosome 20q13 were considered as candidate loci for brachydactyly type E. In this paper we described a family with isolated autosomal dominant type E brachydactyly in whom molecular analysis excludes linkage to these regions, providing support for further genetic heterogeneity of this trait.

Clinical implications of genetic defects in G proteins. The molecular basis of McCune-Albright syndrome and Albright hereditary osteodystrophy

Inactivating and activating mutations in the gene encoding G alpha s (GNAS1) are known to be the basis for 2 well-described contrasting clinical disorders, Albright hereditary osteodystrophy (AHO) and McCune-Albright syndrome (MAS). AHO is an autosomal dominant disorder due to germline mutations in GNAS1 that decrease expression or function of G alpha s protein. Loss of G alpha s function leads to tissue resistance to multiple hormones whose receptors couple to G alpha s. By contrast, MAS results from postzygotic somatic mutations in GNAS1 that lead to enhanced function of G alpha s protein. Acquisition of the activating mutation early in life leads to a more generalized distribution of the mosaicism and is associated with the classic clinical triad of polyostotic fibrous dysplasia, endocrine hyperfunction, and café au lait skin lesions described in MAS. Acquisition of a similar activating mutation in GNAS1 later in life presumably accounts for the restricted distribution of the gsp oncogene, and is associated with the development of isolated lesions (for example, fibrous dysplasia, pituitary or thyroid tumors) without other manifestations of MAS. Tissues that are affected by loss of G alpha s function in AHO are also affected by gain of G alpha s function in MAS, thus identifying specific tissues in which the second messenger cAMP plays a dominant role in cell growth, proliferation, or function. Further investigations of the functions of G alpha s and other members of the GTPase binding protein family will provide more insight into the pathogenesis and clinical manifestations of human disease.

A 4-base pair deletion mutation of Gs alpha gene in a Japanese patient with pseudohypoparathyroidism

Mutations in the guanine nucleotide binding protein alpha subunit (Gs alpha) have been found in patients with pseudohypoparathyroidism (PHP). We have screened the Gs alpha gene for mutations in a Japanese patient with this disorder and identified a novel 4-base pair deletion in exon 7 in codons 189-190. This deletion causes a frameshift and if synthesis of a truncated form of Gs alpha occurred, it would likely be biologically inactive. The patient was heterozygous for this deletion. The patient's mother and an unaffected brother were tested for the presence of this mutation. His mother had the same mutation, and although her serum calcium and parathyroid hormone levels were within the normal range, she had subcutaneous calcifications. Thus, this mutation appears to be necessary but not sufficient to cause the complete pseudohypoparathyroidism phenotype and thus, other unknown factors, either genetic or acquired, may be necessary for the full syndrome to occur.

Pseudohypoparathyroidism type Ia: two new heterozygous frameshift mutations in exons 5 and 10 of the Gs alpha gene

Pseudohypoparathyroidism type Ia (PHP-Ia) is a hereditary disease characterized by resistance to PTH and other hormones that act via cAMP. Patients have deficient activity of Gs alpha, the alpha subunit of the G protein, which couples hormone receptors to stimulation of adenylate cyclase. We describe two new mutations discovered in two sporadic patients with PHP-Ia. Using genomic DNA, we have amplified exons 2-13 of the Gs alpha gene (GNAS1) by PCR, and sequenced the resulting products. Both patients had Albright's hereditary osteodystrophy, resistance to multiple hormones, and deficient Gs alpha activity. In the first patient, a deletion of a C in exon 5 at codon 115 was found. In the second patient, an insertion of a C in exon 10 at codon 267 was detected. Both these heterozygous mutations cause frameshift, and predict decreased production of Gs alpha. This report adds two new Gs alpha mutations to the known ten mutations recently described.

Parental origin of Gs alpha gene mutations in Albright's hereditary osteodystrophy

Heterozygous mutations of the Gs alpha gene leading to reduced Gs alpha activity have been identified in patients with Albright's hereditary osteodystrophy (AHO). However, AHO may be associated with hormone resistance (pseudohypoparathyroidism type Ia, PHPIa) or a normal response (pseudo-pseudohypoparathyroidism, PPHP). As both disorders may occur within the same family, the relationship between Gs alpha genotype and phenotype remains unresolved. The AHO phenotype may be dependent upon the sex of the parent transmitting the Gs alpha mutation, perhaps through a gene imprinting mechanism. We have used an intragenic Gs alpha FokI polymorphism to determine the parental origin of Gs alpha gene mutations in sporadic and familial AHO. We now show that a de novo G-->A substitution at the exon 5 donor splice junction in a child with PPHP was paternally derived. Furthermore, in a female with PPHP, the Gs alpha abnormality was shown to be of paternal origin, while subsequent maternal processing and transmission resulted in PHPIa in two offspring. As transmission of PPHP has rarely been reported, determining parental origin of the disease allele in sporadic cases may provide insight into the mechanism of hormone resistance in AHO.

Albright's hereditary osteodystrophy

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