Normal erythrocyte membrane Gs alpha bioactivity in two unrelated patients with acrodysostosis

Protein Kinase A Downregulation Delays the Development and Progression of Polycystic Kidney Disease

Background: Upregulation of cAMP-dependent and -independent PKA signaling is thought to promote cystogenesis in polycystic kidney disease (PKD). PKA-I regulatory subunit RIα is increased in kidneys of orthologous mouse models. Kidney-specific knockout of RIα upregulates PKA activity, induces cystic disease in wild-type mice, and aggravates it in Pkd1 ^RC/RC mice. Methods: PKA-I activation or inhibition was compared to EPAC activation or PKA-II inhibition using Pkd1 ^RC/RC metanephric organ cultures. The effect of constitutive PKA (preferentially PKA-I) downregulation in vivo was ascertained by kidney-specific expression of a dominant negative RIαB allele in Pkd1 ^RC/RC mice obtained by crossing Prkar1α ^R1αB/WT, Pkd1 ^RC/RC, and Pkhd1-Cre mice (C57BL/6 background). The effect of pharmacologic PKA inhibition using a novel, selective PRKACA inhibitor (BLU2864) was tested in mIMCD3 3D cultures, metanephric organ cultures, and Pkd1 ^RC/RC mice on a C57BL/6 x 129S6/Sv F1 background. Mice were sacrificed at 16 weeks of age. Results: PKA-I activation promoted and inhibition prevented ex vivo P-Ser133 CREB expression and cystogenesis. EPAC activation or PKA-II inhibition had no or only minor effects. BLU2864 inhibited in vitro mIMCD3 cystogenesis and ex vivo P-Ser133 CREB expression and cystogenesis. Genetic downregulation of PKA activity and BLU2864 directly and/or indirectly inhibited many pro-proliferative pathways and were both protective in vivo BLU2864 had no detectable on- or off-target adverse effects. Conclusions: PKA-I is the main PKA isozyme promoting cystogenesis. Direct PKA inhibition may be an effective strategy to treat PKD and other conditions where PKA signaling is upregulated. By acting directly on PKA, the inhibition may be more effective than or substantially increase the efficacy of treatments that only affect PKA activity by lowering cAMP.

A novel de novo PDE4D gene mutation identified in a Chinese patient with acrodysostosis

Acrodysostosis is an extremely rare disorder at birth, that is, characterized by skeletal dysplasia with short stature and midfacial hypoplasia, which has been reported to be caused by PDE4D and PRKAR1A gene mutations. Here, a Chinese boy with acrodysostosis, ventricular septal defect, and pulmonary hypertension was recruited for our study, and his clinical and biochemical characteristics were analyzed. A novel de novo heterozygous missense mutation (NM_001104631: c.2030A>C, p.Tyr677Ser) of the PDE4D gene was detected by whole exome sequencing and confirmed by Sanger sequencing. The c.2030A>C (p.Tyr677Ser) variant was located in exon 15 of the PDE4D gene, predicted to be damaging by a functional prediction program and shown to be highly conserved among many species. Further functional analysis showed that the p.Tyr677Ser substitution changes the function of the PDE4D protein, affects its subcellular localization in transfected cells, increases PDE4 activity in the regulation of cAMP signaling and affects cell proliferation. Our study identified a novel de novo PDE4D mutation in acrodysostosis of Chinese origin that not only contributes a deeper appreciation of the phenotypic characteristics of patients with PDE4D mutations but also expands the spectrum of PDE4D mutations.

Endocrinological and phenotype evaluation in a patient with acrodysostosis

Acrodysostosis is characterized by distinctive facial features and severe brachydactyly. Mutations in PRKAR1A or PDE4D are known to be responsible for this disease. Cases of hormonal resistance have been reported, particularly in patients with PRKAR1A mutations. The physical characteristics and endocrine function of pseudohypoparathyroidism type Ia is known to resemble acrodysostosis. We report the case of a 4-yr-old patient with a PRKAR1A mutation. He had characteristic facies with an upturned nose and cone-shaped epiphyses of most phalanges. These findings have not been reported as extensive for cases of pseudohypoparathyroidism type Ia. He also had TSH resistance from birth. We performed endocrinological stimulation tests to further evaluate his endocrine status. These examinations revealed resistance to TSH and PTH, but there was normal secretion of ACTH, GH, and cortisol. An Ellsworth-Howard test resulted in normal urinary cAMP excretion. This response differs from that of pseudohypoparathyroidism type Ia. In summary, the constellation of an upturned nose, cone-shaped epiphyses of most if not all phalanges, and PTH resistance with a normal urinary cAMP response may satisfactorily enable clinical diagnosis of acrodysostosis.

Pseudohypoparathyroidism: one gene, several syndromes

Pseudohypoparathyroidism (PHP) and pseudopseudohypoparathyroidism (PPHP) are caused by mutations and/or epigenetic changes at the complex GNAS locus on chromosome 20q13.3 that undergoes parent-specific methylation changes at several sites. GNAS encodes the alpha-subunit of the stimulatory G protein (Gsα) and several splice variants thereof. Heterozygous inactivating mutations involving the maternal GNAS exons 1-13 cause PHP type Ia (PHP1A). Because of much reduced paternal Gsα expression in certain tissues, such as the proximal renal tubules, thyroid, and pituitary, there is little or no Gsα protein in the presence of maternal GNAS mutations, thus leading to PTH-resistant hypocalcemia and hyperphosphatemia. When located on the paternal allele, the same or similar GNAS mutations are the cause of PPHP. Besides biochemical abnormalities, patients affected by PHP1A show developmental abnormalities, referred to as Albrights hereditary osteodystrophy (AHO). Some, but not all of these AHO features are encountered also in patients affected by PPHP, who typically show no laboratory abnormalities. Autosomal dominant PHP type Ib (AD-PHP1B) is caused by heterozygous maternal deletions within GNAS or STX16, which are associated with loss-of-methylation (LOM) at exon A/B alone or at all maternally methylated GNAS exons. LOM at exon A/B and the resulting biallelic expression of A/B transcripts reduces Gsα expression, thus leading to hormonal resistance. Epigenetic changes at all differentially methylated GNAS regions are also observed in sporadic PHP1B, the most frequent disease variant, which remains unresolved at the molecular level, except for rare cases with paternal uniparental isodisomy or heterodisomy of chromosome 20q (patUPD20q).

Pseudohypoparathyroidism vs. tricho-rhino-phalangeal syndrome: patient reclassification

OBJECTIVES

Given that tricho-rhino-phalangeal syndrome (TRPS) and pseudohypoparathyroidism/pseudopseudohypoparathyroidism (PHP/PPHP) are very rare monogenic disorders that share some features (distinctive facies, short stature, brachydactyly and, in some patients, intellectual disability) that lead to their misdiagnosis in some cases, our objective was to identify clinical, biochemical or radiological signs that could help to distinguish these two syndromes.

METHODS AND RESULTS

We report on two cases, which were referred to the Endocrinology and Pediatric Endocrinology Services for obesity. Clinical evaluation initially suggested the diagnosis of PHP-Ia [phenotype suggestive of Albright hereditary osteodystrophy (AHO) with parathyroid hormone (PTH) resistance] and PPHP (phenotype resembling AHO, without PTH resistance), but (epi)genetic analysis of the GNAS locus ruled out the suspected diagnosis. Further clinical re-evaluation prompted us to suspect TRPS, and this was confirmed genetically.

CONCLUSION

TRPS was mistakenly identified as PHP/PPHP because of the coexistence of obesity and brachydactyly, with PTH resistance in one of the cases. Specific traits such as sparse scalp hair and a pear-shaped nose, present in both cases, can be considered pathognomonic signs of TRPS, which could help us to reach a correct diagnosis.

Heterozygous mutations in cyclic AMP phosphodiesterase-4D (PDE4D) and protein kinase A (PKA) provide new insights into the molecular pathology of acrodysostosis

Acrodysostosis without hormone resistance is a rare skeletal disorder characterized by brachydactyly, nasal hypoplasia, mental retardation and occasionally developmental delay. Recently, loss-of-function mutations in the gene encoding cAMP-hydrolyzing phosphodiesterase-4D (PDE4D) have been reported to cause this rare condition but the pathomechanism has not been fully elucidated. To understand the pathogenetic mechanism of PDE4D mutations, we conducted 3D modeling studies to predict changes in the binding efficacy of cAMP to the catalytic pocket in PDE4D mutants. Our results indicated diminished enzyme activity in the two mutants we analyzed (Gly673Asp and Ile678Thr; based on PDE4D4 residue numbering). Ectopic expression of PDE4D mutants in HEK293 cells demonstrated this reduction in activity, which was identified by increased cAMP levels. However, the cells from an acrodysostosis patient showed low cAMP accumulation, which resulted in a decrease in the phosphorylated cAMP Response Element-Binding Protein (pCREB)/CREB ratio. The reason for this discrepancy was due to a compensatory increase in expression levels of PDE4A and PDE4B isoforms, which accounted for the paradoxical decrease in cAMP levels in the patient cells expressing mutant isoforms with a lowered PDE4D activity. Skeletal radiographs of 10-week-old knockout (KO) rats showed that the distal part of the forelimb was shorter than in wild-type (WT) rats and that all the metacarpals and phalanges were also shorter in KO, as the name acrodysostosis implies. Like the G-protein α-stimulatory subunit and PRKAR1A, PDE4D critically regulates the cAMP signal transduction pathway and influences bone formation in a way that activity-compromising PDE4D mutations can result in skeletal dysplasia. We propose that specific inhibitory PDE4D mutations can lead to the molecular pathology of acrodysostosis without hormone resistance but that the pathological phenotype may well be dependent on an over-compensatory induction of other PDE4 isoforms that can be expected to be targeted to different signaling complexes and exert distinct effects on compartmentalized cAMP signaling.

PRKAR1A mutation affecting cAMP-mediated G protein-coupled receptor signaling in a patient with acrodysostosis and hormone resistance

CONTEXT

Acrodysostosis is a rare autosomal dominant disorder characterized by short stature, peculiar facial appearance with nasal hypoplasia, and short metacarpotarsals and phalanges with cone-shaped epiphyses. Recently, mutations of PRKAR1A and PDE4D downstream of GNAS on the cAMP-mediated G protein-coupled receptor (GPCR) signaling cascade have been identified in acrodysostosis with and without hormone resistance, although functional studies have been performed only for p.R368X of PRKAR1A.

OBJECTIVE

Our objective was to report a novel PRKAR1A mutation and its functional consequence in a Japanese female patient with acrodysostosis and hormone resistance.

PATIENT

This patient had acrodysostosis-compatible clinical features such as short stature and brachydactyly and mildly elevated serum PTH and TSH values.

RESULTS

Although no abnormality was detected in GNAS and PDE4D, a novel de novo heterozygous missense mutation (p.T239A) was identified at the cAMP-binding domain A of PRKAR1A. Western blot analysis using primary antibodies for the phosphorylated cAMP-responsive element (CRE)-binding protein showed markedly reduced CRE-binding protein phosphorylation in the forskolin-stimulated lymphoblastoid cell lines of this patient. CRE-luciferase reporter assays indicated significantly impaired response of protein kinase A to cAMP in the HEK293 cells expressing the mutant p.T239A protein.

CONCLUSIONS

The results indicate that acrodysostosis with hormone resistance is caused by a heterozygous mutation at the cAMP-binding domain A of PRKAR1A because of impaired cAMP-mediated GPCR signaling. Because GNAS, PRKAR1A, and PDE4D are involved in the GPCR signal transduction cascade and have some different characters, this would explain the phenotypic similarity and difference in patients with GNAS, PRKAR1A, and PDE4D mutations.

Recurrent PRKAR1A mutation in acrodysostosis with hormone resistance

The skeletal dysplasia characteristic of acrodysostosis resembles the Albright's hereditary osteodystrophy seen in patients with pseudohypoparathyroidism type 1a, but defects in the α-stimulatory subunit of the G-protein (GNAS), the cause of pseudohypoparathyroidism type 1a, are not present in patients with acrodysostosis. We report a germ-line mutation in the gene encoding PRKAR1A, the cyclic AMP (cAMP)-dependent regulatory subunit of protein kinase A, in three unrelated patients with acrodysostosis and resistance to multiple hormones. The mutated subunit impairs the protein kinase A response to stimulation by cAMP; this explains our patients' hormone resistance and the similarities of their skeletal abnormalities with those observed in patients with pseudohypoparathyroidism type 1a.