Phenotypic and molecular genetic aspects of pseudohypoparathyroidism type Ib in a Greek kindred: evidence for enhanced uric acid excretion due to parathyroid hormone resistance

Autosomal dominant pseudohypoparathyroidism type 1b due to STX16 deletion: a case presentation and literature review

INTRODUCTION

Pseudohypoparathyroidism (PHP) is a heterogeneous group of rare, genetically related, endocrine disorders, characterized by end-organ resistance to parathyroid hormone (PTH) action and other G protein-coupled receptors (GPCRs) related hormones. The clinical variants of PHP are classified according to the presence of features of Albright's hereditary osteodystrophy (AHO) and in vivo response to exogenous PTH. Autosomal dominant PHP1b is often caused by a deletion in the syntaxin-16 (STX16) gene, leading to a loss of methylation in the A/B exon of the guanine nucleotide-binding protein a-stimulating polypeptide (GNAS) complex. Herein, we present a case of a 41-year-old man with familiar PHP1b due to a maternal inherited 3-kb STX16 deletion, who was referred to us for consultation by artificial reproductive technology specialists.

EVIDENCE ACQUISITION

A bibliographic search was performed in electronic databases (PubMed and Cochrane Library) to identify similar cases.

EVIDENCE SYNTHESIS

Twenty studies (case-series or reports) were eligible. These studies included collectively 120 patients; 46 patients (38.3%) presented with symptoms of hypocalcemia; 38 were asymptomatic (31.7%); data for 36 patients (30%) were unavailable. Thyroid-stimulating hormone (TSH) resistance was documented in 25 occasions (21%); growth hormone deficiency in 2 (1.7%); 3 patients shared features of the AHO (2.5%); 6 had abnormal bone mineral density test (5%). Notable is the development of tertiary hyperparathyroidism in 3 individuals (2.5%).

CONCLUSIONS

The present review confirms the heterogeneity in the clinical spectrum of familiar PHP1b. Future research should focus on the molecular characterization of the GNAS disorders, leading to a facile diagnosis and appropriate genetic counseling.

Hereditary and non-hereditary etiologies associated with extensive brain calcification: case series

Cerebral calcification may be caused by several potentially treatable conditions, however, in most cases it does not receive special attention in clinical practice. From the point of view of etiology, the diseases associated with cerebral calcification can be divided into two main groups: idiopathic (mostly incurable) and secondary (potentially treatable). The first group includes mainly the hereditary diseases identified before 2021 (primary familial brain calcification subtypes, previously known as Fahr's disease or Fahr's syndrome). In contrast, the second group includes diseases with cerebral calcification that develop generally as a consequence of metabolic/endocrine/autoimmune abnormalities. The aim of our research was to present hereditary and non-hereditary etiologies associated with extensive brain calcification. We compare the detailed clinical, radiological and laboratory results of 6 patients with prominent cerebral calcification identified in our clinic in the last 3 years (idiopathic and secondary etiologies as well). Our research draws attention to the complexity of the etiologies in the context of cerebral calcification. We recommend, beside NGS-based sequence analyses, the application of array comparative genomic hybridization as well, to identify potential genetic etiologies associated with brain calcification.

Urine and Serum Electrolytes and Biochemical Values Associated with Osteoporosis in Premenopausal and Postmenopausal Women: A Longitudinal and Cross-Sectional Study Using Korean Genome and Epidemiology Study (KoGES) Cohort

Osteoporosis is a major public health concern, especially in women. This study aims to identify early biomarkers from biochemical measurements of serum and urine for recognizing the development of osteoporosis and osteopenia in premenopausal and postmenopausal women. From the Korean Genome and Epidemiology Study (KoGES) cohort, longitudinal study participants with normal bone density were enrolled and assessed for the association of baseline clinical and biochemical factors with osteoporosis development over 4 years. In addition, a cross-sectional study between normal bone density and osteopenia/osteoporosis was conducted to validate the risk factors found in the longitudinal cohort. Of the 5272 female participants in the KoGES cohort, 813 women (501 premenopausal and 312 menopausal) who had normal bone density at baseline were included in the longitudinal study. During the 4 years of follow-up, 64 patients developed osteoporosis and 354 developed osteopenia. In a multivariate logistic regression analysis, serum calcium and urine uric acid levels were significantly associated with elevated osteoporosis risk in premenopausal and postmenopausal women, respectively (risk of osteoporosis by serum calcium levels in premenopausal women: 4.03 (1.09–14.93), p = 0.037; risk of osteoporosis by urine uric acid levels in postmenopausal women: 24.08 (1.79–323.69), p = 0.016). For the cross-sectional study, serum and urine parameters were compared between women with osteopenia or osteoporosis at baseline and those with normal bone density. Urine uric acid levels were found to be significantly higher in both premenopausal and postmenopausal women with bone loss than in women with normal bone density (p < 0.001 and p = 0.004, respectively). Uric acid level in urine may be an early marker for the development of osteoporosis in women, especially after menopause.

Preferential Maternal Transmission of STX16-GNAS Mutations Responsible for Autosomal Dominant Pseudohypoparathyroidism Type Ib (PHP1B): Another Example of Transmission Ratio Distortion

Preferential transmission of a genetic mutation to the next generation, referred to as transmission ratio distortion (TRD), is well established for several dominant disorders, but underlying mechanisms remain undefined. Recently, TRD was reported for patients affected by pseudohypoparathyroidism type Ia or pseudopseudohypoparathyroidism. To determine whether TRD is observed also for autosomal dominant pseudohypoparathyroidism type Ib (AD-PHP1B), we analyzed kindreds with the frequent 3-kb STX16 deletion or other STX16/GNAS mutations. If inherited from a female, these genetic defects lead to loss-of-methylation at exon A/B alone or at all three differentially methylated regions (DMR), resulting in parathyroid hormone (PTH)-resistant hypocalcemia and hyperphosphatemia and possibly resistance to other hormones. In total, we investigated 212 children born to 80 females who are unaffected carriers of a STX16/GNAS mutation (n = 47) or affected by PHP1B (n = 33). Of these offspring, 134 (63.2%) had inherited the genetic defect (p = .00012). TRD was indistinguishable for mothers with a STX16/GNAS mutation on their paternal (unaffected carriers) or maternal allele (affected). The mechanisms favoring transmission of the mutant allele remain undefined but are likely to include abnormalities in oocyte maturation. Search for mutations in available descendants of males revealed marginally significant evidence for TRD (p = .038), but these analyses are less reliable because many more offspring of males than females with a STX16/GNAS mutation were lost to follow-up (31 of 98 versus 6 of 218). This difference in follow-up is probably related to the fact that inheritance of a mutation from a male does not have clinical implications, whereas inheritance from an affected or unaffected female results in PHP1B. Lastly, affected PHP1B females had fewer descendants than unaffected carriers, but it remains unclear whether abnormal oocyte development or impaired actions of reproductive hormones are responsible. Our findings highlight previously not recognized aspects of AD-PHP1B that are likely to have implications for genetic testing and counseling. © 2020 American Society for Bone and Mineral Research (ASBMR).

Association between serum uric acid and bone health in adolescents

Previous studies are suggestive of the protective role of uric acid on bone in the middle-aged and elderly. Whether this association exists in younger individuals has not been examined. This investigation showed a significant positive association between serum uric acid and bone parameters among Iranian adolescents.

INTRODUCTION

Uric acid (UA) might be linked to bone health, but it is unclear whether its effects on bone are limited to certain population subgroups. This study is aimed at investigating the correlation between serum uric acid levels and bone mineral density (BMD) in Iranian adolescents.

METHODS

This cross-sectional study was conducted on 413 (221 girls and 192 boys) Iranian adolescents aged 9-19 years. An analysis of anthropometric, biochemical parameters and bone density was performed on the participants. Measurements included serum uric acid, calcium, phosphorus, alkaline phosphatase, albumin, and vitamin D. They were divided according to their serum UA into the low UA group who had UA ≤ 6 mg/dL and the high UA group with UA > 6 mg/dL. BMD and bone mineral content (BMC) were measured in the total body, lumbar spine, and left femoral neck, using dual energy X-ray absorptiometry (DXA), and bone mineral apparent density (BMAD) was calculated.

RESULTS

A Pearson correlation analysis revealed a significant correlation between UA and bone parameters. In multiple regression analyses adjusted for potential confounders, serum UA was proven to be associated with BMD and BMC at all sites. There was no association between UA, serum calcium, and vitamin D concentrations.

CONCLUSION

Our study, as the first research on adolescents, demonstrated a higher bone density in those who had higher UA levels.

Diagnosis and management of pseudohypoparathyroidism and related disorders: first international Consensus Statement

This Consensus Statement covers recommendations for the diagnosis and management of patients with pseudohypoparathyroidism (PHP) and related disorders, which comprise metabolic disorders characterized by physical findings that variably include short bones, short stature, a stocky build, early-onset obesity and ectopic ossifications, as well as endocrine defects that often include resistance to parathyroid hormone (PTH) and TSH. The presentation and severity of PHP and its related disorders vary between affected individuals with considerable clinical and molecular overlap between the different types. A specific diagnosis is often delayed owing to lack of recognition of the syndrome and associated features. The participants in this Consensus Statement agreed that the diagnosis of PHP should be based on major criteria, including resistance to PTH, ectopic ossifications, brachydactyly and early-onset obesity. The clinical and laboratory diagnosis should be confirmed by a molecular genetic analysis. Patients should be screened at diagnosis and during follow-up for specific features, such as PTH resistance, TSH resistance, growth hormone deficiency, hypogonadism, skeletal deformities, oral health, weight gain, glucose intolerance or type 2 diabetes mellitus, and hypertension, as well as subcutaneous and/or deeper ectopic ossifications and neurocognitive impairment. Overall, a coordinated and multidisciplinary approach from infancy through adulthood, including a transition programme, should help us to improve the care of patients affected by these disorders.

Role of Parathyroid Hormone in Regulating Transporter and Metabolizing Enzyme Function

Recent studies demonstrate that parathyroid hormone (PTH) not only maintains mineral homeostasis through targeting the kidneys and bone, but also exerts its effects on other organs. For instance, PTH induces urate accumulation through inhibiting the expression of the ABCG2 in both the intestine and the kidney. In addition, PTH downregulates the expression of cytochrome P450 (CYP) 3A, a major enzyme for drug metabolism in both the intestine and liver, resulting in the increase of substrate drug exposure. These functions of PTH are mediated through the PTH receptor (PTHR) signaling. Since PTHR exists in various organs, PTH may regulate other, still unspecified transporters or enzymes in the organs that express PTHR.

A rare cause of hypocalcemia

Background:

Case Report:

Conclusions:

De novo STX16 deletions: an infrequent cause of pseudohypoparathyroidism type Ib that should be excluded in sporadic cases

CONTEXT

Maternally inherited 3-kb STX16 deletions cause autosomal dominant pseudohypoparathyroidism type Ib (PHP-Ib) characterized by PTH resistance with loss of methylation restricted to the GNAS exon A/B.

OBJECTIVE

The objective of the study was to search for the 3-kb STX16 deletion and to establish haplotypes for the GNAS region for two PHP-Ib patients and their families.

SETTING

The study was conducted at a research laboratory and tertiary care hospitals.

PATIENTS

The index cases presented at the ages 8 and 9.5 yr, respectively, with hypocalcemia, hyperphosphatemia, and elevated PTH.

INTERVENTIONS

There were no interventions.

RESULTS

DNA analyses of the index cases revealed an isolated loss of the GNAS exon A/B methylation and the 3-kb STX16 deletion. In the first family, the patient's healthy mother and sister showed no genetic or epigenetic abnormality, yet microsatellite analysis of the GNAS region indicated that both siblings share the same maternal allele, with the exception of an allelic loss for marker 261P9-CA1 (located within STX16), leading to the conclusion that a de novo mutation had occurred on the maternal allele. In the second family, three siblings of the index case are also affected, and an analysis of their DNA revealed the 3-kb STX16 deletion, which was also found in the healthy mother and a maternal uncle. Analysis of the siblings of the deceased maternal grandfather and some of their descendants excluded the 3-kb STX16 deletion, but haplotype analysis of the GNAS region suggested that he had acquired the mutation de novo.

CONCLUSIONS

De novo 3-kb STX16 deletions, reported only once previously, are infrequent but should be excluded in all cases of PHP-Ib, even when the family history is negative for an inherited form of this disorder.

Uric acid and IGF1 as possible determinants of FGF23 metabolism in children with normal renal function

BACKGROUND

Fibroblast growth factor 23 (FGF23) is a phosphaturic hormone and a suppressor of renal 1α hydroxylase. Although circulating values of FGF23 are increased in early chronic kidney disease (CKD), the interplay between FGF23 levels, growth and nutritional biomarkers has not been evaluated in children with normal renal function.

METHODS

We performed a secondary analysis of the cross-sectional observational INU23 study in 98 children (51 boys, mean age 10.5 ± 3.9 years) with preserved renal function (glomerular filtration rate (GFR) 114 ± 14 ml/min/1.73 m(2)).

RESULTS

In bivariate analyses, C-terminal FGF23 levels were positively related to phosphorus and uric acid levels. Intact FGF23 levels were positively associated with uric acid and insulin growth factor 1 (IGF1) levels, with similar results for age, body mass index (BMI), and 25OH vitamin D (25(OH)D). By multivariable analyses, 25(OH)D, uric acid, and phosphorus were independent predictors of C-terminal FGF23, while 25(OH)D, uric acid, and IGF1 were independent predictors of intact FGF23.

CONCLUSIONS

In children with preserved kidney function, the association between FGF23, uric acid, and IGF1 suggests that FGF23 could be an early nutritional indicator of high protein and phosphate intake. The association between FGF23 and IGF1 also suggests a relationship between FGF23 and growth, and warrants further investigation.

Clinical and functional characterization of URAT1 variants

Idiopathic renal hypouricaemia is an inherited form of hypouricaemia, associated with abnormal renal handling of uric acid. There is excessive urinary wasting of uric acid resulting in hypouricaemia. Patients may be asymptomatic, but the persistent urinary abnormalities may manifest as renal stone disease, and hypouricaemia may manifest as exercise induced acute kidney injury. Here we have identified Macedonian and British patients with hypouricaemia, who presented with a variety of renal symptoms and signs including renal stone disease, hematuria, pyelonephritis and nephrocalcinosis. We have identified heterozygous missense mutations in SLC22A12 encoding the urate transporter protein URAT1 and correlate these genetic findings with functional characterization. Urate handling was determined using uptake experiments in HEK293 cells. This data highlights the importance of the URAT1 renal urate transporter in determining serum urate concentrations and the clinical phenotypes, including nephrolithiasis, that should prompt the clinician to suspect an inherited form of renal hypouricaemia.

Clinical characterization and identification of two novel mutations of the GNAS gene in patients with pseudohypoparathyroidism and pseudopseudohypoparathyroidism

OBJECTIVE

Pseudohypoparathyroidism (PHP) and pseudopseudohypoparathyroidism (PPHP) are rare disorders resulting from genetic and epigenetic aberrations in the GNAS locus.

DESIGN

Investigation of clinical characteristics and molecular analysis in PHP and PPHP.

PATIENTS

Fourteen subjects from 13 unrelated families including subjects with PPHP (n = 1), PHP-Ia (n = 6) and PHP-Ib (n = 7) were enrolled.

MEASUREMENTS

Clinical data, including age at presentation, presenting symptom, auxological findings, family history, presence of Albright hereditary osteodystrophy (AHO) features and hormonal and biochemical findings, were analysed. The GNAS locus was subjected to direct sequencing and methylation analysis using methylation-specific multiplex ligation-dependent probe amplification (MS-MLPA).

RESULTS

Of the 13 PHP subjects, 10 (three PHP-Ia and seven PHP-Ib) presented with hypocalcemic tetany at ages ranging from 7 to 14·8 years. Subcutaneous calcification was observed as an early manifestation of AHO in one PHP-Ia patient (age, 2·9 years) and one PPHP patient (age, 7 months). Six PHP-Ia and one PPHP harboured four different heterozygous mutations within the coding region of GNAS, p.Asp189_Tyr190delinsMetfxX14, p.Val117fsX23, p.Tyr190CysfsX19, and a splicing mutation (c.659 + 1G>A), of which the latter two were novel. Five subjects with PHP-Ib exhibited complete loss of the maternal-specific methylation pattern. The remaining two PHP-Ib showed a loss of methylation of exon 1A on the maternal allele as a consequence of heterozygous 3-kb microdeletions within the STX16 gene.

CONCLUSIONS

GNAS mutation analyses and MS-MLPA assays are useful molecular tools for understanding the molecular bases and confirming the diagnosis of PHP and PPHP.

The GNAS Locus: Quintessential Complex Gene Encoding Gsalpha, XLalphas, and other Imprinted Transcripts

The currently estimated number of genes in the human genome is much smaller than previously predicted. As an explanation for this disparity, most individual genes have multiple transcriptional units that represent a variety of biologically important gene products. GNAS exemplifies a gene of such complexity. One of its products is the alpha-subunit of the stimulatory heterotrimeric G protein (Gsalpha), a ubiquitous signaling protein essential for numerous different cellular responses. Loss-of-function and gain-of-function mutations within Gsalpha-coding GNAS exons are found in various human disorders, including Albright's hereditary osteodystrophy, pseudohypoparathyroidism, fibrous dysplasia of bone, and some tumors of different origin. While Gsalpha expression in most tissues is biallelic, paternal Gsalpha expression is silenced in a small number of tissues, playing an important role in the development of phenotypes associated with GNAS mutations. Additional products derived exclusively from the paternal GNAS allele include XLalphas, a protein partially identical to Gsalpha, and two non-coding RNA molecules, the A/B transcript and the antisense transcript. The maternal GNAS allele leads to NESP55, a chromogranin-like neuroendocrine secretory protein. In vivo animal models have demonstrated the importance of each of the exclusively imprinted GNAS products in normal mammalian physiology. However, although one or more of these products are also disrupted by most naturally occurring GNAS mutations, their roles in disease pathogenesis remain unknown. To further our understanding of the significance of this gene in physiology and pathophysiology, it will be important to elucidate the cellular roles and the mechanisms regulating the expression of each GNAS product.

Serum uric acid is associated with bone health in older men: a cross-sectional population-based study

Serum uric acid (UA) is a strong endogenous antioxidant. Since oxidative stress has been linked to osteoporosis, we examined the association between serum UA levels and bone mineral density (BMD), prevalent vertebral and nonvertebral fractures, and laboratory measures such as calcitropic hormones and bone turnover marker levels. This cross-sectional analysis consisted of 1705 community-dwelling men aged 70 years or over who participated in the baseline part of the Concord Health and Ageing in Men Project (CHAMP), a population-based study of older men in Sydney, Australia. BMD at all sites was significantly higher among men with serum UA levels above the group median than among men with UA levels below the median. In multiple regression analyses adjusted for potential confounders, serum UA remained associated with BMD at all sites (β = 0.12 to 0.14, p < .001), serum calcium (β = 0.11, p = .001), parathyroid hormone (β = 0.09, p = .002), 25-hydroxyvitamin D (β = 0.09, p = .005), and was negatively associated with urinary excretion amino-terminal cross-linked telopeptide of type 1 collagen (β = -0.09, p = .006). Overall, serum UA accounted for 1.0% to 1.44% of the variances in BMD (R(2)  = 0.10 to 0.22). In multiple logistic regression analyses, above-median serum UA levels were associated with a lower prevalence of osteoporosis at the femoral neck [odds ratio (OR) = 0.42, 95% confidence interval (CI) 0.22-0.81, p = .010) and lumbar spine (OR = 0.44, 95% CI 0.23-0.86, p = .016) and a lower prevalence of vertebral (OR = 0.62, 95% CI 0.43-0.91, p = .015) and nonvertebral (OR = 0.51, 95% CI 0.29-0.89, p = .018) fractures. In conclusion, higher serum UA levels are associated with higher BMD at all skeletal sites and with a lower prevalence of vertebral and nonvertebral fractures in older men.

Osteosclerosis in two brothers with autosomal dominant pseudohypoparathyroidism type 1b: bone histomorphometric analysis

OBJECTIVE

Pseudohypoparathyroidism (PHP) is a heterogeneous disorder characterized by hypocalcemia and hyperphosphatemia resulting from selective renal resistance to parathyroid hormone (PTH). One autosomal dominant form of PHP type 1b (PHP-Ib) is most frequently caused by a maternally inherited 3-kb deletion within STX16, the gene encoding syntaxin 16. To date, increased bone mineral density (BMD) has been described only in PHP type 1a, and there is a lack of detailed information on bone histomorphometry in PHP-Ib. The objective of this report was to present trans-iliac static and dynamic histomorphometry in two brothers with the 3-kb deletion in the STX16 region and elevated BMD.

DESIGN

Observational study of two brothers (age 18.0 and 22.7 years) with the 3-kb STX16 deletion and increased BMD.

RESULTS

The brothers had elevated PTH (146 pg/ml (15.6 pmol/l) and 102 pg/ml (10.9 pmol/l); normal: 10-64 pg/ml (1.1-6.8 pmol/l)) and striking osteosclerosis (lumbar spine areal BMD Z-scores: +5.4 and +4.9). Bone histomorphometry showed marked elevations in cortical width for both brothers (241 and 209% of the mean result expected for age), with elevations in the bone formation rate on the endocortical (119 and 260% of the healthy mean) and trabecular (220 and 190% of mean) surfaces.

CONCLUSION

Our findings suggest that PTH in this PHP-Ib genotype can increase cortical thickness due to its anabolic effect on endocortical bone, and underscore the heterogeneity in the skeletal phenotype among patients with PHP-Ib.

Recessive versus imprinted disorder: consanguinity can impede establishing the diagnosis of autosomal dominant pseudohypoparathyroidism type Ib

Hypocalcemia and hyperphosphatemia with low/normal parathyroid hormone (PTH) levels can be observed in hypoparathyroidism (HP), a disorder that may follow an autosomal dominant (AD) or autosomal recessive (AR) mode of inheritance. Similar biochemical changes are also observed in pseudohypoparathyroidism (PHP) type Ia and Ib, but affected patients usually show elevated PTH levels indicative of hormonal resistance. Features of Albright's hereditary osteodystrophy (AHO) are typically not observed in patients affected by familial forms of PHP-Ib, which are most frequently caused by maternally inherited, heterozygous microdeletions within STX16 and are associated with isolated loss of methylation at GNAS exon A/B. We established the molecular defect in two children of consanguineous Turkish parents, who presented with hypocalcemia, hyperphosphatemia, and low 25-OH vitamin D levels, but initially normal or only mildly elevated PTH levels, i.e. findings that do not readily exclude HP. After normalizing serum magnesium levels, hypocalcemia and hyperphosphatemia persisted, and PTH levels increased, suggesting PTH resistance rather than PTH deficiency. Because of the absence of AHO and parental consanguinity, an AR form of PHP-Ib appeared plausible, which had previously been suggested for sporadic cases. However, loss of GNAS methylation was restricted to exon A/B, which led to the identification of the 3-kb STX16 microdeletion. The same mutation was also detected in the healthy mother, who did not show any GNAS methylation abnormality, indicating that her deletion resides on the paternal allele. Our findings emphasize the importance of considering a parentally imprinted, AD disorder even if consanguinity suggests an AR mode of inheritance.

Deletion of the noncoding GNAS antisense transcript causes pseudohypoparathyroidism type Ib and biparental defects of GNAS methylation in cis

CONTEXT

GNAS encodes the alpha-subunit of the stimulatory G protein as well as additional imprinted transcripts including the maternally expressed NESP55 and the paternally expressed XLalphas, antisense, and A/B transcripts. Most patients with pseudohypoparathyroidism type Ib (PHP-Ib) exhibit imprinting defects affecting the maternal GNAS allele, which are thought to reduce/abolish Gsalpha expression in renal proximal tubules and thereby cause resistance to PTH.

OBJECTIVE

Our objective was to define the genetic defect in a previously unreported family with autosomal dominant PHP-Ib.

DESIGN AND SETTING

Analyses of serum and urine chemistries and of genomic DNA and lymphoblastoid-derived RNA were conducted at a tertiary hospital and research laboratory.

PATIENTS

Affected individuals presented with muscle weakness and/or paresthesia and showed hypocalcemia, hyperphosphatemia, and elevated serum PTH. Obligate carriers were healthy and revealed no obvious abnormality in mineral ion homeostasis.

RESULTS

A novel 4.2-kb microdeletion was discovered in the affected individuals and the obligate carriers, ablating two noncoding GNAS antisense exons while preserving the NESP55 exon. On maternal transmission, the deletion causes loss of all maternal GNAS imprints, partial gain of NESP55 methylation, and PTH resistance. Paternal transmission of the mutation leads to epigenetic alterations in cis, including a partial loss of NESP55 methylation and a partial gain of A/B methylation.

CONCLUSIONS

The identified deletion points to a unique cis-acting element located telomeric of the NESP55 exon that is critical for imprinting both GNAS alleles. These findings provide novel insights into the molecular mechanisms underlying PHP and GNAS imprinting.

Molecular diagnosis and clinical characterization of pseudohypoparathyroidism type-Ib in a patient with mild Albright's hereditary osteodystrophy-like features, epileptic seizures, and defective renal handling of uric acid

We describe a patient who presented with epileptic seizures unresponsive to anticonvulsive treatment. Laboratory investigations demonstrated epileptiform seizure activity in the brain but also revealed severe hypocalcemia, hyperphosphatemia, and elevated serum parathyroid hormone. In addition, the patient showed a reduced serum level of 25-[OH]-vitamin D. The diagnosis of pseudohypoparathyroidism type-Ib (PHP-Ib) was made based on these clinical findings and upon identification of a 3-kb deletion within the STX16 locus, a genetic defect frequently associated with autosomal dominant PHP-Ib. This mutation was also present in the patient's unaffected mother and her affected sister. Despite the molecular diagnosis of PHP-Ib, which is characterized by parathyroid hormone resistance in the absence of Albright's hereditary osteodystrophy (AHO), the patient had a round face, slightly short stature, and short fourth metacarpals, which were consistent with mild AHO. The patient and her affected sister, who lacked AHO-like features, showed reduced serum levels of uric acid and increased fractional excretion of uric acid, a finding that was reported only once previously for PHP-Ib. Unlike the previous report, the fractional uric acid excretion and serum uric acid levels returned to normal in our patient and her sister after 3 months of treatment period. These findings underscore several important points with respect to the pathogenesis and clinical presentation of PHP-Ib. Furthermore, the findings in the index case present interesting novel aspects, including a previously undescribed coexistence of the 3-kb STX16 deletion and AHO-like features and a clinical course complicated by concomitant 25-[OH]-vitamin D deficiency, which may have resulted, at least partly, from long-term use of antiepileptic drugs.

The GNAS locus and pseudohypoparathyroidism

Pseudohypoparathyroidism (PHP) is a disorder of end-organ resistance primarily affecting the actions of parathyroid hormone (PTH). Genetic defects associated with different forms of PHP involve the alpha-subunit of the stimulatory G protein (Gsalpha), a signaling protein essential for the actions of PTH and many other hormones. Heterozygous inactivating mutations within Gsalpha-encoding GNAS exons are found in patients with PHP-Ia, who also show resistance to other hormones and a constellation ofphysical features called Albright's hereditary osteodystrophy (AHO). Patients who exhibit AHO features without evidence for hormone resistance, who are said to have pseudopseudohypoparathyroidism (PPHP), also carry heterozygous inactivating Gsalpha mutations. Maternal inheritance of such a mutation leads to PHP-Ia, i.e., AHO plus hormone resistance, while paternal inheritance of the same mutation leads to PPHP, i.e., AHO only. This imprinted mode of inheritance for hormone resistance can be explained by the predominantly maternal expression of Gsalpha in certain tissues, including renal proximal tubules. Patients with PHP-Ib lack coding Gsalpha mutations but display epigenetic defects of the GNAS locus, with the most consistent defect being a loss of imprinting at the exon A/B differentially methylated region (DMR). This epigenetic defect presumably silences, in cis, Gsalpha expression in tissues where this protein is derived from the maternal allele only, leading to a marked reduction of Gsa levels. The familial form of PHP-Ib (AD-PHP-Ib) is typically associated with an isolated loss of imprinting at the exon A/B DMR. A unique 3-kb microdeletion that disrupts the neighboring STX16 1ocus has been identified in this disorder and appears to be the cause of the loss of imprinting. In addition, deletions removing the entire NESP55 DMR, located within GNAS, have been identified in some AD-PHP-Ib kindreds in whom affected individuals show loss of all the maternal GNAS imprints. Mutations identified in different forms of PHP-Ib thus point to different cis-acting elements that are apparently required for the proper imprinting of the GNAS locus. Most sporadic PHP-Ib cases also have imprinting abnormalities of GNAS that involve multiple DMRs, but the genetic lesion(s) responsible for these imprinting abnormalities remain to be discovered.

Similar clinical and laboratory findings in patients with symptomatic autosomal dominant and sporadic pseudohypoparathyroidism type Ib despite different epigenetic changes at the GNAS locus

OBJECTIVE

Most patients with autosomal dominant pseudohypoparathyroidism type Ib (AD-PHP-Ib) carry an identical maternally inherited 3-kb microdeletion up-stream of GNAS (STX16del4-6(mat)), which is associated with a methylation loss restricted to exon A/B. STX16del4-6(mat) is not found in sporadic PHP-Ib (sporPHP-Ib) patients, who show broad GNAS methylation changes. Because of the epigenetic differences between both groups, we searched for clinical and/or laboratory differences.

PATIENTS AND METHODS

Age at diagnosis, calcium, phosphorus and PTH were analysed in 43 patients with AD-PHP-Ib due to STX16del4-6(mat) and in 22 patients with sporPHP-Ib.

RESULTS

All AD-PHP-Ib patients with STX16del4-6(mat) showed only loss of exon A/B methylation. Of the 43 individuals, 26 were symptomatic when diagnosis was established at age 12.1 +/- 1.34 years (mean +/- SEM); laboratory findings at presentation were calcium 1.69 +/- 0.06 mmol/l, phosphorus 2.25 +/- 0.12 mmol/l and PTH 442 +/- 54.1 pg/ml. The remaining 17 individuals with STX16del4-6(mat) were asymptomatic when diagnosed at age 23.5 +/- 3.93 years (calcium 2.18 +/- 0.05 mmol/l, phosphorus 1.63 +/- 0.10 mmol/l, PTH 222 +/- 40.3 pg/ml). Patients with sporPHP-Ib showed methylation changes at two or more GNAS exons, presented at age 10.0 +/- 1.01 years and had, as a group, similar laboratory findings as patients with symptomatic AD-PHP-Ib (calcium 1.51 +/- 0.06 mmol/l, phosphorus 2.65 +/- 0.10 mmol/l, PTH 634 +/- 162.1 pg/ml). However, sporPHP-Ib females appeared to be more severely affected.

CONCLUSIONS

Patients with symptomatic AD-PHP-Ib due to STX16del4-6(mat) and sporPHP-Ib have similar changes in calcium, phosphate and PTH. STX16del4-6(mat) often leads to asymptomatic disease and screening of all siblings of affected individuals is therefore advised. The cause of the apparent sexual dimorphism in patients with sporPHP-Ib remains uncertain.

Lack of Gnas epigenetic changes and pseudohypoparathyroidism type Ib in mice with targeted disruption of syntaxin-16

Pseudohypoparathyroidism type Ib (PHP-Ib) is characterized by hypocalcemia and hyperphosphatemia due to proximal renal tubular resistance to PTH but without evidence for Albright's hereditary osteodystrophy. The disorder is paternally imprinted and affected individuals, but not unaffected carriers, show loss of GNAS exon A/B methylation, a differentially methylated region upstream of the exons encoding Gsalpha. Affected individuals of numerous unrelated kindreds with an autosomal dominant form of PHP-Ib (AD-PHP-Ib) have an identical 3-kb microdeletion removing exons 4-6 of syntaxin-16 (STX16) (STX16del4-6), which is thought to disrupt a cis-acting element required for exon A/B methylation. To explore the mechanisms underlying the regulation of exon A/B methylation, we generated mice genetically altered to carry the equivalent of STX16del4-6 (Stx16(Delta4-6)). Although the human GNAS locus shows a similar organization as the murine Gnas ortholog and although the human and mouse STX16/Stx16 regions show no major structural differences, no phenotypic or epigenotypic abnormalities were detected in mice with Stx16(Delta4-6) on one or both parental alleles. Furthermore, calcium and PTH levels in Stx16(Delta4-6) mice were indistinguishable from those in wild-type animals, indicating that ablation of the murine equivalent of human STX16del4-6 does not contribute to the development of PTH resistance. The identification of a novel intragenic transcript from within the STX16/Stx16 locus in total RNA from kidneys of Stx16(Delta4-6) mice and lymphoblastoid cell-derived RNA of a patient with AD-PHP-Ib raises the question whether this transcript contributes, if deleted or altered, to the development of AD-PHP-Ib in humans.

Hypocalcemia due to tubular dysfunction in a patient with holoprosencephaly

This is the first report of a case of hypocalcemia in a female infant with holoprosencephaly. Hypocalcemia developed 60 days after birth, secondary to decreased serum 1,25-dihydroxyvitamin D, as a result of renal tubular dysfunction which may have been induced by prerenal acute renal failure, the administration of anticonvulsants, and hypothyroidism. However, there was no evidence of rickets, and her serum 25-hydroxyvitamin D value was normal. She was treated with high-dose (0.5 microg/kg) 1alpha-hydroxyvitamin D3 and calcium lactate, and her calcium and 1,25-dihydroxyvitamin D values were consequently, normalized. However, she died at 268 days after birth.