Constitutional deletion of chromosome 20q in two patients affected with albright hereditary osteodystrophy

Whole-genome sequencing revealed a novel long-range deletion mutation spanning GNAS in familial pseudohypoparathyroidism

BACKGROUND

Pseudohypoparathyroidism (PHP) is a series of diseases related to pathological changes and neurocognitive and endocrine abnormalities, mainly due to the GNAS mutation on chromosome 20q13.2, which weakens receptor-mediated hormone signal transduction. Considering its complex genetic and epigenetic characteristics, GNAS may produce complex clinical phenotypes in families or sporadic cases. This study presented a case of familial PHP caused by a deletion mutation in the 20q13.2 region.

METHODS AND RESULTS

The proband and her second daughter had PHP, and the proband's mother had pseudo-PHP. Whole-genome sequencing revealed that the proband had an 849.81 kb deletion spanning GNAS near the maternal 20q13.2 chromosome. Multiplex ligation-dependent probe amplification methylation analysis indicated that the proband as well as her mother and second daughter had seemingly abnormal GNAS methylation. This is different from the phenotype (feeding difficulty, slow growth, and special facial features) of previously reported cases with the deletion of fragments near the 20q13.2 chromosome.

CONCLUSIONS

This report demonstrated the variability of 20q13.2 deletion phenotypes and the clinical importance of using multiple molecular genetic detection methods.

Case Report: A Paternal 20q13.2-q13.32 Deletion Patient With Growth Retardation Improved by Growth Hormone

Interstitial chromosome 20q deletions, containing GNAS imprinted locus, are rarely reported in the past. Hereby, we presented a Chinese boy with a novel 4.36 Mb deletion at paternal 20q13.2-13.32, showing feeding difficulty, malnutrition, short stature, lower limb asymmetry, sightly abnormal facial appearance and mild intellectual abnormality. With 3 years’ growth hormone treatment, his height was increased from 90 to 113.5 cm. This report is the first time to describe the outcome of clinical treatment on a patient with this rare chromosomal 20 long arm interstitial deletion, containing GNAS locus, which may facilitate the diagnosis and treatment of this type of patient in the future.

Maternal GNAS Contributes to the Extra-Large G Protein α-Subunit (XLαs) Expression in a Cell Type-Specific Manner

GNAS encodes the stimulatory G protein alpha-subunit (Gsα) and its large variant XLαs. Studies have suggested that XLαs is expressed exclusively paternally. Thus, XLαs deficiency is considered to be responsible for certain findings in patients with paternal GNAS mutations, such as pseudo-pseudohypoparathyroidism, and the phenotypes associated with maternal uniparental disomy of chromosome 20, which comprises GNAS. However, a study of bone marrow stromal cells (BMSC) suggested that XLαs could be biallelically expressed. Aberrant BMSC differentiation due to constitutively activating GNAS mutations affecting both Gsα and XLαs is the underlying pathology in fibrous dysplasia of bone. To investigate allelic XLαs expression, we employed next-generation sequencing and a polymorphism common to XLαs and Gsα, as well as A/B, another paternally expressed GNAS transcript. In mouse BMSCs, Gsα transcripts were 48.4 ± 0.3% paternal, while A/B was 99.8 ± 0.2% paternal. In contrast, XLαs expression varied among different samples, paternal contribution ranging from 43.0 to 99.9%. Sample-to-sample variation in paternal XLαs expression was also detected in bone (83.7-99.6%) and cerebellum (83.8 to 100%) but not in cultured calvarial osteoblasts (99.1 ± 0.1%). Osteoblastic differentiation of BMSCs shifted the paternal XLαs expression from 83.9 ± 1.5% at baseline to 97.2 ± 1.1%. In two human BMSC samples grown under osteoinductive conditions, XLαs expression was also predominantly monoallelic (91.3 or 99.6%). Thus, the maternal GNAS contributes significantly to XLαs expression in BMSCs but not osteoblasts. Altered XLαs activity may thus occur in certain cell types irrespective of the parental origin of a GNAS defect.

Intragenic Deletions of GNAS in Pseudohypoparathyroidism Type 1A Identify a New Region Affecting Methylation of Exon A/B

CONTEXT

Pseudohypoparathyroidism type 1A (PHP1A) and pseudopseudohypoparathyroidism (PPHP) are caused by inactivating mutations in the exons of GNAS that encode the alpha-subunit of the stimulatory G protein (Gsα). In some cases abnormal methylation of exon A/B of GNAS, a hallmark of PHP1B, has been reported.

OBJECTIVE

To identify the underlying genetic basis for PHP1A/PPHP in patients in whom molecular defects were not detected by GNAS sequencing and microarray-based analysis of copy number variations.

METHODS

Whole genome sequencing (WGS) and pyrosequencing of differentially methylated regions (DMRs) of GNAS using genomic deoxyribonucleic acid from affected patients.

RESULTS

We identified 2 novel heterozygous GNAS deletions: a 6.4 kb deletion that includes exon 2 of GNAS in the first proband that was associated with normal methylation (57%) of exon A/B DMR, and a 1438 bp deletion in a second PHP1A patient that encompasses the promoter region and 5' untranslated region of Gsα transcripts, which was inherited from his mother with PPHP. This deletion was associated with reduced methylation (32%) of exon A/B DMR.

CONCLUSIONS

WGS can detect exonic and intronic mutations, including deletions that are too small to be identified by microarray analysis, and therefore is more sensitive than other techniques for molecular analysis of PHP1A/PPHP. One of the deletions we identified led to reduced methylation of exon A/B DMR, further refining a region needed for normal imprinting of this DMR. We propose that deletion of this region can explain why some PHP1A patients have reduced of methylation of the exon A/B DMR.

Candidate Genes Associated with Delayed Neuropsychomotor Development and Seizures in a Patient with Ring Chromosome 20

Ring chromosome 20 (r20) is characterized by intellectual impairment, behavioral disorders, and refractory epilepsy. We report a patient presenting nonmosaic ring chromosome 20 followed by duplication and deletion in 20q13.33 with seizures, delayed neuropsychomotor development and language, mild hypotonia, low weight gain, and cognitive deficit. Chromosomal microarray analysis (CMA) enabled us to restrict a chromosomal segment and thus integrate clinical and molecular data with systems biology. With this approach, we were able to identify candidate genes that may help to explain the consequences of deletions in 20q13.33. In our analysis, we observed five hubs (ARFGAP1, HELZ2, COL9A3, PTK6, and EEF1A2), seven bottlenecks (CHRNA4, ARFRP1, GID8, COL9A3, PTK6, ZBTB46, and SRMS), and two H-B nodes (PTK6 and COL9A3). The candidate genes may play an important role in the developmental delay and seizures observed in r20 patients. Gene ontology included microtubule-based movement, nucleosome assembly, DNA repair, and cholinergic synaptic transmission. Defects in these bioprocesses are associated with the development of neurological diseases, intellectual disability, neuropathies, and seizures. Therefore, in this study, we can explore molecular cytogenetic data, identify proteins through network analysis of protein-protein interactions, and identify new candidate genes associated with the main clinical findings in patients with 20q13.33 deletions.

A G protein-coupled, IP3/protein kinase C pathway controlling the synthesis of phosphaturic hormone FGF23

Dysregulated actions of bone-derived phosphaturic hormone fibroblast growth factor 23 (FGF23) result in several inherited diseases, such as X-linked hypophosphatemia (XLH), and contribute substantially to the mortality in kidney failure. Mechanisms governing FGF23 production are poorly defined. We herein found that ablation of the Gq/11α-like, extralarge Gα subunit (XLαs), a product of GNAS, exhibits FGF23 deficiency and hyperphosphatemia in early postnatal mice (XLKO). FGF23 elevation in response to parathyroid hormone, a stimulator of FGF23 production via cAMP, was intact in XLKO mice, while skeletal levels of protein kinase C isoforms α and δ (PKCα and PKCδ) were diminished. XLαs ablation in osteocyte-like Ocy454 cells suppressed the levels of FGF23 mRNA, inositol 1,4,5-trisphosphate (IP3), and PKCα/PKCδ proteins. PKC activation in vivo via injecting phorbol myristate acetate (PMA) or by constitutively active Gqα-Q209L in osteocytes and osteoblasts promoted FGF23 production. Molecular studies showed that the PKC activation-induced FGF23 elevation was dependent on MAPK signaling. The baseline PKC activity was elevated in bones of Hyp mice, a model of XLH. XLαs ablation significantly, but modestly, reduced serum FGF23 and elevated serum phosphate in Hyp mice. These findings reveal a potentially hitherto-unknown mechanism of FGF23 synthesis involving a G protein-coupled IP3/PKC pathway, which may be targeted to fine-tune FGF23 levels.

A Novel de Novo Paracentric Inversion [inv(20)(q13.1q13.3)] Accompanied by an 11q14.3-q21 Microdeletion in a Pediatric Patient with an Intellectual Disability

A novel de novo paracentric inversion of the long arm of chromosome 20 [inv(20)(q13.1q13.3)], detected by conventional karyotyping in a 14-year-old boy with mental retardation is described. Further investigation by array comparative genomic hybridization (aCGH) revealed that the 20q inversion was not accompanied by microdeletions/microduplications containing disease-associated genes near or at the breakpoints. Two deletions at chromosomal regions 11q14.3q21 and 20q12 of 4.5 and 1.97 Mb size, respectively, containing important online Mendelian inheritance in man (OMIM) genes, were detected. The 4.5Mb 11q14.3q21 microdeletion was contained within a region that is involved, in most of the reported cases, with the interstitial 11q deletion and may be related to the mental retardation and developmental delay present in the patient. On the other hand, the published data about the 20q12 microdeletion are very few and it is not possible to correlate this finding with our patient’s phenotype. This case report contributes to the description of a new chromosomal entity, not previously reported, and is therefore important, especially in prenatal diagnosis and management of patients. Array comparative genomic hybridization has proven a useful technique for detecting submicroscopic rearrangements and should be offered prenatally, especially in cases of de novo karyotypically balanced chromosomal inversions or translocations in order to unveil other unbalanced chromosomal abnormalities such as deletions and amplifications.

First Report of Low-Rate Mosaicism for 20q11.21q12 Deletion and Delineation of the Associated Disorder

Interstitial deletions of the long arm of chromosome 20 are very rare, with only 12 reported patients harboring the 20q11.2 microdeletion and presenting a disorder characterized by psychomotor and growth delay, dysmorphisms, and brachy-/clinodactyly. We describe the first case of mosaic 20q11.2 deletion in a 5-year-old girl affected by mild psychomotor delay, feeding difficulties, growth retardation, craniofacial dysmorphisms, and finger anomalies. SNP array analysis disclosed 20% of cells with a 20q11.21q12 deletion, encompassing the 20q11.2 minimal critical region and the 3 OMIM disease-causing genes GDF5, EPB41L1, and SAMHD1. We propose a pathogenic role of other genes mapping outside the small region of overlap, in particular GHRH (growth hormone releasing hormone), whose haploinsufficiency could be responsible for the prenatal onset of growth retardation which is shared by half of these patients. Our patient highlights the utility of chromosomal microarray analysis to identify low-level mosaicism.

GNAS mutations and heterotopic ossification

GNAS is a complex imprinted gene encoding the alpha-subunit of the stimulatory heterotrimeric G protein (Gsα). GNAS gives rise to additional gene products that exhibit exclusively maternal or paternal expression, such as XLαs, a large variant of Gsα that shows exclusively paternal expression and is partly identical to the latter. Gsα itself is expressed biallelically in most tissues, although the expression occurs predominantly from the maternal allele in a small set of tissues, such as renal proximal tubules. Inactivating mutations in Gsα-coding GNAS exons are responsible for Albright's hereditary osteodystrophy (AHO), which refers to a constellation of physical and developmental disorders including obesity, short stature, brachydactyly, cognitive impairment, and heterotopic ossification. Patients with Gsα mutations can present with AHO in the presence or absence of end-organ resistance to multiple hormones including parathyroid hormone. Maternal Gsα mutations lead to AHO with hormone resistance (i.e. pseudohypoparathyroidism type-Ia), whereas paternal mutations cause AHO alone (i.e. pseudo-pseudohypoparathyroidism). Heterotopic ossification associated with AHO develops through intramembranous bone formation and is limited to dermis and subcutis. In rare cases carrying Gsα mutations, however, ossifications progress into deep connective tissue and skeletal muscle, a disorder termed progressive osseous heteroplasia (POH). Here I briefly review the genetic, clinical, and molecular aspects of these disorders caused by inactivating GNAS mutations, with particular emphasis on heterotopic ossification.

Heterotrimeric G proteins in the control of parathyroid hormone actions

Parathyroid hormone (PTH) is a key regulator of skeletal physiology and calcium and phosphate homeostasis. It acts on bone and kidney to stimulate bone turnover, increase the circulating levels of 1,25 dihydroxyvitamin D and calcium and inhibit the reabsorption of phosphate from the glomerular filtrate. Dysregulated PTH actions contribute to or are the cause of several endocrine disorders. This calciotropic hormone exerts its actions via binding to the PTH/PTH-related peptide receptor (PTH1R), which couples to multiple heterotrimeric G proteins, including G_s and G_q/11 Genetic mutations affecting the activity or expression of the alpha-subunit of G_s, encoded by the GNAS complex locus, are responsible for several human diseases for which the clinical findings result, at least partly, from aberrant PTH signaling. Here, we review the bone and renal actions of PTH with respect to the different signaling pathways downstream of these G proteins, as well as the disorders caused by GNAS mutations.

Ablation of the Stimulatory G Protein α-Subunit in Renal Proximal Tubules Leads to Parathyroid Hormone-Resistance With Increased Renal Cyp24a1 mRNA Abundance and Reduced Serum 1,25-Dihydroxyvitamin D

PTH regulates serum calcium, phosphate, and 1,25-dihydroxyvitamin D (1,25(OH)2D) levels by acting on bone and kidney. In renal proximal tubules (PTs), PTH inhibits reabsorption of phosphate and stimulates the synthesis of 1,25(OH)2D. The PTH receptor couples to multiple G proteins. We here ablated the α-subunit of the stimulatory G protein (Gsα) in mouse PTs by using Cre recombinase driven by the promoter of type-2 sodium-glucose cotransporter (Gsα(Sglt2KO) mice). Gsα(Sglt2KO) mice were normophosphatemic but displayed, relative to controls, hypocalcemia (1.19 ±0.01 vs 1.23 ±0.01 mmol/L; P < .05), reduced serum 1,25(OH)2D (59.3 ±7.0 vs 102.5 ±12.2 pmol/L; P < .05), and elevated serum PTH (834 ±133 vs 438 ±59 pg/mL; P < .05). PTH-induced elevation in urinary cAMP excretion was blunted in Gsα(Sglt2KO) mice (2- vs 4-fold over baseline in controls; P < .05). Relative to baseline in controls, PTH-induced reduction in serum phosphate tended to be blunted in Gsα(Sglt2KO) mice (-0.39 ±0.33 vs -1.34 ±0.36 mg/dL; P = .07). Gsα(Sglt2KO) mice showed elevated renal vitamin D 24-hydroxylase and bone fibroblast growth factor-23 (FGF23) mRNA abundance (∼3.4- and ∼11-fold over controls, respectively; P < .05) and tended to have elevated serum FGF23 (829 ±76 vs 632 ±60 pg/mL in controls; P = .07). Heterozygous mice having constitutive ablation of the maternal Gsα allele (E1(m-/+)) (model of pseudohypoparathyroidism type-Ia), in which Gsα levels in PT are reduced, also exhibited elevated serum FGF23 (474 ±20 vs 374 ±27 pg/mL in controls; P < .05). Our findings indicate that Gsα is required in PTs for suppressing renal vitamin D 24-hydroxylase mRNA levels and for maintaining normal serum 1,25(OH)2D.

The G protein α subunit variant XLαs promotes inositol 1,4,5-trisphosphate signaling and mediates the renal actions of parathyroid hormone in vivo

GNAS, which encodes the stimulatory G protein (heterotrimeric guanine nucleotide-binding protein) α subunit (Gαs), also encodes a large variant of Gαs termed extra-large α subunit (XLαs), and alterations in XLαs abundance or activity are implicated in various human disorders. Although XLαs, like Gαs, stimulates generation of the second messenger cyclic adenosine monophosphate (cAMP), evidence suggests that XLαs and Gαs have opposing effects in vivo. We investigated the role of XLαs in mediating signaling by parathyroid hormone (PTH), which activates a G protein-coupled receptor (GPCR) that stimulates both Gαs and Gαq/11 in renal proximal tubules to maintain phosphate and vitamin D homeostasis. At postnatal day 2 (P2), XLαs knockout (XLKO) mice exhibited hyperphosphatemia, hypocalcemia, and increased serum concentrations of PTH and 1,25-dihydroxyvitamin D. The ability of PTH to reduce serum phosphate concentrations was impaired, and the abundance of the sodium phosphate cotransporter Npt2a in renal brush border membranes was reduced in XLKO mice, whereas PTH-induced cAMP excretion in the urine was modestly increased. Basal and PTH-stimulated production of inositol 1,4,5-trisphosphate (IP3), which is the second messenger produced by Gαq/11 signaling, was repressed in renal proximal tubules from XLKO mice. Crossing of XLKO mice with mice overexpressing XLαs specifically in renal proximal tubules rescued the phenotype of the XLKO mice. Overexpression of XLαs in HEK 293 cells enhanced IP3 generation in unstimulated cells and in cells stimulated with PTH or thrombin, which acts through a Gq/11-coupled receptor. Together, our findings suggest that XLαs enhances Gq/11 signaling to mediate the renal actions of PTH during early postnatal development.

GNAS Spectrum of Disorders

The GNAS complex locus encodes the alpha-subunit of the stimulatory G protein (Gsα), a ubiquitous signaling protein mediating the actions of many hormones, neurotransmitters, and paracrine/autocrine factors via generation of the second messenger cAMP. GNAS gives rise to other gene products, most of which exhibit exclusively monoallelic expression. In contrast, Gsα is expressed biallelically in most tissues; however, paternal Gsα expression is silenced in a small number of tissues through as-yet-poorly understood mechanisms that involve differential methylation within GNAS. Gsα-coding GNAS mutations that lead to diminished Gsα expression and/or function result in Albright's hereditary osteodystrophy (AHO) with or without hormone resistance, i.e., pseudohypoparathyroidism type-Ia/Ic and pseudo-pseudohypoparathyroidism, respectively. Microdeletions that alter GNAS methylation and, thereby, diminish Gsα expression in tissues in which the paternal Gsα allele is normally silenced also cause hormone resistance, which occurs typically in the absence of AHO, a disorder termed pseudohypoparathyroidism type-Ib. Mutations of GNAS that cause constitutive Gsα signaling are found in patients with McCune-Albright syndrome, fibrous dysplasia of bone, and different endocrine and non-endocrine tumors. Clinical features of these diseases depend significantly on the parental allelic origin of the GNAS mutation, reflecting the tissue-specific paternal Gsα silencing. In this article, we review the pathogenesis and the phenotypes of these human diseases.

A novel de novo 20q13.32-q13.33 deletion in a 2-year-old child with poor growth, feeding difficulties and low bone mass

Interstitial deletions of the long arm of chromosome 20 are rarely reported in the literature. We report a 2-year-old child with a 2.6 Mb deletion of 20q13.32-q13.33, detected by microarray-based comparative genomic hybridization, who presented with poor growth, feeding difficulties, abnormal subcutaneous fat distribution with the lack of adipose tissue on clinical examination, facial dysmorphism and low bone mass. This report adds to rare publications describing constitutional aberrations of chromosome 20q, and adds further evidence to the fact that deletion of the GNAS complex may not always be associated with an Albright's hereditary osteodystrophy phenotype as described previously.

Pseudohypoparathyroidism type Ib associated with novel duplications in the GNAS locus

Context

Pseudohypoparathyroidism type 1b (PHP-Ib) is characterized by renal resistance to PTH (and, sometimes, a mild resistance to TSH) and absence of any features of Albright's hereditary osteodystrophy. Patients with PHP-Ib suffer of defects in the methylation pattern of the complex GNAS locus. PHP-Ib can be either sporadic or inherited in an autosomal dominant pattern. Whereas familial PHP-Ib is well characterized at the molecular level, the genetic cause of sporadic PHP-Ib cases remains elusive, although some molecular mechanisms have been associated with this subtype.

Objective

The aim of the study was to investigate the molecular and imprinting defects in the GNAS locus in two unrelated patients with PHP-Ib.

Design

We have analyzed the GNAS locus by direct sequencing, Methylation-Specific Multiplex Ligation-dependent Probe Amplification, microsatellites, Quantitative Multiplex PCR of Short Fluorescent fragments and array-Comparative Genomic Hybridization studies in order to characterize two unrelated families with clinical features of PHP-Ib.

Results

We identified two duplications in the GNAS region in two patients with PHP-Ib: one of them, comprising ∼320 kb, occurred ‘de novo’ in the patient, whereas the other one, of ∼179 kb in length, was inherited from the maternal allele. In both cases, no other known genetic cause was observed.

Conclusion

In this article, we describe the to-our-knowledge biggest duplications reported so far in the GNAS region. Both are associated to PHP-Ib, one of them occurring ‘de novo’ and the other one being maternally inherited.

Evidence of hormone resistance in a pseudo-pseudohypoparathyroidism patient with a novel paternal mutation in GNAS

CONTEXT

Loss-of-function GNAS mutations lead to hormone resistance and Albright's hereditary osteodystrophy (AHO) when maternally inherited, i.e. pseudohypoparathyroidism-Ia (PHPIa), but cause AHO alone when located on the paternal allele, i.e. pseudoPHP (PPHP).

OBJECTIVE

We aimed to establish the molecular diagnosis in a patient with AHO and evidence of hormone resistance.

CASE

The patient is a female who presented at the age of 13.5years with short stature and multiple AHO features. No evidence for TSH or gonadotropin-resistance was present. Serum calcium and vitamin D levels were normal. However, serum PTH was elevated on multiple occasions (64-178pg/mL, normal: 9-52) and growth hormone response to clonidine or L-DOPA was blunted, suggesting hormone resistance and PHP-Ia. The patient had diminished erythrocyte Gsα activity and a novel heterozygous GNAS mutation (c.328 G>C; p.A109P). The mother lacked the mutation, and the father's DNA was not available. Hence, a diagnosis of PPHP also appeared possible, supported by low birth weight and a lack of AHO features associated predominantly with PHP-Ia, i.e. obesity and cognitive impairment. To determine the parental origin of the mutation, we amplified the paternally expressed A/B and biallelically expressed Gsα transcripts from the patient's peripheral blood RNA. While both wild-type and mutant nucleotides were detected in the Gsα amplicon, only the mutant nucleotide was present in the A/B amplicon, indicating that the mutation was paternal.

CONCLUSION

These findings suggest that PTH and other hormone resistance may not be an exclusive feature of PHP-Ia and could also be observed in patients with PPHP.

Clinically relevant known and candidate genes for obesity and their overlap with human infertility and reproduction

PURPOSE

Obesity is a growing public health concern now reaching epidemic status worldwide for children and adults due to multiple problems impacting on energy intake and expenditure with influences on human reproduction and infertility. A positive family history and genetic factors are known to play a role in obesity by influencing eating behavior, weight and level of physical activity and also contributing to human reproduction and infertility. Recent advances in genetic technology have led to discoveries of new susceptibility genes for obesity and causation of infertility. The goal of our study was to provide an update of clinically relevant candidate and known genes for obesity and infertility using high resolution chromosome ideograms with gene symbols and tabular form.

METHODS

We used computer-based internet websites including PubMed to search for combinations of key words such as obesity, body mass index, infertility, reproduction, azoospermia, endometriosis, diminished ovarian reserve, estrogen along with genetics, gene mutations or variants to identify evidence for development of a master list of recognized obesity genes in humans and those involved with infertility and reproduction. Gene symbols for known and candidate genes for obesity were plotted on high resolution chromosome ideograms at the 850 band level. Both infertility and obesity genes were listed separately in alphabetical order in tabular form and those highlighted when involved with both conditions.

RESULTS

By searching the medical literature and computer generated websites for key words, we found documented evidence for 370 genes playing a role in obesity and 153 genes for human reproduction or infertility. The obesity genes primarily affected common pathways in lipid metabolism, deposition or transport, eating behavior and food selection, physical activity or energy expenditure. Twenty-one of the obesity genes were also associated with human infertility and reproduction. Gene symbols were plotted on high resolution ideograms and their name, precise chromosome band location and description were summarized in tabular form.

CONCLUSIONS

Meaningful correlations in the obesity phenotype and associated human infertility and reproduction are represented with the location of genes on chromosome ideograms along with description of the gene and position in tabular form. These high resolution chromosome ideograms and tables will be useful in genetic awareness and counseling, diagnosis and treatment to improve clinical outcomes.

The GNAS1 gene in myelodysplastic syndromes (MDS)

GNAS1 gene is located at the long arm of chromosome 20 (q13.32). GNAS1 gene deletion has never been investigated in MDS. A GNAS1 activating mutation (R201) was recently found in MDS. We applied FISH and DHPLC plus sequencing to investigate GNAS1 gene in MDS cases with and without del(20q) at karyotype.

The smallest de novo deletion of 20q11.21-q11.23 in a girl with feeding problems, retinal dysplasia, and skeletal abnormalities

We report on a de novo interstitial deletion of 20q11.21-q11.23 in a 2-year-old girl with a set of dysmorphic features, cleft palate, heart defect, severe feeding problems, failure to thrive, developmental delay, preaxial polydactyly (right thumb), and retinal dysplasia. Interstitial microdeletions of the long arm of chromosome 20 are rare. Exclusively rare are proximal microdeletions involving 20q11-q12 region. Our patient is the fourth described so far and has the smallest deleted region 20q11.21-q11.23 of 5.7 Mb. The defined clinical phenotype of our patient is very similar to previously published cases and confirms the existence of retinal dysplasia and skeletal abnormalities as a part of phenotypic spectrum for deletion 20q11-q12. Description of four similar patients, including two almost identical, suggests a new distinct, phenotypicaly recognizable microdeletion syndrome associated with the loss of 20q11-q12 region.

The GNAS complex locus and human diseases associated with loss-of-function mutations or epimutations within this imprinted gene

GNAS is a complex imprinted locus leading to several different gene products that show exclusive monoallelic expression. GNAS also encodes the α-subunit of the stimulatory G protein (Gsα), a ubiquitously expressed signaling protein that is essential for the actions of many hormones and other endogenous molecules. Gsα is expressed biallelically in most tissues but its expression is silenced from the paternal allele in a small number of tissues. The tissue-specific paternal silencing of Gsα results in different parent-of-origin-specific phenotypes in patients who carry inactivating GNAS mutations. In this paper, we review the GNAS complex locus and discuss how disruption of Gsα expression and the expression of other GNAS products shape the phenotypes of human disorders caused by mutations in this gene.

Methylation defect in imprinted genes detected in patients with an Albright's hereditary osteodystrophy like phenotype and platelet Gs hypofunction

BACKGROUND

Pseudohypoparathyroidism (PHP) indicates a group of heterogeneous disorders whose common feature is represented by impaired signaling of hormones that activate Gsalpha, encoded by the imprinted GNAS gene. PHP-Ib patients have isolated Parathormone (PTH) resistance and GNAS epigenetic defects while PHP-Ia cases present with hormone resistance and characteristic features jointly termed as Albright's Hereditary Osteodystrophy (AHO) due to maternally inherited GNAS mutations or similar epigenetic defects as found for PHP-Ib. Pseudopseudohypoparathyroidism (PPHP) patients with an AHO phenotype and no hormone resistance and progressive osseous heteroplasia (POH) cases have inactivating paternally inherited GNAS mutations.

METHODOLOGY/PRINCIPAL FINDINGS

We here describe 17 subjects with an AHO-like phenotype that could be compatible with having PPHP but none of them carried Gsalpha mutations. Functional platelet studies however showed an obvious Gs hypofunction in the 13 patients that were available for testing. Methylation for the three differentially methylated GNAS regions was quantified via the Sequenom EpiTYPER. Patients showed significant hypermethylation of the XL amplicon compared to controls (36 ± 3 vs. 29 ± 3%; p<0.001); a pattern that is reversed to XL hypomethylation found in PHPIb. Interestingly, XL hypermethylation was associated with reduced XLalphaS protein levels in the patients' platelets. Methylation for NESP and ExonA/B was significantly different for some but not all patients, though most patients have site-specific CpG methylation abnormalities in these amplicons. Since some AHO features are present in other imprinting disorders, the methylation of IGF2, H19, SNURF and GRB10 was quantified. Surprisingly, significant IGF2 hypermethylation (20 ± 10 vs. 14 ± 7%; p<0.05) and SNURF hypomethylation (23 ± 6 vs. 32 6%; p<0.001) was found in patients vs. controls, while H19 and GRB10 methylation was normal.

CONCLUSION/SIGNIFICANCE

In conclusion, this is the first report of methylation defects including GNAS in patients with an AHO-like phenotype without endocrinological abnormalities. Additional studies are still needed to correlate the methylation defect with the clinical phenotype.

Structural Chromosome Abnormalities Associated with Obesity: Report of Four New subjects and Review of Literature

Obesity in humans is a complex polygenic trait with high inter-individual heritability estimated at 40-70%. Candidate gene, DNA linkage and genome-wide association studies (GWAS) have allowed for the identification of a large set of genes and genomic regions associated with obesity. Structural chromosome abnormalities usually result in congenital anomalies, growth retardation and developmental delay. Occasionally, they are associated with hyperphagia and obesity rather than growth delay. We report four new individuals with structural chromosome abnormalities involving 10q22.3-23.2, 16p11.2 and Xq27.1-q28 chromosomal regions with early childhood obesity and developmental delay. We also searched and summarized the literature for structural chromosome abnormalities reported in association with childhood obesity.

The consequences of uniparental disomy and copy number neutral loss-of-heterozygosity during human development and cancer

UPD (uniparental disomy) describes the inheritance of a pair of chromosomes from only one parent. Mechanisms that lead to UPD include trisomy rescue, gamete complementation, monosomy rescue and somatic recombination. Most of these mechanisms can involve aberrant chromosomes, particularly isochromosomes and Robertsonian translocations. In the last decade, the number of UPD cases reported in the literature has increased exponentially. This is partly due to the advances in genomic technologies that have allowed for high-resolution SNP (single nucleotide polymorphism) studies, which have complemented traditional methods relying on polymorphic microsatellite markers. In this review, we discuss aberrant cellular mechanisms leading to UPD and their impact on gene expression. Special emphasis is placed on the unmasking of mutant recessive alleles and the disruption of imprinted gene dosage, which give rise to specific and recurrent imprinting phenotypes. Finally, we discuss how copy number maps determined from SNP array datasets have helped identify not only deletions and duplications but also recurrent copy number neutral regions of loss-of-heterozygosity, which have been reported in many cancer types and that may constitute an important driving force in cancer. These tiny regions of UPD also alter imprinted gene dosage, which may have cumulative tumourgenic effects in addition to that of unmasking homozygous cancer-associated mutations.

Transgenic overexpression of the extra-large Gsα variant XLαs enhances Gsα-mediated responses in the mouse renal proximal tubule in vivo

XLαs, a variant of the stimulatory G protein α-subunit (Gsα), can mediate receptor-activated cAMP generation and, thus, mimic the actions of Gsα in transfected cells. However, it remains unknown whether XLαs can act in a similar manner in vivo. We have now generated mice with ectopic transgenic expression of rat XLαs in the renal proximal tubule (rptXLαs mice), where Gsα mediates most actions of PTH. Western blots and quantitative RT-PCR showed that, while Gsα and type-1 PTH receptor levels were unaltered, protein kinase A activity and 25-hydroxyvitamin D 1-α-hydroxylase (Cyp27b1) mRNA levels were significantly higher in renal proximal tubules of rptXLαs mice than wild-type littermates. Immunohistochemical analysis of kidney sections showed that the sodium-phosphate cotransporter type 2a was modestly reduced in brush border membranes of male rptXLαs mice compared to gender-matched controls. Serum calcium, phosphorus, and 1,25 dihydroxyvitamin D were within the normal range, but serum PTH was ∼30% lower in rptXLαs mice than in controls (152 ± 16 vs. 222 ± 41 pg/ml; P < 0.05). After crossing the rptXLαs mice to mice with ablation of maternal Gnas exon 1 (E1(m-/+)), male offspring carrying both the XLαs transgene and maternal Gnas exon 1 ablation (rptXLαs/E1(m-/+)) were significantly less hypocalcemic than gender-matched E1(m-/+) littermates. Both E1(m-/+) and rptXLαs/E1(m-/+) offspring had higher serum PTH than wild-type littermates, but the degree of secondary hyperparathyroidism tended to be lower in rptXLαs/E1(m-/+) mice. Hence, transgenic XLαs expression in the proximal tubule enhanced Gsα-mediated responses, indicating that XLαs can mimic Gsα in vivo.

A de novo deletion of 20q11.2-q12 in a boy presenting with abnormal hands and feet, retinal dysplasia, and intractable feeding difficulty

Proximal interstitial deletions involving 20q11-q12 are very rare. Only two cases have been reported. We describe another patient with 20q11.21-q12 deletion. We precisely mapped the 6.5-Mb deletion and successfully determined the deletion landmarks at the nucleotide level. Common clinical features among the three cases include developmental delay, intractable feeding difficulties with gastroesophageal reflux, and facial dysmorphism including triangular face, hypertelorism, and hypoplastic alae nasi, indicating that the 20q11.2-q12 deletion can be a clinically recognizable syndrome. This is also supported by the fact that the three deletions overlap significantly. In addition, unique features such as arthrogryposis/fetal akinesia (hypokinesia) deformation and retinal dysplasia are recognized in the patient reported herein.

Diseases associated with genomic imprinting

Genomic imprinting is the phenomenon where the expression of a locus differs between the maternally and paternally inherited alleles. Typically, this manifests as transcriptional silencing of one of the alleles, although many genes are imprinted in a tissue- or isoform-specific manner. Diseases associated with imprinted genes include various cancers, disorders of growth and metabolism, and disorders in neurodevelopment, cognition, and behavior, including certain major psychiatric disorders. In many cases, the disease phenotypes associated with dysfunction at particular imprinted loci can be understood in terms of the evolutionary processes responsible for the origin of imprinting. Imprinted gene expression represents the outcome of an intragenomic evolutionary conflict, where natural selection favors different expression strategies for maternally and paternally inherited alleles. This conflict is reasonably well understood in the context of the early growth effects of imprinted genes, where paternally inherited alleles are selected to place a greater demand on maternal resources than are maternally inherited alleles. Less well understood are the origins of imprinted gene expression in the brain, and their effects on cognition and behavior. This chapter reviews the genetic diseases that are associated with imprinted genes, framed in terms of the evolutionary pressures acting on gene expression at those loci. We begin by reviewing the phenomenon and evolutionary origins of genomic imprinting. We then discuss diseases that are associated with genetic or epigenetic defects at particular imprinted loci, many of which are associated with abnormalities in growth and/or feeding behaviors that can be understood in terms of the asymmetric pressures of natural selection on maternally and paternally inherited alleles. We next described the evidence for imprinted gene effects on adult cognition and behavior, and the possible role of imprinted genes in the etiology of certain major psychiatric disorders. Finally, we conclude with a discussion of how imprinting, and the evolutionary-genetic conflicts that underlie it, may enhance both the frequency and morbidity of certain types of diseases.

New mechanisms involved in paternal 20q disomy associated with pseudohypoparathyroidism

PURPOSE

Type I pseudohypoparathyroidism (PHP-I) can be subclassified into Ia and Ib, depending on the presence or absence of Albright's hereditary osteodystrophy's phenotype, diminished α-subunit of the stimulatory G protein (G(s)α) activity and multihormonal resistance. Whereas PHP-Ia is mainly associated with heterozygous inactivating mutations in G(s)α-coding exons of GNAS, PHP-Ib is caused by imprinting defects of GNAS. To date, just one patient with PHP and complete paternal uniparental disomy (UPD) has been described. We sought to identify the underlining molecular defect in twenty patients with parathyroid hormone resistance, hypocalcemia and hyperphosphatemia, and abnormal methylation pattern at GNAS locus.

METHODS

Microsatellite typing and comparative genome hybridization were performed for proband and parents.

RESULTS

We describe four patients with partial paternal UPD of chromosome 20 involving pat20qUPD in one case, from 20q13.13-qter in two cases, and pat20p heterodisomy plus interstitial 20q isodisomy in one patient.

CONCLUSIONS

These observations demonstrate that mitotic recombination of chromosome 20 can also give rise to UPD and PHP, a situation similar to other imprinting disorders, such as Beckwith-Wiedemann syndrome or neonatal diabetes.

Association of maternally inherited GNAS alleles with African-American male birth weight

OBJECTIVE

Human birth weight variation has a significant genetic component and important clinical consequences. We performed a survey of single nucleotide polymorphisms (SNPs) in 14 candidate genes to identify associations with birth weight variation.

METHODS

SNP variation was surveyed in 221 healthy African-American mother-newborn pairs. Genes were selected based on previous association with obesity-related traits, significant differences in circulating protein levels in low birth weight pregnancies or association with newborn size in model organisms or growth disorders in humans. Association was tested via multiple linear regression with adjustment for significant covariables.

RESULTS

Under a dominant model SNP rs7754561 of ENPPI was significantly associated with birth weight. Among imprinted loci, maternal genotypes for SNP rs6026576 of GNAS were significantly associated with birth weight (additive and dominant models). This association was restricted to male offspring. Analyses that distinguished between alleles of paternal and maternal origin demonstrated that only maternally-transmitted alleles were associated with birth weight and that this association was restricted to male newborns.

CONCLUSION

The effect of only maternally-transmitted alleles of GNAS may be a consequence of the complex splicing and imprinting pattern of the GNAS gene, although the reason this effect is observed only among male newborns is unclear.

A genotype-first approach for the molecular and clinical characterization of uncommon de novo microdeletion of 20q13.33

Background

Subtelomeric deletions of the long arm of chromosome 20 are rare, with only 11 described in the literature. Clinical features of individuals with these microdeletions include severe limb malformations, skeletal abnormalities, growth retardation, developmental and speech delay, mental retardation, seizures and mild, non-specific dysmorphic features.

Methodology/Principal Findings

We characterized microdeletions at 20q13.33 in six individuals referred for genetic evaluation of developmental delay, mental retardation, and/or congenital anomalies. A comparison to previously reported cases of 20q13.33 microdeletion shows phenotypic overlap, with clinical features that include mental retardation, developmental delay, speech and language deficits, seizures, and behavior problems such as autistic spectrum disorder. There does not appear to be a clinically recognizable constellation of dysmorphic features among individuals with subtelomeric 20q microdeletions.

Conclusions/Significance

Based on genotype-phenotype correlation among individuals in this and previous studies, we discuss several possible candidate genes for specific clinical features, including ARFGAP1, CHRNA4 and KCNQ2 and neurodevelopmental deficits. Deletion of this region may play an important role in cognitive development.

Progressive osseous heteroplasia caused by a novel nonsense mutation in the GNAS1 gene

Progressive osseous heteroplasia (POH), characterized by progressive heterotopic ossifications of the dermis, skeletal muscle and deep connective tissues, is caused by inactivating mutations of GNAS1 of a paternally transmitted allele. We report a novel GNAS1 mutation in a patient with POH. The patient is a 6-year-old boy, whose short stature came to medical attention in infancy. He was diagnosed with growth hormone (GH) deficiency, and subsequent GH therapy resulted in catch-up growth. He developed soft tissue masses in the right heel and right elbow that were calcified or ossified on plain radiographs. MR imaging raised a suspicion of heterotopic ossification; thus, GNAS1 was analyzed. A novel nonsense mutation p.R342X was observed in the patient, but not in his parents. Single nucleotide polymorphism analysis revealed paternal transmission of the mutant allele. RT-PCR analysis demonstrated expression of both normal and mutant GNAS1 transcripts in the patient. Thus, the patient is considered to have developed POH because of the non-functioning truncated Gs(alpha) protein.

Intragenic GNAS deletion involving exon A/B in pseudohypoparathyroidism type 1A resulting in an apparent loss of exon A/B methylation: potential for misdiagnosis of pseudohypoparathyroidism type 1B

CONTEXT

Several endocrine diseases that share resistance to PTH are grouped under the term pseudohypoparathyroidism (PHP). Patients with PHP type Ia show additional hormone resistance, defective erythrocyte G(s)alpha activity, and dysmorphic features termed Albright's hereditary osteodystrophy (AHO). Patients with PHP-Ib show less diverse hormone resistance and normal G(s)alpha activity; AHO features are typically absent in PHP-Ib. Mutations affecting G(s)alpha coding exons of GNAS and epigenetic alterations in the same gene are associated with PHP-Ia and -Ib, respectively. The epigenetic GNAS changes in familial PHP-Ib are caused by microdeletions near or within GNAS but without involving G(s)alpha coding exons.

OBJECTIVE

We sought to identify the molecular defect in a patient who was diagnosed with PHP-Ia based on clinical presentation (hormone resistance and AHO) but displayed the molecular features typically associated with PHP-Ib (loss of methylation at exon A/B) without previously described genetic mutations.

METHODS

Microsatellite typing, comparative genome hybridization, and allelic dosage were performed for proband and her parents.

RESULTS

Comparative genome hybridization revealed a deletion of 30,431 bp extending from the intronic region between exons XL and A/B to intron 5. The same mutation was also demonstrated, by PCR, in the patient's mother, but polymorphism and allele dosage analyses indicated that she had this mutation in a mosaic manner.

CONCLUSION

We discovered a novel multiexonic GNAS deletion transmitted to our patient from her mother who is mosaic for this mutation. The deletion led to different phenotypic manifestations in the two generation and appeared, in the patient, as loss of GNAS imprinting.

Human imprinting syndromes

Human imprinting disorders can provide critical insights into the role of imprinted genes in human development and health, and the molecular mechanisms that regulate genomic imprinting. To illustrate these concepts we review the clinical and molecular features of several human imprinting syndromes including Beckwith–Wiedemann syndrome, Silver–Russell syndrome, Angelman syndrome, Prader–Willi syndrome, pseudohypoparathyroidism, transient neonatal diabetes, familial complete hydatidiform moles and chromosome 14q32 imprinting domain disorders.

Cutaneous and superficial soft tissue lesions associated with Albright hereditary osteodystrophy: clinicopathological and molecular genetic study of 4 cases, including a novel mutation of the GNAS gene

Albright hereditary osteodystrophy is a rare syndrome, in which cutaneous and superficial soft tissue lesions traditionally include osteomas and calcifications. We report 4 patients from 2 families affected with Albright hereditary osteodystrophy and demonstrate that the spectrum of these cutaneous and soft tissue lesions is broader than is usually defined in the literature. In addition to osteomas in the dermis and subcutis, including so-called plaque-like osteoma, we identified the following lesions: calcifying aponeurotic fibroma-like lesion, calcinosis circumscripta-like lesion, and unusual nevi with osteoid and/or peculiar intranuclear pseudoinclusions. One osteoma and the calcifying aponeurotic fibroma-like lesion were analyzed by HUMARA and proved to be clonal. In a family, a novel mutation in the GNAS gene was also identified.

Genetics of pseudohypoparathyroidism: bases for proper genetic counselling

Pseudohypoparathyroidism (PHP) is characterized by hypocalcemia and hyperphosphatemia due to resistance to parathyroid hormone (PTH). Patients with PHP-Ia often show additional hormone resistance and characteristic physical features that are collectively termed Albright's hereditary osteodystrophy (AHO). These features are also present in pseudopseudohypoparathyroidism (PPHP), but patients with this disorder do not show hormone resistance. PHP-Ib patients, on the other hand, predominantly show renal PTH resistance and lack features of AHO. From the genetic point of view, PHP-I is caused by defects in the GNAS gene or in the 5' region of this gene locus. PHP-Ia is caused by heterozygous inactivating mutations in any of the 13 exons codifying the alpha subunit of the stimulatory guanine nucleotide-binding protein (Gsα), while PHP-Ib is due to alterations in the methylation pattern of the 5' regions of the locus, usually associated with upstream microdeletions that are maternally transmitted. The imprinting pattern that affects the GNAS locus has important implications for the inheritance pattern and consequently for appropriate genetic counselling.

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.

Physiological Dysfunctions Associated with Mutations of the Imprinted Gnas Locus

The ubiquitous Gαs-subunit of the trimeric, stimulatory G-protein plays a central role in receptor-mediated signal transduction, coupling receptor activation with the production of cAMP. The Gαs-encoding locus Gnas is now known to consist of a complex arrangement of several protein-coding and noncoding transcripts. We provide an overview of its genomic organization, its regulation by genomic imprinting, and a summary of the physiological roles of the alternative protein variants Gαs and XLαs as determined from deficient mouse models.

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.

Mental retardation in a girl with a subtelomeric deletion on chromosome 20q and complete deletion of the myelin transcription factor 1 gene (MYT1)

A great number of syndromes and inborn errors of metabolism associated with impaired development have been observed, but the aetiology of mental retardation remains unclear in a considerable proportion of cases. Here, we present the clinical and molecular data from a patient with a new de novo subtelomeric deletion on chromosome 20 [46,XX.ish del(20)(qter-)]. For further refinement, bacterial artificial chromosome clones are used. The deletion spans exactly two genes called MYT1 and PCMTD2. Both genes play an important role in myelination and regulating neural differentiation. Loss of these two genes seems to be responsible for the severe mental retardation and mild facial dysmorphic features in our young patient. It might show the phenotypic picture of this specified deletion.

Interstitial del(20)(q11.2q12) - clinical and molecular cytogenetic characterization

A 2-year-old male patient with dysmorphic facial features and multiple congenital anomalies suggestive of a chromosome syndrome is presented. The facial features consisted of a large and high forehead, mild metopic ridging, a small triangular face, depressed nasal bridge, microphthalmia (right more than the left), protruding ears, and mildly prominent anteverted nose with long and smooth philtrum. Cytogenetic analysis showed 46,XY,del(20)(q11.2q12). Parental karyotypes were normal. Molecular characterization of del(20)(q11.2q12) by high-resolution microarray comparative genomic hybridization (arrayCGH) showed an approximately 6.8 Mb deletion. To our knowledge this is the first report of a de novo interstitial del(20)(q11.2q12) characterized by arrayCGH.

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.

A de novo 1.1–1.6 Mb subtelomeric deletion of chromosome 20q13.33 in a patient with learning difficulties but without obvious dysmorphic features

We report on a de novo submicroscopic deletion of 20q13.33 identified by subtelomeric fluorescence in situ hybridization (FISH) in a 4-year-old girl with learning difficulties, hyperlaxity and strabismus, but without obvious dysmorphic features. Further investigations by array-based comparative genomic hybridization (array-CGH) and FISH analysis allowed us to delineate the smallest reported subterminal deletion of chromosome 20q, spanning a 1.1-1.6 Mb with a breakpoint localized between BAC RP5-887L7 and RP11-261N11. The genes CHRNA4 and KCNQ2 implicated in autosomal dominant epilepsy are included in the deletion interval. Subterminal 20q deletions as found in the present patient have, to our knowledge, only been reported in three patients. We review the clinical and behavioral phenotype of such "pure" subterminal 20q deletions.

Major feeding difficulties in the first reported case of interstitial 20q11.22-q12 microdeletion and molecular cytogenetic characterization

We report on a 4-year-old female presenting with intrauterine growth retardation, facial dysmorphic features, major feeding difficulties with severe diarrhea and vomiting, mental retardation with abnormal behavior and hypertonia. Feeding difficulties were the most invalidating features with absent oral intake requiring persistent enteral feeding. Standard cytogenetic studies were normal, but high-resolution chromosome analyses revealed a small de novo interstitial deletion of the long arm of chromosome 20, 46,XX,del(20)(q11.21q12). The deletion was confirmed using metaphase comparative genomic hybridization (CGH) and multicolor high resolution banding (mBAND). The deletion breakpoints were characterized using FISH analyses with YACs, PACs, and BACs clones located in the deleted and adjacent regions. A 6.6-Mb deleted region between markers D20S815 (20q11.22) and D20S435 (20q12) could be delineated. None of the nine previously reported cases with interstitial 20q deletion found in the literature involve the same breakpoints. This report further emphasizes the indication of high-resolution chromosome analyses in children with syndromic mental retardation. The description of additional cases would be useful in order to better characterize the phenotype of patients with proximal interstitial 20q deletion.