Paternal Uniparental Isodisomy Research Articles

Exclusion of the GNAS locus in PHP-Ib patients with broad GNAS methylation changes: Evidence for an autosomal recessive form of PHP-Ib?

Most patients with autosomal dominant pseudohypoparathyroidism type Ib (AD-PHP-Ib) carry maternally inherited microdeletions upstream of GNAS that are associated with loss of methylation restricted to GNAS exon A/B. Only few AD-PHP-Ib patients carry microdeletions within GNAS that are associated with loss of all maternal methylation imprints. These epigenetic changes are often indistinguishable from those observed in patients affected by an apparently sporadic PHP-Ib form that has not yet been defined genetically. We have now investigated six female patients affected by PHP-Ib (four unrelated and two sisters) with complete or almost complete loss of GNAS methylation, whose healthy children (11 in total) showed no epigenetic changes at this locus. Analysis of several microsatellite markers throughout the 20q13 region made it unlikely that PHP-Ib is caused in these patients by large deletions involving GNAS or by paternal uniparental isodisomy or heterodisomy of chromosome 20 (patUPD20). Microsatellite and single-nucleotide variation (SNV) data revealed that the two affected sisters share their maternally inherited GNAS alleles with unaffected relatives that lack evidence for abnormal GNAS methylation, thus excluding linkage to this locus. Consistent with these findings, healthy children of two unrelated sporadic PHP-Ib patients had inherited different maternal GNAS alleles, also arguing against linkage to this locus. Based on our data, it appears plausible that some forms of PHP-Ib are caused by homozygous or compound heterozygous mutation(s) in an unknown gene involved in establishing or maintaining GNAS methylation.

Paternal uniparental isodisomy of the entire chromosome 20 as a molecular cause of pseudohypoparathyroidism type Ib (PHP-Ib)

Pseudohypoparathyoridism type Ib (PHP-Ib) typically defines the presence of end-organ resistance to parathyroid hormone in the absence of Albright's hereditary osteodystrophy. Patients affected by this disorder present with imprinting defects in the complex GNAS locus. Microdeletions within STX16 or GNAS have been identified in familial cases with PHP-Ib, but the molecular cause of the GNAS imprinting defects in sporadic PHP-Ib cases remains poorly defined. We now report a case with sporadic PHP-Ib for whom a SNPlex analysis revealed loss of the maternal GNAS allele. Further analysis of the entire genome with a 100 K SNP chip identified a paternal uniparental isodisomy affecting the entire chromosome 20 without evidence for another chromosomal abnormality. Our findings explain the observed GNAS methylation changes and the patient's hormone resistance, and furthermore suggest that chromosome 20 harbors, besides GNAS, no additional imprinted region that contributes to the clinical and laboratory phenotype.

Familial Focal Congenital Hyperinsulinism

Congenital hyperinsulinism (CHI) is a cause of persistent hypoglycemia. Histologically, there are two subgroups, diffuse and focal. Focal CHI is a consequence of two independent events, inheritance of a paternal mutation in ABCC8/KCNJ11 and paternal uniparental isodisomy of chromosome 11p15 within the embryonic pancreas, leading to an imbalance in the expression of imprinted genes. The probability of both events occurring within siblings is rare. We describe the first familial form of focal CHI in two siblings. The proband presented with medically unresponsive CHI. He underwent pancreatic venous sampling and Fluorine-18-L-dihydroxyphenylalanine positron emission tomography scan, which localized a 5-mm focal lesion in the isthmus of the pancreas. The sibling presented 8 yr later also with medically unresponsive CHI. An Fluorine-18-L-dihydroxyphenylalanine positron emission-computerised tomography scan showed a 7-mm focal lesion in the posterior section of the head of the pancreas. Both siblings were found to be heterozygous for two paternally inherited ABCC8 mutations, A355T and R1494W. Surgical removal of the focal lesions in both siblings cured the Hyperinsulinaemic hypoglycaemia. This is the first report of focal CHI occurring in siblings. Genetic counseling for families of patients with focal CHI should be recommended, despite the rare risk of recurrence of this disease.

Paternal isodisomy of chromosome 2 as a cause of long chain 3‐hydroxyacyl‐CoA dehydrogenase (LCHAD) deficiency

Long-chain 3-hydroxyacyl-CoA dehydrogenase deficiency is an autosomal recessive disorder affecting mitochondrial fatty acid oxidation due to mutations in the HADHA gene. We report on a 22-month-old child who was identified on expanded newborn screening with an abnormal acylcarnitine pattern and increased C14OH. Molecular analysis showed that the child was homozygous for the common mutation, c.1526G > C (p.Glu510Gln) in the HADHA gene. Carrier testing on the parental samples revealed that the father was heterozygous for the mutation whereas the mother did not carry the mutation. Short tandem repeat testing with markers covering both short and long arms of chromosome 2 showed that the child has paternal uniparental isodisomy. We highlight the importance of parental testing in cases of homozygosity in autosomal recessive disorders and its impact on genetic counseling of the family.

Paternal uniparental isodisomy of chromosome 6 causing a complex syndrome including complete IFN‐γ receptor 1 deficiency

Mendelian susceptibility to mycobacterial disease (MSMD) is a rare primary immunodeficiency associated with clinical disease caused by weakly virulent mycobacterial species. Interferon gamma receptor 1 (IFN-gammaR1) deficiency is a genetic etiology of MSMD. We describe the clinical and genetic features of a 7-year-old Italian boy suffering from MSMD associated with a complex phenotype, including neonatal hyperglycemia, neuromuscular disease, and dysmorphic features. The child also developed necrotizing pneumonia caused by Rhodococcus equi. The child is homozygous for a nonsense mutation in exon 3 of IFNGR1 as a result of paternal uniparental disomy (UPD) of the entire chromosome 6. This is the first reported case of uniparental disomy resulting in a complex phenotype including MSMD.

Transient neonatal diabetes mellitus with macroglossia diagnosed by methylation specific PCR (MS-PCR)

= Abstract = Transient neonatal diabetes mellitus (TNDM) has been associated with paternal uniparental isodisomy of chromosome 6, paternally inherited duplication of 6q24, or a methylation defect at a CpG island of the ZAC or HYMAI gene. We expe- rienced a case of TNDM in which the patient presented with hyperglycemia, macroglossia, and intrauterine growth retar- dation, caused by a paternally derived HYMAI. An 18-day-old female infant was admitted to the hospital because of mac- roglossia and recurrent hyperglycemia. In addition to the macroglossia, she also presented with large fontanelles, microg- nathia, and prominent eyes. Serum glucose levels were 200 300 mg/dL and they improved spontaneously 2 days after - admission. To identify the presence of a maternal methylated allele, bisulfite-treated genomic DNA from peripheral blood was prepared and digested with BssHII after polymerase chain reaction (PCR) amplification with methylation-specific HYMAI primers. PCR and restriction fragment length polymorphism analysis showed that the patient had only the paternal origin of the HYMA1 gene. TNDM is associated with a methylation defect in chromosome 6, suggesting that an imprinted gene on chromosome 6 is responsible for this phenotype. (Korean J Pediatr 2010;53:432-436)

Tyrosinemia type 1 and Angelman syndrome due to paternal uniparental isodisomy 15

Uniparental isodisomy arises when an individual inherits two copies of a specific chromosome from a single parent, which can unmask a recessive mutation or cause a problem of genetic imprinting. Here we describe an exceptional case in which the patient simultaneously presents tyrosinemia type 1 and Angelman syndrome. The genetic studies showed that the patient presents paternal uniparental isodisomy of chromosome 15, with absence of the maternal homolog. As a consequence of this isodisomy, the patient is homozygous for the mutation IVS12+5G>A in the FAH gene, located in the chromosomal region 15q23-25, causing tyrosinemia type 1. The mutation was inherited from his father in double dosage, whereas the mother is not a carrier, which implies that the recurrence risk in the family is negligible. On the other hand, the lack of maternal contribution causes Angelman syndrome, a neurodevelopmental disorder associated with a loss of maternal gene expression in chromosome region 15q11-q13, and more specifically, of the UBE3A gene. This gene shows a tissue-specific imprinting, and only the maternally derived allele is expressed in certain areas of the brain. We observed through a literature review that uniparental disomy probably occurs more frequently than suspected, although it is more usually detected when the uniparental disomy implies the appearance of a disease because of the gene imprinting or by reduction to homozygosity of a recessive mutation. The conclusion is that uniparental disomy should always be considered when more than one genetic disease mapping to the same chromosome is present in a patient.

Incidence of neonatal diabetes in Austria-calculation based on the Austrian Diabetes Register

Neonatal diabetes mellitus (NDM) is a rare monogenic form of diabetes which is diagnosed in the first 6 months of life. Several studies in the last few years provide information on genetic causes for NDM. The aim of this study was to identify all patients with diabetes in the first 6 months of life through the Austrian Diabetes Register, which is available since 1989. A retrospective data analyses was performed to calculate the current incidence of NDM. Ten patients were registered with diabetes onset within the first 6 months of life in the Austrian Diabetes Register. Evaluation of detailed clinical data was performed by sending a questionnaire to all diabetes centers. Ten patients from nine different families with NDM were diagnosed in Austria from 1989 until September 2007. Seven patients (one male, six females) had transient NDM (TNDM), three (two males, one female) showed a permanent course [permanent neonatal diabetes mellitus (PNDM)]. One had immunodeficiency, polyendocrinopathy and enteropathy X-linked (IPEX) syndrome and another showed aplasia of the pancreas; no genetic etiology was found in the third case. In three out of seven patients with a transient course of NDM a genetic diagnosis was possible. Two female siblings had activating point mutations in the ABCC8 gene, although one patient had paternal uniparental isodisomy of chromosome 6q24. One patient's family did not consent to genetic testing. The incidence of NDM in Austria is 1/160 949, with an incidence of 1/ 536 499 for PNDM and 1/229 928 for TNDM.

Donnai–Barrow syndrome (DBS/FOAR) in a child with a homozygous LRP2 mutation due to complete chromosome 2 paternal isodisomy

Donnai-Barrow syndrome [Faciooculoacousticorenal (FOAR) syndrome; DBS/FOAR] is a rare autosomal recessive disorder resulting from mutations in the LRP2 gene located on chromosome 2q31.1. We report a unique DBS/FOAR patient homozygous for a 4-bp LRP2 deletion secondary to paternal uniparental isodisomy for chromosome 2. The propositus inherited the mutation from his heterozygous carrier father, whereas the mother carried only wild-type LRP2 alleles. This is the first case of DBS/FOAR resulting from uniparental disomy (UPD) and the fourth published case of any paternal UPD 2 ascertained through unmasking of an autosomal recessive disorder. The absence of clinical symptoms above and beyond the classical phenotype in this and the other disorders suggests that paternal chromosome 2 is unlikely to contain imprinted genes notably affecting either growth or development. This report highlights the importance of parental genotyping in order to give accurate genetic counseling for autosomal recessive disorders.

-thalassemia major evolution from -thalassemia minor is associated with paternal uniparental isodisomy of chromosome 11p15

beta-thalassemia major can be caused by homozygosity or compound heterozygosity for beta-globin gene mutations (HBB gene). Most cases are inherited from parents who both have diseased alleles of the HBB gene. We report a patient with late-onset beta-thalassemia major that evolved from beta-thalassemia minor in which only one of her parents had the diseased HBB gene. To study the cause of beta-thalassemia major in this patient, we performed the 100K single nucleotide polymorphism genotyping assay, fluorescence in situ hybridization, and DNA methylation analysis of the imprinting genes near the HBB gene. The results showed a loss of heterozygosity in the region of chromosome 11p14.3 to 11p15.5, which perfectly matched one allele of her father. Our study demonstrates that paternal uniparental isodisomy of chromosomal 11p15.5 is associated with the beta-thalassemia major in this patient. Key words: beta-thalassemia major, uniparental isodisomy, mosaicism.

Mosaic Paternal Uniparental Isodisomy and an ABCC8 Gene Mutation in a Patient With Permanent Neonatal Diabetes and Hemihypertrophy

Activating mutations in the KCNJ11 and ABCC8 genes encoding the Kir6.2 and SUR1 subunits of the pancreatic ATP-sensitive K(+) channel are the most common cause of permanent neonatal diabetes. In contrast to KCNJ11, where only dominant heterozygous mutations have been identified, recessively acting ABCC8 mutations have recently been found in some patients with neonatal diabetes. These genes are co-located on chromosome 11p15.1, centromeric to the imprinted Beckwith-Wiedemann syndrome (BWS) locus at 11p15.5. We investigated a male with hemihypertrophy, a condition classically associated with neonatal hyperinsulinemia and hypoglycemia, who developed neonatal diabetes at age 5 weeks. The KCNJ11 and ABCC8 genes and microsatellite markers on chromosome 11 were analyzed in DNA samples from the patient and his parents. A paternally inherited activating mutation (N72S) in the ABCC8 gene was identified in the proband. The mutation was present at 70% in the patient's leukocytes and 50% in buccal cells. Microsatellite analysis demonstrated mosaic segmental paternal uniparental isodisomy (UPD) of 11pter-11p14 in the proband that encompassed the ABCC8 gene and the BWS locus. We report a patient with neonatal diabetes, hemihypertrophy, and relatively high birth weight resulting from telomeric segmental paternal UPD of chromosome 11, which unmasks a recessively acting gain-of-function mutation in the ABCC8 gene and causes deregulation of imprinted genes at the BWS locus on 11p15.5.

An ABCC8 Gene Mutation and Mosaic Uniparental Isodisomy Resulting in Atypical Diffuse Congenital Hyperinsulinism

Congenital hyperinsulinism (CHI) may be due to diffuse or focal pancreatic disease. The diffuse form is associated with an increase in the size of beta-cell nuclei throughout the whole of the pancreas and most commonly results from recessive ATP-sensitive K(+) channel (K(ATP) channel) mutations. Focal lesions are the consequence of somatic uniparental disomy for a paternally inherited K(ATP) channel mutation with enlargement of the beta-cell nuclei confined to the focal lesion. Some "atypical" cases defy classification and show pancreatic beta-cell nuclear enlargement confined to discrete regions of the pancreas. We investigated an atypical case with normal morphology within the tail of the pancreas but occasional enlarged endocrine nuclei in parts of the body and head. The KCNJ11 and ABCC8 genes encoding the K(ATP) channel subunits and microsatellite markers on chromosome 11 were analyzed in DNA samples from the patient and her parents. A mosaic ABCC8 nonsense mutation (Q54X) was identified in the proband. The paternally inherited mutation was present at 90% in lymphocytes and 50% in normal pancreatic sections but between 64 and 74% in abnormal sections. Microsatellite analysis showed mosaic interstitial paternal uniparental isodisomy (UPD) for chromosome 11p15.1. We report a novel genetic mechanism to explain atypical histological diffuse forms of CHI due to mosaic UPD in patients with dominantly inherited ABCC8 (or KCNJ11) gene mutations.

Paternal uniparental isodisomy for chromosome 14 with mosaicism for a supernumerary marker chromosome 14

Uniparental disomy (UPD) describes the inheritance of two homologous chromosomes from a single parent. Disease phenotypes associated with UPD and chromosomal imprinting, rather than with mutations, include Beckwith-Wiedemann syndrome (paternal UPD11p), Angelman syndrome (paternal UPD15), Prader-Willi syndrome (maternal UPD15), and transient neonatal diabetes (paternal UPD6). Here we report on the first case of paternal uniparental isodisomy of chromosome 14 with a mosaicism for a supernumerary marker chromosome 14. The patient demonstrated a small thorax with a 'coat hanger' shape of the ribs, kyphoscoliosis, hypoplasia of the maxilla and mandible, a broad nasal bridge with anteverted nares, contractures of the wrists with ulnar deviation bilaterally, diastasis recti, and marked muscle hypotonia. Vertical skin creases under the chin and stippled epiphyses of the humeri were features not previously described in patients with paternal UPD14. This case illustrates that as with the finding of an isochromosome, a supernumerary marker chromosome can be an important clue to the presence of UPD14.

Unusual Clinical Severity of Complement Membrane Cofactor Protein–Associated Hemolytic-Uremic Syndrome and Uniparental Isodisomy

Atypical hemolytic-uremic syndrome (aHUS; OMIM 235400) is genetically and clinically heterogeneous. Mutations in membrane cofactor protein (MCP; CD46), a widely expressed complement regulator, predispose to recurrent forms of the disease. Patients carrying MCP mutations have a favorable clinical outcome in comparison to those with factor H (CFH) or factor I (IF) mutations, which lead in most cases to end-stage renal failure. We identified 1 patient who presented at 1 year of age with a first episode of aHUS requiring dialysis therapy. After 2 recurrences of the disease, the patient developed end-stage renal failure. No mutation in the CFH and IF genes was found. A novel homozygous mutation (IVS10+2 T-->C) in the splice-donor of exon 10 encoding the transmembrane region of the MCP gene was associated with dramatically decreased cell-surface expression of MCP. Because the nucleotide substitution was inherited from the patient's father, but not her mother, a large deletion or uniparental disomy was suspected. Both karyotyping and cytogenetic analysis of chromosome 1q32 were performed, for which MCP maps showed no abnormalities. Subsequent genotype analysis using microsatellite markers spanning chromosome 1 showed that the affected child was homozygous for the entire series of markers tested and that all alleles originated from the father. Complete paternal uniparental isodisomy of chromosome 1 is a novel mechanism resulting in severe deficiency of MCP expression. The outcome of the disease reported here indicates that MCP mutation and complete paternal uniparental disomy of chromosome 1 could have an additive effect in determining the severity of the HUS phenotype.

231 INVITED Activation of p53 by small molecules from fish to man

Whole chromosomal and segmental uniparental disomy (UPD) is one of the causes of imprinting disorder and other recessive disorders. Most investigations of UPD were performed only using cases with relevant phenotypic features and included few markers. However, the diagnosis of cases with segmental UPD requires a large number of molecular investigations. Currently, the accurate frequency of whole chromosomal and segmental UPD in a normal developing embryo is not well understood. Here, we present whole chromosome and segmental UPD analysis using single nucleotide polymorphism (SNP) microarray data of 173 mother-father-child trios (519 individuals) from six populations (including 170 HapMap trios). For two of these trios, we also investigated the possibility of shorter segmental UPD as a consequence of homologous recombination repair (HR) for DNA double strand breaks (DSBs) during the early developing stage using high-coverage whole-genome sequencing (WGS) data from 1000 Genomes Project. This could be overlooked by SNP microarray. We identified one obvious segmental paternal uniparental isodisomy (iUPD) (8.2 mega bases) in one HapMap sample from 173 trios using Genome-Wide Human SNP Array 6.0 (SNP6.0 array) data. However, we could not identify shorter segmental iUPD in two trios using WGS data. Finally, we estimated the rate of segmental UPD to be one per 173 births (0.578%) based on the UPD screening for 173 trios in general populations. Based on the autosomal chromosome pairs investigated, we estimate the rate of segmental UPD to be one per 3806 chromosome pairs (0.026%). These data imply the possibility of hidden segmental UPD in normal individuals.

Prenatal diagnostic indicators of paternal uniparental disomy 14

To present clinical findings of a child with paternal uniparental isodisomy 14 (pat UPD14) focusing on relevant prenatal characteristics. Ultrasonography at 23 weeks of gestation of a 37-year-old multigravid woman revealed a fetus with polyhydramnios, small thorax, and short, distinctively angled ribs. Fetal karyotype was 46,XY. The child was born spontaneously at 35 weeks with poor neonatal adaptation. From birth, he presented with severe respiratory insufficiency due to severe thoracic malformation. Clinical examination revealed a small, bell-shaped thorax, redundant lax skin, mild contractures of the fingers and dysmorphic facial features. Chest X rays showed short, abnormally curved ribs that suggested the possibility of pat UPD14, which was confirmed by molecular analysis. Pat UPD14 is associated with a distinct clinical phenotype. Prognosis is poor because of severe respiratory insufficiency and neurodevelopmental retardation. Our report confirms salient postnatal signs of previous descriptions, especially the characteristic radiological abnormalities with ribs showing a 'coat-hanger' configuration. Retrospective fetal ultrasound of our case allowed the identification of this pathognomonic feature prenatally, which makes it possible to consider pat UPD14 at routine prenatal sonography, in particular in combination with a small bell-shaped thorax and polyhydramnios.

Paternal uniparental isodisomy of the entire chromosome 3 revealed in a person with no apparent phenotypic disorders

Uniparental disomy (UPD) is a rare genetic abnormality. During a whole genome linkage study we identified a case of paternal uniparental isodisomy 3 serendipitously. This is the first ascertained human paternal UPD for chromosome 3 (UPD3pat). The finding of this paternal UPD case of the entire chromosome 3 with no apparent phenotypic disorders suggests that there are no paternal imprinted genes causing rare genetic disorders on chromosome 3.

Constitutional Segmental Uniparental Disomy in a Twin Pair with t(12;21) Positive Acute Lymphoblastic Leukemia Characterized by the Same Prenatal Clone and Divergent Clonal Evolution.

We previously reported (Tussiwand R. et al., ASH 2002) the case of a monozygotic twin pair with concordant ALL aged 3.0 years at diagnosis, with only 13 days difference in latency. Twin 1 was classified as pre-B ALL and twin 2 as common-ALL, based on standard immunophenotyping criteria. A large screening for TcR and Ig gene rearrangements was performed, resulting in only one common VKII-Kde rearrangement, the others being not related to each others. Highly sensitive RQ-PCR was performed for all markers in both twins. The result of the crossed analysis was consistent with the hypothesis that after a prenatal event resulting in a preleukemic clone, at least a second independent event must have occurred before overt leukemia. To further identify markers of the clonal evolution, high-resolution single nucleotide polymorphisms (SNP) genotype analysis was performed on the DNA using the 10K SNP array (Affymetrix). Remission bone marrow was taken as a germ line samples. Array-based analysis of SNPs allows the rapid determination of genome-wide allelic information at high density, including the identification of submicroscopical copy number changes and/or loss of heterozygosity (LOH). SNP array analysis have been so far successfully applied to demonstrate allelic imbalance in ALL and AML blasts. A 12p12-13 deletion was observed on twin 2. The deletion of the TEL allele not involved in the t(12;21) was confirmed in twin 2 by FISH and microsatellite analyses. The twin 1 did not show any 12p deletion. Similar observations have been made in late-relapses occurring in ALL carrying the t(12;21) translocation: the predominant clone did not correspond to the same clone observed at diagnosis, but represented a second, independent transformation event within the fetal pre-leukemic clone, even when in the presence of the same genetic background. More interestingly, we found by SNP array a 13Mb area of LOH in remission and presentation samples of both twins involving the 2q13-14.3 region. As further confirmed by FISH with 2q probes, LOH was not associated with chromosomal loss, implying a recombination event resulting in Uniparental Isodisomy (UPD). The UPD area includes 57 known genes, several of them implicated in oncogenesis; they include translin, a gene involved in the control of chromosomal translocation and implicated in lymphoid malignancy. UPD of this region has not been reported in other tumors or in remission samples of leukemia; in genetic diseases few cases have been reported with maternal or paternal UPD 2, never associated with haematological disorders. By contrast, it has been shown that a transmitted deletion of 2q13 to 2q14.1 causes no phenotypic abnormalities. This is the first report on constitutional UPD in leukemia patients. Further analyses are necessary to understand the clinical meaning of this chromosomal abnormality; one hypothesis could be that the twins were born with a genetic predisposition to develop leukaemia. In this context, t(12;21) and additional events (i.e. TEL deletion) may be responsible for the overt leukemia.

84 A Rare Association of Transient Neonatal Diabetes and Trisomy X

Background: Neonatal diabetes mellitus (NDM) has an incidence of 1/400,000–500,000 live births and is characterized by hyperglycemia with onset in the first 6 weeks after delivery. It may be transient, resolving spontaneously in the first four months or may persist for more than a year (30% of cases). Case report: Preterm newborn, born SGA at 33 weeks of gestational age by emergency caesarian section for acute fetal distress. Family history: paternal grandmother and uncles of mother with diabetes type II, mother with gestational diabetes. Genetic prenatal diagnosis: trisomy X. Apgar at 1 min= 1, intubated and admitted to NICU, assisted ventilation for RDS. Clinical features: patent ductus arteriosus, craniofacial dysmorphism (small ears, retrognathia, cleft palate), left valgus foot. Hyperglycemia at 2 h of life treated with continuous insulin infusion until 3 months of age, than replaced with subcutaneous therapy. At present, insulin therapy has been suspended and glycemic control is good. Clinical and laboratory investigations: High levels of LH, FSH, PRL, E2, PG, testosterone, DEA-S, 17-OHPG, ACTH and glucagons; low levels of basal cortisol; normal levels of GH, FT3, FT4, TSH were detected. Persistent low levels of C-Peptide, absence of anti-insulin, anti-islet and anti-GAD antibodies were also found. Abdominal ultrasonography was normal. The DNA analysis for polymorphism of chromosome 6 showed paternal uniparental isodisomy. Discussion: Transient NDM is associated with uniparental isodisomy of chromosome 6 of paternal origin. Persistent NDM seems to be associated with an inactivating mutation of IPF1, which is necessary for the development of the pancreas as well as for glucose responsive stimulation of insulin gene transcription. The discrimination between transient and persistent NDM is important for both prognosis and therapy. In our opinion the association we found between trisomy X and transient NDM is likely casual. This report is the second one in the literature concerning a patient with transient NDM, uniparental isodisomy of chromosome 6 of paternal origin and trisomy X.

A Novel STX16 Deletion in Autosomal Dominant Pseudohypoparathyroidism Type Ib Redefines the Boundaries of a cis-Acting Imprinting Control Element of GNAS

A unique heterozygous 3-kb microdeletion within STX16, a closely linked gene centromeric of GNAS, was previously identified in multiple unrelated kindreds as a cause of autosomal dominant pseudohypoparathyroidism type Ib (AD-PHP-Ib). We now report a novel heterozygous 4.4-kb microdeletion in a large kindred with AD-PHP-Ib. Affected individuals from this kindred share an epigenetic defect that is indistinguishable from that observed in patients with AD-PHP-Ib who carry the 3-kb microdeletion in the STX16 region (i.e., an isolated loss of methylation at GNAS exon A/B). The novel 4.4-kb microdeletion overlaps with the previously identified deletion by 1,286 bp and, similar to the latter deletion, removes several exons of STX16 (encoding syntaxin-16). Because these microdeletions lead to AD-PHP-Ib only after maternal transmission, we analyzed expression of this gene in lymphoblastoid cells of affected individuals with the 3-kb or the 4.4-kb microdeletion, an individual with a NESP55 deletion, and a healthy control. We found that STX16 mRNA was expressed in all cases from both parental alleles. Thus, STX16 is apparently not imprinted, and a loss-of-function mutation in one allele is therefore unlikely to be responsible for this disorder. Instead, the region of overlap between the two microdeletions likely harbors a cis-acting imprinting control element that is necessary for establishing and/or maintaining methylation at GNAS exon A/B, thus allowing normal G alpha(s) expression in the proximal renal tubules. In the presence of either of the two microdeletions, parathyroid hormone resistance appears to develop over time, as documented in an affected individual who was diagnosed at birth with the 4.4-kb deletion of STX16 and who had normal serum parathyroid hormone levels until the age of 21 mo.