Gene structure, DNA methylation, and imprinted expression of the human SNRPN gene

The human SNRPN (small nuclear ribonucleoprotein polypeptide N) gene is one of a gene family that encode proteins involved in pre-mRNA splicing and maps to the smallest deletion region involved in the Prader-Willi syndrome (PWS) within chromosome 15q11-q13. Paternal only expression of SNRPN has previously been demonstrated by use of cell lines from PWS patients (maternal allele only) and Angelman syndrome (AS) patients (paternal allele only). We have characterized two previously unidentified 5' exons of the SNRPN gene and demonstrate that exons -1 and 0 are included in the full-length transcript. This gene is expressed in a wide range of somatic tissues and at high, approximately equal levels in all regions of the brain. Both the first exon of SNRPN (exon -1) and the putative transcription start site are embedded within a CpG island. This CpG island is extensively methylated on the repressed maternal allele and is unmethylated on the expressed paternal allele, in a wide range of fetal and adult somatic cells. This provides a quick and highly reliable diagnostic assay for PWS and AS, which is based on DNA-methylation analysis that has been tested on > 100 patients in a variety of tissues. Conversely, several CpG sites approximately 22 kb downstream of the transcription start site in intron 5 are preferentially methylated on the expressed paternal allele in somatic tissues and male germ cells, whereas these same sites are unmethylated in fetal oocytes. These findings are consistent with a key role for DNA methylation in the imprinted inheritance and subsequent gene expression of the human SNRPN gene.

DNA methylation patterns in human tissues of uniparental origin using a zinc-finger gene (ZNF127) from the Angelman/Prader-Willi region

In order to further our understanding of the epigenetic modifications of DNA and its role in imprinting, we examined DNA methylation patterns of human tissues of uniparental origin. We used complete hydatidiform moles (CHM), which are totally androgenetic conceptions, to examine the paternal methylation pattern in the absence of a maternal contribution and we used ovarian teratomas to represent the maternal counterpart. We carried out an analysis of DNA methylation of a gene which has been shown to contain sites which are differentially methylated in a parent-specific fashion. The gene, ZNF127, is located on chromosome 15q11-q13 in the region associated with Prader-Willi and Angelman syndromes. The parent-of-origin DNA methylation has been postulated to reflect the presence of an imprint and recent studies have confirmed that ZNF127 is differentially expressed only from the paternal chromosome. We identified a unique pattern of hyper- and hypomethylated sites in androgenetic conceptions which was nearly identical to the paternal pattern found in sperm. This may represent the paternal germ-line methylation imprint. We also studied partial hydatidiform moles, non-molar triploid conceptions, normal chorionic villi, and somatic tissue. These all demonstrated a modified DNA methylation pattern characteristic of normal chorionic villi with only limited findings of the imprint. Our results suggest that human androgenetic conceptions may provide an excellent model to analyze epigenetic DNA modifications, such as methylation, in imprinted genes. The paternal allele-specific methylation imprint will also be useful clinically to confirm the androgenetic nature of suspected molar conceptions in which parental blood samples may not be available.

Lack of X inactivation associated with maternal X isodisomy: evidence for a counting mechanism prior to X inactivation during human embryogenesis

We have previously reported functional disomy for X-linked genes in females with tiny ring X chromosomes and a phenotype significantly more abnormal than Turner syndrome. In such cases the disomy results from failure of these X chromosomes to inactivate because they lack DNA sequences essential for cis X inactivation. Here we describe a novel molecular mechanism for functional X disomy that is associated with maternal isodisomy. In this case, the severe mental retardation and multiple congenital abnormalities in a female with a mosaic 45,X/ 46,X,del(X)(q21.3-qter)/ 46X,r(X) karyotype are associated with overexpression of the genes within Xpter to Xq21.31 in many of her cells. Her normal X, ring X, and deleted linear X chromosomes originate from the same maternal X chromosome, and all are transcriptionally active. None expresses X inactive specific transcript (XIST), although the locus and region of the putative X inactivation center (XIC) are present on both normal and linear deleted X chromosomes. To our knowledge, this is the first report of a functional maternal X isodisomy, and the largest X chromosome to escape inactivation. In addition, these results (1) show that cis inactivation does not invariably occur in human females with two X chromosomes, even when the XIC region is present on both of them; (2) provide evidence for a critical time prior to the visible onset of X inactivation in the embryo when decisions about X inactivation are made; and (3) support the hypothesis that the X chromosome counting mechanism involves chromosomal imprinting, occurs prior to the onset of random inactivation, and is required for subsequent inactivation of the chromosome.

Frequency of familial dilated cardiomyopathy

Based on evaluation of 59 probands and their families, we previously demonstrated that over 20% of patients with idiopathic dilated cardiomyopathy (DCM) have familial disease. We acquired 36 additional probands and found familial disease in 24.2% of the 95 probands in this expanded consecutively ascertained cohort. The family history, as reported by the patient and relatives, is often an unreliable indicator of familial disease because patients may be unaware of the significance of a family history of sudden death, arrhythmia or stroke. We demonstrate that careful family history with review of medical records identifies more familial cases than merely asking the patient if there is a family history of DCM. However, even such a careful family history does not identify all familial cases. Some familial cases are identified only by echocardiographic investigation of asymptomatic relatives. We found no clinical attributes of probands, other than family history, which predicted familial disease.

DNA fluorescent probes for diagnosis of velocardiofacial and related syndromes

OBJECTIVE

To study the usefulness of fluorescent in situ hybridization (FISH) with the DNA probe D22S75 for detecting microdeletions in chromosome 22q11.2 in metaphases from patients with features of "CATCH 22" (cardiac anomalies, abnormal facies, thymic hypoplasia or aplasia, cleft palate, and hypocalcemia).

METHODS

High-resolution chromosome analysis and FISH were performed on metaphases from 10 control subjects, 42 patients with features of CATCH 22, and 6 parents of children with CATCH 22. Patients were screened for conotruncal heart defect, palatal abnormality, and facial features. We correlated the phenotype, karyotype, and deletion of a D22S75 locus.

RESULTS

Specimens from nine patients with one or more features of CATCH 22 had a single hybridization signal for D22S75, indicating a deletion of chromosome 22q11.2. Four patients had all the major features of the syndrome and a chromosomal deletion. Thirteen patients had two CATCH 22 features, five of whom had a deletion. None of the 25 patients with a single CATCH 22 feature had a deletion. One patient with a deletion detected by FISH also had a deletion noted on high-resolution banding. All six parents who had blood samples studied by FISH had normal hybridization patterns.

CONCLUSION

FISH is a useful adjunct to chromosome analysis for assessing patients with features of CATCH 22. Detecting a chromosomal deletion by FISH provides a definitive diagnosis and helps to ensure appropriate medical management and genetic counseling.

Takayasu's arteritis presenting with unilateral digital clubbing

Unilateral clubbing has been associated with local vascular lesions of the arm, axilla, and thoracic outlet and with hemiplegia. We describe a patient with Takayasu's arteritis, a disease that affects the aorta and its major branches, who presented with unilateral digital clubbing. A review of the literature on unilateral clubbing is included.

Idiopathic restrictive cardiomyopathy in childhood: diagnostic features and clinical course

OBJECTIVE

To describe the clinical course and outcome of children with idiopathic restrictive cardiomyopathy (IRCM) and to present the Doppler echocardiographic features of this disease in childhood.

DESIGN

We reviewed the Mayo Clinic patient database for the period from 1975 to 1993 to identify children who underwent assessment for IRCM.

MATERIAL AND METHODS

Clinical records and diagnostic studies, including two-dimensional (2-D), M-mode, and Doppler echocardiograms, were reviewed for each patient. Characteristics were analyzed statistically to determine potential predictors of outcome.

RESULTS

Eight children (five girls and three boys) were diagnosed with IRCM between 1975 and 1993 at our institution. The median age at diagnosis was 11 years, and the median duration of follow-up was 11.5 years. Of the eight patients, five died (the median time from initial examination to death was 1 year). All five of these patients had clinical and radiographic evidence of pulmonary venous congestion. In all patients, 2-D and M-mode echocardiography revealed atrial enlargement without ventricular dilatation or hypertrophy. The four patients who underwent detailed diastolic Doppler assessment had findings consistent with restrictive filling and increased left ventricular end-diastolic pressure: (1) short mitral deceleration time, (2) increased pulmonary vein atrial reversal velocity and duration, and (3) pulmonary vein atrial reversal duration greater than mitral A-wave duration.

CONCLUSION

The prognosis for children with IRCM is poor. In this small group of patients, absence of pulmonary venous congestion most consistently predicted extended survival. A combined 2-D and Doppler echocardiographic examination provides a reliable noninvasive means of assessing the physiologic and morphologic features of IRCM in children.

Repair of congenital and acquired pulmonary vein stenosis

BACKGROUND

Congenital pulmonary vein stenosis is a rare cause of obstruction of pulmonary venous blood flow with a high mortality. Acquired pulmonary vein stenosis is an equally serious condition.

METHODS

Eight patients (age range, 3 months to 43 years; median age, 1.5 years) underwent surgical relief of pulmonary vein stenosis. Two had congenital pulmonary vein stenosis, 5 had pulmonary vein stenosis that was acquired after surgical treatment of total anomalous pulmonary venous connection, and 1 had pulmonary vein stenosis associated with idiopathic mediastinal fibrosis and calcification.

RESULTS

One infant died 2 months after correction of acquired pulmonary vein stenosis. At follow-up extending to 16 years (median follow-up, 6.5 years), 6 patients are in New York Heart Association functional class I, and 1 patient is in class II.

CONCLUSIONS

In view of the dismal natural history of untreated pulmonary vein stenosis, prompt surgical relief of the stenosis may be a rewarding undertaking.

Growth patterns of hyperlipidemic children enrolled in a preventive cardiovascular health clinic

The objective of this study was to assess growth patterns of hyperlipidemic children enrolled in a preventive cardiovascular health clinic. A retrospective chart review of hyperlipidemic children enrolled in the Mayo Clinic Cardiovascular Health Clinic for the Young was performed. All participants were counseled to eat an American Heart Association Step-One Diet and exercise regularly. Weight and height were measured every 3 months. Growth was assessed using attained heights and weights and body-mass index Z scores compared to standard distributions for North American children. Sixty-three patients (33 males and 30 females) were enrolled in the study. Mean age at clinic entrance was 7.8 +/- 3.5 years (range: 2 to 16 years). We conclude that participation in a preventive health clinic is generally safe for hyperlipidemic children. However, medical management of hyperlipidemic children must include meticulous surveillance to detect the infrequent occurrence of excessive weight loss or weight stabilization resulting from inappropriate response to dietary counseling.

Enteric protein loss after the Fontan operation

OBJECTIVE

The current study was undertaken to assess the frequency of excessive enteric protein loss and protein-losing enteropathy in the relatively early period after the Fontan operation.

DESIGN

Protein excretion was determined in 26 of 27 consecutive patients who underwent the Fontan procedure between January and June 1990 at the Mayo Clinic.

MATERIAL AND METHODS

At two testing intervals during the first 4 months after the Fontan operation, alpha 1-antitrypsin clearance and fecal alpha 1-antitrypsin concentration studies were done.

RESULTS

All results were normal for the first postoperative test period (2 to 8 weeks). For the second study period, all 17 patients tested had normal alpha 1-antitrypsin clearances. One of the 17 patients had an appreciably increased fecal alpha 1-antitrypsin concentration and transient protein-losing enteropathy.

CONCLUSION

Excessive enteric protein loss and protein-losing enteropathy are relatively uncommon during the first 4 months after the Fontan operation.

Intrauterine closure of membranous ventricular septal defects: mechanism of closure in two autopsy specimens

Anatomic evidence of intrauterine closure of ventricular septal defects (VSDs) has been reported rarely. Between 1985 and 1990, 112 autopsies were performed at the Mayo Clinic on third trimester stillborns and infants who died during the first week of life. There were 21 (19%) cases of congenital heart disease. VSD was found in 12 (11%) cases: in eight (7%) as a part of a more complex heart defect and in four (4%) as an isolated lesion. Two cases with membranous VSDs with tricuspid valve tissue partially occluding the ventricular septal defect were found. A 2280-g female infant (case 1) with trisomy 18 died at 4 days of age. Autopsy revealed bilateral superior venae cavae, a large atrial septal defect, cor triatriatum, an atypical tricuspid valve with large septal leaflet partially obstructing a large membranous VSD, a hypoplastic right ventricle, and severe pulmonic stenosis. A 2610-g female infant (case 2), born with congenital heart block died at 4 days of age. Autopsy revealed cor triatriatum dexter obstructing the tricuspid orifice, a large membranous VSD partially obstructed by the septal leaflet of the tricuspid valve, four small muscular VSDs, and pulmonic stenosis. These cases suggest that closure of membranous VSDs may begin in utero and the mechanism of closure is similar to that reported postnatally.

Circulating heart autoantibodies in familial as compared with nonfamilial idiopathic dilated cardiomyopathy

BACKGROUND

Idiopathic dilated cardiomyopathy (DCM) is a serious heart disease characterized by enlargement of one or both ventricles and ventricular dysfunction. Although most patients have sporadic disease, 20% have been found to have familial DCM when relatives are investigated by echocardiography. No other factors have been identified to date that consistently distinguish familial from nonfamilial DCM. Although some patients have a family history of DCM, a "negative" family history does not exclude familial DCM because affected family members may be presymptomatic or undiagnosed. Because some patients have life-threatening complications at the time of initial assessment of DCM, identifying a serum marker predictive of familial disease would help determine which families would most likely benefit from echocardiographic investigation.

OBJECTIVE

In this study, our objective was to determine whether antiheart autoantibodies could be used to distinguish familial from nonfamilial idiopathic DCM.

METHODS

We analyzed serum specimens for antiheart antibodies from 19 patients categorized as having familial DCM and 15 classified as having nonfamilial DCM on the basis of echocardiographic investigation of first-degree relatives. The mean duration of disease in these 34 patients was 50 months at the time the serum specimens were obtained.

RESULTS

Titers of antibodies against the adenine nucleotide translocator, branched-chain keto acid dehydrogenase, and cardiac myosin did not distinguish between familial and nonfamilial cases of DCM.

Functional imprinting and epigenetic modification of the human SNRPN gene

The SNRPN gene encodes a small nuclear ribonucleoprotein subunit, SmN, thought to be involved in splicing of pre-mRNA. A closely related protein, SmB/B', is constitutively expressed in all tissues except the brain, where SmN is predominantly expressed. The mouse homolog of the SNRPN gene has been shown to be functionally imprinted in mouse brain, being expressed only from the paternally derived chromosome. SNRPN has been mapped to human chromosome 15q11-q13 within the shortest region of deletion overlap for the Prader-Willi syndrome. We have now demonstrated functional imprinting of the human SNRPN gene using reverse transcription followed by the polymerase chain reaction (RT-PCR). No expression was observed in cultured skin fibroblasts of Prader-Willi patients, but was found in all Angelman patients and normal controls examined. We have also demonstrated a parent-specific DNA methylation imprint within intron 5 of the SNRPN gene, which suggests an epigenetic mechanism by which parent-specific expression of this gene might be inherited. Our findings indicate that SNRPN is expressed only from the paternally derived chromosome 15 in humans and therefore may fulfill one major criterion for being involved in the pathogenesis of the Prader-Willi syndrome.

Dystrophin analysis in idiopathic dilated cardiomyopathy

Idiopathic dilated cardiomyopathy (DCM) is characterised by ventricular dilatation and impaired systolic function resulting in congestive heart failure and frequently death. A dilated cardiomyopathy is common in patients with symptomatic Duchenne/Becker muscular dystrophy, a disease caused by dystrophin gene defects. However, cardiomyopathy is rarely the predominant clinical feature of this form of muscular dystrophy. To determine whether dystrophin gene defects might account for a significant number of patients with apparently isolated idiopathic DCM, we performed dystrophin gene analysis in 27 DCM patients, who were ascertained as part of a prospective study on idiopathic DCM. No dystrophin gene defects were found in our patients, whose average age was 50 years. These data suggest that dystrophin defects are not a common cause of idiopathic DCM in this age group in the absence of skeletal muscle cramps or weakness.

Difference in methylation patterns within the D15S9 region of chromosome 15q11-13 in first cousins with Angelman syndrome and Prader-Willi syndrome

Abnormalities of chromosome region 15q11-13 are associated with Angelman syndrome (AS) and Prader-Willi syndrome (PWS). Differences between the methylation patterns of the region of chromosome 15q11-13 which hybridizes to the highly conserved DNA, DN34, in normal individuals and in patients with AS and PWS have been described. We report on a family in which first cousins are affected by AS and PWS as a result of a familial paracentric inversion of 15q11-q13. The results of the studies on this family demonstrate the differences in the methylation patterns in the 2 conditions and the phenomenon of genomic imprinting, whereby genetic information is expressed differently dependent on the parent of origin.

Pulmonary microthrombi. Caveat for successful modified Fontan operation

We reviewed medical records from eight patients (4 to 29 years of age) with a functional single ventricle and pulmonary microthrombosis as observed on open lung biopsy specimens. Hemodynamic assessment before biopsy revealed pulmonary hypertension (mean pulmonary artery pressure 19 to 53 mm Hg) in the seven patients tested and severely increased pulmonary arteriolar resistance (6 to 13 U.m2) in three of the five patients in whom these measurements were performed. Pulmonary blood flow varied (< 4 L/min per square meter in two patients and > 5 L/min per square meter in four patients), and the pulmonary/systemic blood flow ratio was low or normal (0.6 to 1.1) in five of six cases. Hemoglobin values were increased (16.4 to 22 gm/dl) in seven of eight patients. Lung biopsy specimens revealed thrombotic obstructive lesions in 56% (range 28% to 96%) of muscular pulmonary arteries. Coexistent medial hypertrophy was absent or mild in all but one specimen, and none had features of plexogenic pulmonary arteriopathy. Microscopic pulmonary thrombi accounted for unexpected pulmonary hypertension, precluding the Fontan operation, in five patients and unsuccessful Fontan operation in two. Clinically significant thrombi should be considered in patients with severe polycythemia (hemoglobin value > 20 gm/dl) and unobtainable or unreliable pulmonary blood flow or pulmonary resistance determinations.

Clinical, cytogenetic, and molecular evidence for an infant with Smith-Magenis syndrome born from a mother having a mosaic 17p11.2p12 deletion

We describe an infant with del(17) (p11.2p12) whose deleted chromosome was inherited from a mosaic mother. The child had manifestations consistent with Smith-Magenis syndrome. The mother appeared to be of normal intelligence and she had minimal findings of Smith-Magenis syndrome. Separation of chromosome 17 homologues in somatic cell hybrids and molecular studies confirmed the cytogenetic diagnoses and the fact that the mother was mosaic. Furthermore, molecular analysis demonstrated novel breakpoints in this family, with the deletion extending into and completely encompassing the markers duplicated in Charcot-Marie-Tooth (CMT) disease. Although this Smith-Magenis syndrome patient is completely deleted for the CMT region, her electrophysiological findings are different from those found in CMT. This is the only reported case of Smith-Magenis syndrome with transmission from a partially affected mosaic mother. Transmission of interstitial deletions from mosaic parents may be more common than thought; therefore, parental chromosomes should be examined when interstitial deletions are identified.

Modification of 15q11-q13 DNA methylation imprints in unique Angelman and Prader-Willi patients

The clearest example of genomic imprinting in humans comes from studies of the Angelman (AS) and Prader-Willi (PWS) syndromes. Although these are clinically distinct disorders, both typically result from a loss of the same chromosomal region, 15q11-q13. AS usually results from either a maternal deletion of this region, or paternal uniparental disomy (UPD; both chromosomes 15 inherited from the father). PWS results from paternal deletion of 15q11-q13 or maternal UPD of chromosome 15. We have recently described a parent-specific DNA methylation imprint in a gene at the D15S9 locus (new gene symbol, ZNF127), within the 15q11-q13 region, that identifies AS and PWS patients with either a deletion or UPD. Here we describe an AS sibship and three PWS patients in which chromosome 15 rearrangements alter the methylation state at ZNF127, even though this locus is not directly involved in the rearrangement. Parent-specific DNA methylation imprints are also altered at ZNF127 and D15S63 (another locus with a parent-specific methylation imprint) in an AS sibship which have no detectable deletion or UPD of chromosome 15. These unique patients may provide insight into the imprinting process that occurs in proximal chromosome 15 in humans.

Allele-specific replication timing of imprinted gene regions

Several lines of evidence suggest that the paternal and maternal genomes may have different expression patterns in the developing organism and this has been confirmed by the identification of endogenous genes that are parentally imprinted in the mouse. Little is known about the precise mechanisms involved in the process, but structural differences between the two alleles must somehow provide cis-acting signals for directing parental-specific transcription. Cell-cycle replication time is one parameter that has been shown to be associated with both tissue-specific gene expression and the allele-specific transcription patterns of the X chromosomes in female cells. For this reason we have examined the replication timing patterns for the chromosomal regions containing the imprinted genes Igf2, Igf2r, H19 and Snrpn in the mouse. At all of these sites, and their corresponding positions in the human genome, the two homologous alleles replicate asynchronously and it is always the paternal allele that is early-replicating. Thus imprinted genes appear to be embedded in large DNA domains with differential replication patterns, which may provide a structural imprint for parental identity.

Autosomal dominant supravalvular aortic stenosis: localization to chromosome 7

Supravalvular aortic stenosis (SVAS) is a localized or diffuse congenital narrowing of the ascending aorta which may occur sporadically, as a familial defect, or in association with Williams syndrome. Familial cases suggest an autosomal dominant gene defect but the underlying molecular basis of SVAS is unknown. In this study, we sought to localize the genetic defect in familial SVAS by linkage analysis in a large three generation family. A total of 44 polymorphic markers were examined for linkage, including 17 Southern blot-based RFLPs, 2 PCR-based RFLPs, and 25 microsatellites, primarily of the (CA)n repeat type. We report linkage of the disease phenotype to a highly informative (CA)n repeat marker, Mfd 50, at locus D7S440 which has been localized to chromosome arm 7q. Using a 100% penetrance model, which was more conservative than lower values of penetrance, a peak LOD score of 4.66 at a recombination frequency of 0.043 was found. A number of candidate genes have been localized to this region, including collagen 1A2, laminin B1, and elastin. Based on our preliminary linkage data, the abnormal microscopic appearance of aortic elastic fibers in SVAS, and analogous animal and human diseases associated with elastic fiber and vascular abnormalities, there is indirect evidence suggesting elastin as a possible candidate gene for this disorder.

Cardiorespiratory response to exercise after the Fontan operation: a serial study

OBJECTIVES

The purpose of this study was to assess whether there is deterioration of aerobic capacity over time after the Fontan operation in individual patients.

BACKGROUND

We previously observed that maximal aerobic capacity after the Fontan operation was lower in older patients than in younger patients. It was unclear whether this represented a decrease in aerobic capacity with time after operation or was a function of studying patients of different ages at different times postoperatively.

METHODS

All patients who had more than one postoperative exercise study were included. There were 25 patients (19 male, 6 female), aged 3.8 to 39 years at the time of the operation. The first exercise test was performed, on average, 2.2 years after the Fontan operation, and the last exercise test was performed, on average, 5.9 years (range 1.8 to 13) after the operation. In 11 patients, coronary sinus drainage was left on the pulmonary venous side. Five patients had had a previous Glenn operation. Exercise was performed to exhaustion with use of a 3-min incremental cycle protocol.

RESULTS

Exercise duration, oxygen uptake, blood pressure, respiratory rate, minute ventilation, pulmonary blood flow index, exercise factor, ST-T wave changes and the prevalence of arrhythmias were similar during the first and last tests. Exercise systemic arterial blood oxygen saturation decreased from the first to the last postoperative test (p < 0.006) regardless of age. The percent of predicted heart rate, at rest and during maximal exercise, decreased more in older patients from the first to the last test (p < 0.05 for rest and exercise).

CONCLUSIONS

In this select group of patients, exercise tolerance remained relatively unchanged over the range of 13 years after the Fontan operation. Heart rate at rest and during maximal exercise decreased more than predicted for age in older patients. A small but significant progressive decrease in systemic arterial blood oxygen saturation was found. The former might represent abnormal sinus node function in patients with functional single ventricle, and the latter might represent a tendency toward development of abnormal ventilation/perfusion patterns resulting from the development of small pulmonary arteriovenous fistulas or an effect of position of the coronary sinus postoperatively.