Molecular diagnosis of Prader-Willi syndrome: parent-of-origin dependent methylation sites and non-isotopic detection of (CA)n dinucleotide repeat polymorphisms

Growth patterns of Chinese patients with Prader-Willi syndrome

The aim of the present study was to investigate the spontaneous development of growth and weight gain of patients with Prader-Willi syndrome (PWS) in Mainland China. We retrospectively analyzed 120 cases of PWS diagnosed from 1994 to 2014 in Mainland China. Scatter diagram of the growth data was compared to standardized growth curve. The length at birth was similar to the normal population and the mean birthweight is under the 50th centile of normal population. Heights in 43% (27/62) of patients had dropped off below the 3(rd) centile of their peers after 5 years of age. Weights in 65.9% (58/88) of patients had exceeded the 97(th) centile of their peers after 3 years of age. Early obesity is obvious in 92.9% (66/71) of patients with body mass index (BMI) up the 97(th) centile of their peers from 2-3 years of age. Final mean height is 149.3 ± 11.2 cm for females (n = 7) and 146.2 ± 9.8 cm for males (n = 6), showing 11.3 cm and 26.6 cm below the average height for normal Chinese girls and boys. The growth pattern of PWS in Mainland China is comparable to those in Caucasians. Early intervention with recombinant human growth hormone is warranted considering the early onset of growth retardation and obesity.

Proximal 19q trisomy: a new syndrome of morbid obesity and mental retardation

AIMS

To report on the clinical and metabolic characteristic and the unique chromosomal defect of a mentally retarded and morbidly obese patient.

METHODS

A 13-year follow-up, including insulin sensitivity, lipid profile and polysomnography studies and various therapeutic interventions are described. The presence of a supernumerary marker in karyotype preparation was further studied by fluorescence in situ hybridization (FISH). Comparative genomic hybridization (CGH) was used to identify the source of the chromosomal marker.

RESULTS

Insulin resistance was found by the homeostatic model assessment (HOMA) and the quantitative insulin sensitivity check index (QUICKI). M-FISH identified euchromatin derived from chromosome 19, and CGH confirmed the FISH results and demonstrated that the supernumerary marker derived from 19q12 to 19q13.2.

CONCLUSION

The clinical and metabolic characteristics in association with partial chromosomal trisomy differ our patient from the currently known syndromes of obesity and mental retardation. The metabolic impairments in this case can derive from unbalanced expression of several genes in the 19q12-19q13.2 region, genes that are related to adipose tissue homeostasis and insulin resistance. The clinical and genetic similarities to a previously reported case may suggest that partial 19q trisomy is a new syndrome of obesity and mental retardation.

Deletion of the ANKRD15 gene at 9p24.3 causes parent-of-origin-dependent inheritance of familial cerebral palsy

A four-generation family was studied in which nine children had congenital cerebral palsy (CP), characterized by quadriplegia and mental retardation. All the affected children were born to healthy, related fathers, whereas the children of their healthy female relatives were unaffected. Linkage analysis attributed the condition to chromosome 9p24.3, where a 225 kb deletion was identified. The deletion spans a single gene, ANKRD15 (ankyrin repeat domain 15), which is ubiquitously expressed. In the affected children, the ANKRD15 is not expressed in lymphoblastoid cells, whereas in their healthy fathers, who harbor the same deletion, the expression of ANKRD15 did not deviate from controls. This expression pattern can be interpreted as a maternal imprinted gene that is expressed only from the paternal allele. The expression of ANKRD15 in lymphoblastoid cells from the control group was monoallelic but not imprinted. The monoallelic expression was restricted to the ANKRD15 gene, whereas biallelic expression was found in the DOCK8 gene, which resides at the telomeric side of the deletion. No correlation was found between the expression of the ANKRD15 gene and the pattern of DNA methylation in the CpG islands 5' of the gene. However, differences in methylation pattern were found in the CpG islands flanking the DMRT1 gene, which is located at the 3' side of the ANKRD15 gene. In the affected individuals, as in the control group, the CpG islands were hypo-methylated, whereas in the healthy fathers, the CpG islands were hyper-methylated in cis with the deletion. This unique family demonstrates a phenomenon of a deletion that creates imprinting-like inheritance. The implication of this family to sporadic CP is discussed.

A patient with Prader-Willi syndrome and a supernumerary marker chromosome r(15)(q11.1-13p11.1)pat and maternal heterodisomy

We report on a Prader-Willi patient with a de novo supernumerary marker chromosome (SMC) in 16% of the cells. The SMC was a ring chromosome and it included the PWS/AS critical region as was demonstrated by FISH. Segregation analysis indicated that the SMC originated from a paternal chromosome 15 and the two normal chromosomes 15 of the patients were of the maternal homologues. Namely, the patient had maternal heterodisomy in 85% of the cells and triplication of the PWS/AS region in 15% of the cells. The Prader-Willi features were the result of the low mosaicism of the SMC. The evolution of the maternal heterodisomy and the SMC were two unrelated events, the occurrence of both events in the same embryo rescued it from lethality.

Prevalence of myotonic dystrophy in Israeli Jewish communities: inter-community variation and founder premutations

In a comprehensive epidemiological survey among Jews living in Israel, the average prevalence of myotonic dystrophy (DM) was 15.7/10(5) (1 case in 6369) with intercommunity variations; the Ashkenazi Jews had the lowest rate, 5.7/10(5) (1 case in 17544) as compared to the rate in the Sephardim/Oriental Jews 20/10(5) (1 case in 5000) and the in the Yemenite Jews 47.3/10(5) (1 case in 2114). The rate of unrelated DM-sibships per 10(6) people of each community was used as an estimate of the transition rate from stable to unstable DMPK-(CTG)(n) alleles assuming that each transition is a beginning of a new DM sibship. This study indicated that the difference in the incidence of DM is a result of higher mutation rate in the non-Ashkenazi Jews (>50/10(6)) as compared to the rate in the Ashkenazi Jews (16.3/10(6)). The intragenic haplotype of the DM alleles was the same as that of the DM in many populations all over the world. However, two DM closely linked markers D19S207 and D19S112 were in linkage disequilibrium with the DM mutation in patients of Yemenite and Moroccan (the largest subgroup in the Sephardim Jews) extractions and not in the Ashkenazi patients. This observation indicated a common ancestral origin for the DM premutation in patients of the same ethnic origin. We concluded that the difference in the prevalence of DM among the Jewish communities is a consequence of founder premutations in the non-Ashkenazi Jewish communities.

Mucolipidosis III type C: first-trimester biochemical and molecular prenatal diagnosis

OBJECTIVES

Mucolipidosis IIIC (MLIIIC) is a rare autosomal recessive lysosomal storage disease resulting from defective mannose 6-phosphate-dependent lysosomal enzyme trafficking; mutations of the gamma subunit of N-acetylglucosamine-1 phosphotransferase (GINAcPT) were recently found to cause its pathogenesis. We report here for the first time prenatal diagnosis (PND) for MLIIIC by means of chorionic villous sampling (CVS).

METHODS AND RESULTS

A fetus in a large Bedouin-Moslem family was found to be homozygous for the founder haplotype and the mutational SSCP pattern of MLIIIC. The diagnosis was confirmed by markedly reduced lysosomal enzyme activities in cultured chorionic villi. The molecular identification of the disease-causing mutation in this large Bedouin-Moslem kindred permitted, for the first time, identification of carriers and couples at risk.

CONCLUSIONS

The feasibility of early PND for a progressive disabling disease is important for its prevention. Nevertheless, the feasibility of PND raises a serious dilemma since affected individuals might have a variable phenotype and the disease is progressive and non-lethal. In addition, religious and social constraints are important factors to be taken into consideration in the genetic counseling of couples at risk.

Uniparental disomy in fetuses diagnosed with balanced Robertsonian translocations: risk estimate

Forty-two fetuses with non-homologous Robertsonian translocations were analyzed for uniparental disomy (UPD). One fetus with a de novo translocation t(13q;14q) had maternal isodisomy of chromosome 14. In a summary of the published data (including the present study), 315 cases were analyzed for UPD after prenatal diagnosis of balanced Robertsonian translocations, of these two fetuses had UPD, giving a risk estimate of 0.65% (CI 0.2-2.3). This risk justifies the recommendation of UPD analysis in fetuses diagnosed prenatally with Robertsonian translocations, with the emphasis on the chromosomes known to contain imprinted genes, such as 14 and 15. We also discuss the possibility of UPD in offspring of Robertsonian translocation carriers with normal karyotype. Based on the risk for UPD in fetuses with Robertsonian translocation we suggest to test these fetuses for UPD and to do so on amniocytes rather than chorionic villi when the risk for unbalanced karyotype is approximately 1%, comparable to the risk for UPD.

High level of microsatellite instability but not hypermethylation of mismatch repair genes in therapy-related and secondary acute myeloid leukaemia and myelodysplastic syndrome

Microsatellite instability (MSI) is associated with defects in the DNA mismatch repair (MMR) system, such as mutation or epigenetic silencing of the genes by promoter hypermethylation. We investigated the presence of MSI and promoter hypermethylation of hMLH1 and hMSH2 genes in 82 patients (68 acute myeloid leukaemia, AML; 14 myelodysplastic syndromes, MDS). Twelve separate microsatellite loci, including three mononucleotide repeat markers, were used. Mutator phenotype (RER+) was detected in 20 AML (29.4%) and 3 MDS (21.4%) patients. RER+ rate was much higher in the therapy-related and secondary cases compared with the de novo cases. Three out of 7 (42.9%) secondary (s-AML) and 8 out of 17 (47.1%) therapy-related (t-AML) showed RER+ in comparison with 9 out of 44 (20.5%) de novo cases. Similar rates were detected in MDS patients (2/2 therapy-related and 1/12 de novo). The promoter hypermethylation was found in three hMLH1 (3.7%) and two hMSH2 (2.4%) genes. All these five patients had AML and were older than 60 years of age. Two of them had s-AML and one had t-AML. RER+ was detected in three of these five patients. Our data suggest that genetic instability is associated with AML and MDS, especially t-AML and s-AML. In addition, our results indicate that the hMSH2 and hMLH1 promoter hypermethylation is not a common event in these malignancies, but may play a role in the development of AML in elderly patients.

Wiedemann-Beckwith syndrome: further prenatal characterization of the condition

We describe three unrelated cases of Wiedemann-Beckwith syndrome (WBS). Two of them were diagnosed postnatally while the third was detected during pregnancy that resulted in elective termination. Amniotic karyotypes were normal in all. PCR amplification of polymorphic loci mapping to 11p15.5 region documented partial trisomy of 11p15.5 due to paternal translocation in one, and segmental and mosaic segmental unipaternal disomy (UPD) in the second and third cases, respectively. Based on findings documented in these cases and the literature, we tabulated the anomalies that might be detected prenatally by ultrasound and that may suggest the syndrome. Constant findings included fetal overgrowth, polyhydramios, enlarged placenta, and specifically a distended abdomen. As most described signs developed after 22 weeks of gestation, a careful follow-up should be carried on until late stages of pregnancy. An amniotic karyotype might not detect subtle chromosomal rearrangements. We therefore recommend utilizing PCR of polymorphic loci on 11p15.5, in addition to conventional cytogenetic analysis of the fetus and both parents to detect possible maternal deletions or inversions, paternal duplications, and UPD that may account for the largest subset of sporadic WBS reaching 25% of cases. An early diagnosis of WBS is important for counseling the parents concerning potential risk for developing embryonic tumors, selection of the mode of delivery due to potential adrenal cysts that might bleed during labor, and prevention of neonatal hypoglycemia.

Molecular characterization of four alpha-1-antitrypsin variant alleles found in a Japanese population: a mutation hot spot at the codon for amino acid 362

In this study alpha-1-antitrypsin (AAT) phenotypes at the protease inhibitor (PI) locus were determined by isoelectric focusing of native and desialylated serum samples from 236 Japanese subjects living in the western part of Japan. The shifts in relative mobility between some PI types were observed before and after desialylation. This technique was useful in distinguishing between some PI M subtypes and variants. The molecular basis of four variant alleles, including two new alleles found in this study, was characterized: PI E(tokyo) [Lys(335)(AAG)--> Glu(GAG)] and PI N(nagato) [Leu(276)(CTG)-->Pro(CCG)] arose from PI M1(Val(213)) and PI M2, respectively. A new PI P(yonago) [Asp(19)(GAT)-->Ala(GCT)] originated from PI M1(Val(213)). A new PI M5(gunma) [Pro(362)(CCC)-->Ser(TCC)], arising from PI M3, was the sixth allele involving a mutation at codon 362, which is suggested to be a mutation hot spot. PI M5(gunma) was likely to show normal AAT levels and function although the mutations occurred near codon 358 for Met(358). The molecular basis of PI variant alleles found in Japanese was different from that reported in previous studies.

SSCP analysis by RT-PCR for the prenatal diagnosis of Niemann-Pick disease type C

The molecular prenatal diagnosis of Niemann-Pick disease type C (NPC) is presented. The proband with a late infantile type of NPC was a compound heterozygote of a paternal missense mutation, T529G, and a maternal 2 bp deletion at nt 350 of the NPC1 gene. These mutations were detected by single-strand conformation polymorphism (SSCP) analysis of RT-PCR products. When the proband was aged 4 years 3 months, prenatal diagnosis for the second child was performed using both biochemical and molecular methods. SSCP analysis for the parental mutations using cDNA from cultured amniotic fluid cells revealed the absence of both mutations and the fetus was diagnosed as being unaffected. This diagnosis was supported by a normal level of cholesterol esterification using cultured amniotic fluid cells. After the child's birth, when he was 21 months old, the diagnosis was confirmed by SSCP analysis of genomic DNAs of his family. This analysis also revealed a unique variation of intron 13, IVS13+753-758 del TTTTTT, that was shared only by the proband and the father, and was suspected as being linked to the T529G missense mutation. A combination of both biochemical and molecular analyses is very useful and reliable for prenatal diagnosis of Niemann-Pick disease type C.

The 28-kb deletion spanning D15S63 is a polymorphic variant in the Ashkenazi Jewish population

D15S63 is one of the loci, on chromosome 15q11-q13, that exhibit parent-of-origin dependent methylation and that is commonly used in the diagnosis of Prader-Willi or Angelman syndromes (PWS/AS). A 28-kb deletion spanning the D15S63 locus was identified in five unrelated patients; in each of them the deletion was inherited from a normal parent. Three of the five families segregating the deletion were reported to be of Jewish Ashkenazi ancestry, and in the other two families the ancestral origin was unknown. To determine whether the 28-kb deletion is a benign variant, we screened for the deletion in 137 unselected Ashkenazi individuals and in 268 patients who were referred for molecular diagnosis of PWS/AS, of whom 89 were Ashkenazi and 47 were of mixed origin (Ashkenazi and non-Ashkenazi Jews). In the control group, three individuals were carriers of the deletion; among the patients, three were carriers, all of whom were Ashkenazi Jews. There was no significant difference between the control group and the Ashkenazi patients, indicating that the deletion is not a cause of PWS- and AS-like syndromes. The frequency of the 28-kb deletion in the Ashkenazi population was 1/75. Since methylation analysis at the D15S63 locus may lead to misdiagnosis, we suggest the use of SNRPN, either in a PCR-based assay or as a probe in Southern hybridization, as the method of choice in the diagnosis of PWS/AS.

Contribution of connexin 26 mutations to nonsyndromic deafness in Ashkenazi patients and the variable phenotypic effect of the mutation 167delT

Twenty-seven unrelated Jewish Ashkenazi patients with nonsyndromic prelingual deafness (NSD) were analyzed for mutations in the coding sequence of the connexin 26 (Cx26) gene. Biallelic mutations were identified in 19 of the 27 patients (70.4%); 12 were homozygous for the mutation 167delT, 2 were homozygous for the mutation 35delG, and 5 were compound 167delT/35delG heterozygotes. In addition three patients were heterozygous with no second identified mutation in the Cx26 gene. Biallelic mutations in the Cx26 gene account for 83% of familial cases and 44% of the sporadic cases. Among 268 unselected Ashkenazi individuals, 20 were 167delT/N heterozygotes, giving an estimate of 7.5% carrier frequency. Based on the 167delT carrier frequency in three studies (including the present one), it is expected that 167delT/167delT homozygotes account for 70% of all patients with NSD (1 in 1300). The hearing capacity of 30 patients (probands and their sibs) with biallelic Cx26 mutations and at least one allele with 167delT demonstrated inter- and intrafamilial variability from profound to mild hearing impairment.

Diversity of alpha-globin mutations and clinical presentation of alpha-thalassemia in Israel

Alpha-thalassemia is among the world's most common single gene disorders, caused primarily by gene deletions. In Israel, where alpha(o)-trait thalassemia is uncommon, it is of particular importance because of its phenotypic interactions with beta-thalassemia in hetero- and homozygotes. In a study of 232 individuals referred for molecular evaluation of anemia, 303 chromosomes carried alpha-globin gene abnormalities; 6 gene rearrangements and 11 point mutations were identified. This unexpected heterogeneity is in part due to the many ethnic subgroups represented by these patients. Our findings include nine unique Israeli alleles, 3 of which are described here for the first time. An equal number of point mutations was found in the alpha2-globin gene as compared to alpha1. A threonine deletion in codon 39 of the alpha1-globin gene, found frequently in Arabs, is unique to Israel and probably represents one of several indigenous alleles. Among Arabs, point mutations were more frequent than large deletions. Surprisingly, in Ashkenazi Jews, who resided for many centuries in a nonmalarial environment, a single alpha-globin gene deletion -alpha(3.7) was found in many cases. The clinical presentation of individuals carrying two or more alpha-globin lesions was highly variable. In general, the severity correlated inversely with the number of functional alpha-globin genes. In some cases, impairment of two alpha-globin genes by point mutations led to a thalassemia-intermedia-like picture which could be misdiagnosed as beta-thalassemia. We conclude that alpha-thalassemia is phenotypically and genotypically more heterogeneous than previously recognized. DNA analysis is invaluable as it provides a specific diagnosis and enables reliable genetic counseling.

Loss of heterozygosity of the retinoblastoma gene in liver cirrhosis accompanying hepatocellular carcinoma

Carcinogenesis is a multistep process. Most hepatocellular carcinoma (HCC) is preceded by liver cirrhosis, but the genetic changes involved in cirrhosis are not known well. The present study was conducted to evaluate aberration of the retinoblastoma (RB) gene in HCC and adjacent non-tumorous liver using 22 patients with chronic liver damage accompanying HCC. The specimens obtained by microdissection from paraffin-embedded tissues were analyzed using an assay based on the polymerase chain reaction for highly polymorphic nucleotide sequences of microsatellites in the RB gene. Out of 22 cases, 15 showed constitutional heterozygosity for the microsatellite markers. In 11 (73.3%) of these 15 informative cases, the primary HCC foci showed loss of heterozygosity (LOH). In 8 of these 11 doubly informative (informative and LOH-positive in primary HCC) cases, LOH was found in 20 (64.5%) of 31 microdissected non-tumorous foci. All of the non-tumorous foci showing RB loss were cirrhotic lesions but there were no foci of chronic hepatitis. The remaining 4 cases without LOH in HCC foci showed no LOH in non-tumorous lesions. In our study, LOH of the RB gene was frequently observed in liver cirrhosis surrounding tumor.

Clinical features and molecular genetic analysis of a boy with Prader-Willi syndrome caused by an imprinting defect

Prader-Willi syndrome (PWS) is a neuroendocrine disorder caused by a non-functioning paternally derived gene(s) within the chromosome region 15q11-q13. Most cases result from microscopically visible deletions of paternal origin, or maternal uniparental disomy of chromosome 15. In both instances no recurrence has been reported. In rare cases, PWS is associated with lack of gene expression from the paternal allele due to an imprinting defect. We report the clinical features and the molecular genetic analysis of the first Danish child with PWS due to a defect of the putative imprinting centre (IC). When the imprinting mutation is inherited from a carrier father, the risk that future children will be affected is theoretically 50%. It is therefore important that these families are referred to a geneticist for counselling and further investigation. Prenatal diagnosis is currently only feasible when the mutation has been identified in the affected child.

Familial translocation t(Y;15)(q12;p11) and de novo deletion of the Prader-Willi syndrome (PWS) critical region on 15q11-q13

We describe a 17-year-old girl with mild Prader-Willi syndrome (PWS) due to 15q11-q13 deletion. The deletion occurred on a paternal chromosome 15 already involved in a translocation, t(Y;15)(q12;p11), the latter being present in five other, phenotypically normal individuals in three generations. This appears to be the first case of PWS in which the causative 15q11-q13 deletion occurred on a chromosome involved in a familial translocation, but with breakpoints considerably distal to those of the familial rearrangement. The translocation could predispose to additional rearrangements occurring during meiosis and/or mitosis or, alternatively, the association of two cytogenetic anomalies on the same chromosome could be fortuitous.

Bilateral non-communicating paraurethral meatus in Prader-Willi syndrome

Prader-Willi syndrome (PWS) is a complex multisystemic congenital disorder due to an interstitial deletion of chromosome 15q11-13 or to maternal uniparental disomy. Molecular genetic testing is complex, and often requires DNA from both parents, which is not always available. An accurate medical history and presenting clinical signs are frequently the only tools for the clinical diagnosis of this syndrome, therefore it is important to have complete and accurate criteria. The presence of a bilateral non-communicating paraurethral meatus in a 9-year-old female patient affected by PWS, previously unreported in the literature, should induce clinicians to look for this sign when examining such patients.

Trisomy 2: confined placental mosaicism in a fetus with intrauterine growth retardation

In a pregnancy that was monitored due to increased risk for Down syndrome in the triple test, a normal karyotype was found in amniocentesis. Follow-up by serial ultrasound examinations revealed intrauterine growth retardation (IUGR) at 20 weeks of gestation. The parents decided to terminate the pregnancy and the karyotype of the placental fibroblasts was 47,XX,+2. Analysis of polymorphic markers of chromosome 2 demonstrated (a) that trisomy 2 was confined to the placenta (CPM), (b) that the trisomy 2 cell line was a result of a meiotic I error of paternal origin, and (c) that the fetal tissues with a normal karyotype were biparental disomy 2.

t(15;21)(q15;q22.1) pat resulting in partial trisomy and partial monosomy of chromosomes 15 and 21 in two offspring

Two sibs, carriers of unbalanced products of the translocation t(15;21)(q15;q22.1) pat, are described. The sister had Prader-Willi syndrome due to deletion 15 (pter > q15) and partial trisomy 21 (pter > q22.1); her brother had partial trisomy 15 (pter > q15) and partial monosomy 21 (pter > q22.1). The translocation breakpoint on chromosome 21 was located proximal to the SOD1 gene, within a region of 4.0 cM (2.3 Mb) between the loci D21S217 and D21S213. The correlations between the clinical presentation and the molecular findings of the two sibs are discussed in relation to other patients with partial trisomy and monosomy 21.

Comparison of phenotype in uniparental disomy and deletion Prader-Willi syndrome: sex specific differences

Prader-Willi syndrome (PWS) results primarily from either a paternal deletion of 15q11-q13 or maternal uniparental disomy (UPD) 15. Birth parameters and clinical presentation of 79 confirmed UPD cases and 43 deletion patients were compared in order to test whether any manifestations differ between the two groups. There were no major clinical differences between the two classes analyzed as a whole, other than the presence of hypopigmentation predominantly in the deletion group. However, there was a significant bias in sex-ratio (P < .001) limited to the UPD group with a predominance (68%) of males. An equal number of males and females was observed in the deletion group. When analyzed by sex, several significant differences between the UPD and deletion groups were observed. Female UPD patients were found to be less severely affected than female deletion patients in terms of length of gavage feeding and a later onset of hyperphagia. Although these traits are likely to be influenced by external factors, they may reflect a milder presentation of female UPD patients which could explain the observed sex bias by causing under-ascertainment of female UPD. Alternatively, there may be an effect of sex on either early trisomy 15 survival or the probability of somatic loss of a chromosome from a trisomic conceptus.

Frequent loss in chromosome 8p loci in liver cirrhosis accompanying hepatocellular carcinoma

Most hepatocellular carcinoma (HCC) is preceded by liver cirrhosis, but the genetic changes involved in cirrhosis are not well understood. We therefore studied loss of heterozygosity (LOH) in cirrhotic and neoplastic foci in livers of 14 patients with HCC. The samples, microdissected from paraffin-embedded tissues, were analyzed using a polymerase-chain-reaction-based assay for dinucleotide repeat polymorphisms on 8p. Of the 14 cases, 13 showed constitutional heterozygosity for the microsatellite markers. In 7 (54%) of these 13 informative cases, LOH was detected in the primary HCC and, in these 7 doubly informative (informative and LOH-positive in primary HCC) cases, LOH was found in 16 (70%) of 23 liver cirrhotic foci. The pattern of 8p allelic loss was identical in each doubly informative tumor; however, some of the liver cirrhotic foci harbored an 8p loss identical to that seen in the primary HCC, some harbored a different 8p loss, and some did not harbor any 8p loss. The remaining 6 cases without LOH on 8p in HCC showed no 8p loss in any cirrhotic foci. Presumably HCC could develop from cirrhotic cells harboring 8p loss.

Cytogenetic and age-dependent risk factors associated with uniparental disomy 15

Prader-Willi syndrome (PWS) results primarily from either a paternal deletion of 15q11-q13 or maternal uniparental disomy (UPD) of chromosome 15. Including the present and published cases, more than 120 patients with maternal UPD of human chromosome 15 have been ascertained. Investigation of chromosome 15 markers indicates that approximately 71 per cent of the additional maternal chromosomes were the result of meiosis I segregation errors, 13 per cent were the result of meiosis II errors, and 16 per cent resulted from post-zygotic duplication of one chromosome 15. An increase in maternal age is associated with UPD cases due to meiotic errors. The age-specific risk for UPD(15) is analysed and shows an exponential increase with maternal age which is similar to that observed for trisomy 21. For women greater than or equal to 40 years of age, the risk for UPD(15) is approximately 1/3400 livebirths. The frequency of chromosome aberrations associated with UPD(15) is also discussed. Two types of aberrations are at significantly increased risk of fetal UPD(15): de novo (or inherited) isochromosome 15 and confined placental mosaicism for trisomy 15. Two additional abnormalities, de novo small marker chromosomes derived from 15, e.g., idic15(pter-q11:q11-pter), and familial Robertsonian translocations involving chromosome 15, appear to have a mildly increased risk of UPD(15).

Localization of two X-linked mental retardation (XLMR) genes to Xp: MRX37 gene at Xp22.31-p22.32 and a putative MRX gene on Xp22.11-p22.2

MRX genes of 2 families with X-linked mental retardation (XLMR) were localized by linkage analysis. In family A, the gene was mapped to Xp22.31-p22.32, with significant LOD scores to various Xp22 markers within a distance of 6 Mb between DXS1223 and DXS1224. The MRX gene of this family was designated MRX37. In a mentally retarded female who is a carrier of the MRX37 gene, a random pattern of X inactivation was demonstrated. In family B, a positive LOD score, although not significant (< + 2), was found with the marker DXS1202 at Xp22.11-p22.2.

Why is there no diploid overdose effect in Prader-Willi syndrome due to uniparental disomy?

Due to DNA technology, it is now apparent that the mechanisms of genetic disease are more complex than the model of a gene with biallelic expression in the diploid state. If a gene is imprinted, monoallelic expression is the norm when the chromosomes of a pair are inherited normally from each parent. Uniparental disomy (UPD) is the abnormal situation where both chromosomes of a pair come from the same parent. When the chromosome contains an imprinted gene, UPD may result in nullisomy or disomy for a functional copy of that gene. If there are two imprinted loci on the same chromosome, UPD for that chromosome results in nullisomy for one imprinted gene but functional disomy for the other a "diploid overdose" (DO). This situation has been well demonstrated in the Prader-Willi syndrome (PWS) which is the nullisomic phenotype for the PWS gene(s) on chromosome 15q11-13. Chromosome 15q11-13 also contains the gene for Angelman syndrome (AS) which has a phenotype distinct from PWS. Both loci are subject to imprinting--in PWS, the imprint is on the maternal chromosome 15, in AS it is on the paternal chromosome 15. All individuals with PWS due to maternal UPD, while functionally nullisomic for the PWS locus, are functionally disomic for the AS locus--a DO situation. Assuming that biallelic expression of an imprinted gene is harmful, one would expect DO for an imprinted gene to produce a phenotypic effect. Cases of PWS due to UPD do not appear to differ from those due to deletion (hypopigmentation in deletional cases can be explained by loss of D15S12 downstream from the critical region). There is no good evidence of DO for the AS locus in PWS due to UPD. Why then was it 'necessary' in evolutionary terms to imprint the AS locus and maintain the imprint faithfully for life. A similar situation of two imprinted genes on the same chromosome occurs with IGF2 and H19 on chromosome 11p15. Maternal imprinting for IGF2 and paternal imprinting for H19 is the norm. Paternal UPD in this situation does lead to a DO effect, namely Beckwith-Wiedemann syndrome. The possibility of a DO effect needs to be considered when assessing the phenotypic spectrum of UPD for other chromosomes currently under investigation.

Predominant 45,X,--Y karyotype in donor cells after allogeneic BMT: cytogenetic and molecular analysis

We describe two women; one (patient 1) with the diagnosis of acute myeloblastic leukemia (AML), the second (patient 2) with myelodysplastic syndrome (MDS). Both patients underwent allogeneic bone marrow transplantation (BMT), from their HLA-matched brothers. Cytogenetic analysis after the BMT revealed a chromosomal mosaicism in both patients, with the karyotype 46,XX/45,X with no sign of the Y chromosome. The origin of the clone with monosomy X was determined using cytogenetic analysis including heteromorphism and segregation of DNA polymorphic markers. The results led us to the conclusion that in both patients the origin of the 45,X clone was that of the donors. Patient 1 had MDS-like syndrome after the BMT and was stabilized in the chimeric state; to date she is doing well. Patient II also had MDS. However, in her case, it was her primary disease. The graft in patient II was rejected and she died 6 months after BMT.

Comparison of high resolution cytogenetics, fluorescence in situ hybridisation, and DNA studies to validate the diagnosis of Prader-Willi and Angelman's syndromes

Eighty seven referrals with Prader-Willi syndrome and 49 with Angelman's syndrome were studied. High resolution cytogenetics was performed on all probands. Molecular studies, performed on the proband and both parents in each case, utilised multiple probes from within and distal to the 15(q11-13) region in order to establish the presence of DNA deletion or uniparental disomy. In addition, FISH, with probes at D15S11 and GABR beta 3 from the Prader-Willi syndrome/Angelman's syndrome region, was performed on a subset of 25 of these patients. In the referral group with Prader-Willi syndrome, 62 patients had a normal karyotype and 25 were deleted on high resolution cytogenetics. Twenty nine were found to be deleted with DNA techniques. In the Angelman's syndrome group, 37 had a normal karyotype and 12 were deleted on high resolution cytogenetics while 26 were deleted on molecular studies. The diagnosis was reassessed in 35 referrals with Prader-Willi syndrome and 11 with Angelman's syndrome following a non-deleted, non-disomic result. Of individuals who were neither deleted nor disomic on DNA studies, a false positive rate of 11.4% (4/35) for Prader-Willi syndrome and 16.7% (2/12) for Angelman's syndrome was found for a cytogenetically detected deletion. The false negative rate for deletion detected on high resolution cytogenetics was 19.5% (12/62) for Prader-Willi syndrome and 35% (13/37) for Angelman's syndrome. Thus high resolution cytogenetics was shown to be unreliable for deletion detection and should not be used alone to diagnose either syndrome. There were no discrepancies with FISH in 25 cases when FISH was compared with the DNA results, indicating that FISH can be used reliably for deletion detection in both syndromes.

DNA methylation based testing of 450 patients suspected of having Prader-Willi syndrome

Using a test based on parent of origin specific DNA methylation at the D15S63 (PW71) locus, we studied 385 patients (aged 1 to 36 years) for diagnostic confirmation of Prader-Willi syndrome (PWS) and 65 infants (aged 0 to 12 months) with severe hypotonia of unknown cause. Fifty eight of 385 patients were examined personally; 28/58 patients had PWS and lacked the paternal PW71 band and 30/58 patients, who did not have PWS, had a normal methylation pattern. In five of these patients, a differential diagnosis was made (Ohdo-like blepharophimosis syndrome, Alstrøm syndrome, Cohen syndrome, Bardet-Biedl syndrome, and pseudohypoparathyroidism). A total of 327/385 blood samples was sent to us from outside. The test confirmed the diagnosis of PWS in 112/327 patients. Most of the other 215 patients lacked the major diagnostic criteria such as neonatal hypotonia, feeding problems, characteristic facies, and hypogenitalism. On the other hand, 29/65 hypotonic infants tested positive for PWS. We conclude that the PW71 methylation test detects most, if not all, patients with typical PWS and that PWS is often not recognised in infants and wrongly suspected in obese and mentally retarded patients.

Paracentric inversion X(q21.2q24) associated with mental retardation in males and normal ovarian function in females

We describe a familial paracentric inversion (X)(q21.2 q24) in a family with 2 male and 2 female carriers. The males were mentally retarded and the females were normal with normal ovarian function. It is suggested that a recessive mental retardation (MR) gene was disrupted by one of the inversion breakpoints, although an X-linked MR gene which by chance is linked to the inv(X) could not be ruled out. In the female carriers of the paracentric inversion a random X-inactivation was demonstrated. The normal ovarian function is an exception to the concept of "critical region" at Xq13-q26.

Neurofibromatosis type I (NFI) in Israeli families: linkage analysis as a diagnostic tool

Linkage analysis of 18 neurofibromatosis type I (NFI) families was performed using intragenic and flanking polymorphic markers. The aims of the analysis were prenatal diagnosis of at-risk fetuses, and of asymptomatic individuals who were relatives of NFI patients. Prenatal diagnosis was performed in 9 pregnancies of 7 families; 5 fetuses were diagnosed as affected. In 6 families with an affected spouse, the request was to identify informative polymorphisms to be used in future pregnancies. Presymptomatic diagnosis was performed in 4 families. One individual, a brother of an NFI patient, was found to have Lisch nodules as the only NFI symptom. Linkage analysis indicated that if this person is a carrier of the NFI gene, he must be a product of intragenic crossover. In 2 individuals with a new NFI mutation, the origin of the NFI-bearing chromosomes was paternal. The same observation was noted by others. A summary of published cases shows that some 90% of the NFI-bearing chromosomes of patients with new mutations were of paternal origin. We therefore suggest that for the purpose of prenatal diagnosis in carriers of NFI new (and unidentified) mutations, the paternal chromosome will be considered as the NFI-bearing chromosome.