Achondroplasia in Sweden caused by the G1138A mutation in FGFR3

Achondroplasia, an autosomal dominant inherited disorder, is one of the most common forms of skeletal dysplasia resulting in disproportionate extreme shortness. Recently, two point mutations, both affecting nucleotide 1138 in the fibroblast growth factor receptor type 3 (FGFR3) gene, were found to be the cause of the disorder. We investigated DNA from 16 Swedish patients with achondroplasia for the presence of these mutations. All patients were found to be heterozygous for the G to A transition at nucleotide 1138. Our data thus support previous reports showing a striking genetic homogeneity, in that almost all achondroplasia patients have the FGFR3 G380R mutation at the protein level.

No evidence of WT1 gene mutations in children with congenital diaphragmatic hernia

Children with congenital diaphragmatic hernia are born with a defect of the diaphragm that usually leads to a herniation of abdominal organs up into the thoracic cavity on the same side. The condition is often fatal, usually because of concomitant lung hypoplasia. The cause of this condition is unknown. The majority of cases are sporadic, but familial aggregation has been observed, indicating a genetic background. The tumor suppressor gene WT1 is involved in normal early urogenital development, and it is expressed in the mesothelium in the early human fetus. Recently, knockout mice homozygous for WT1 gene mutations were constructed. The embryos died between days 13 and 15 of gestation. Besides serious urogenital and thoracic malformations, the mice had defects in the diaphragm that caused herniation of lung tissue into the abdomen. These findings prompted the authors to screen for WT1 gene mutations in 27 children who had congenital diaphragmatic hernia. Using exon-per-exon polymerase chain reaction (PCR) amplifications and denaturing gradient gel electrophoresis, no WT1 mutations were detected. Southern blot analysis did not show any large rearrangements in the WT1 gene. These results exclude WT1 gene mutations as a major etiological factor for the isolated diaphragmatic defect. However, it is possible that other genes in the WT1 pathway are involved in this defect.

Lack of evidence of permanent engraftment after in utero fetal stem cell transplantation in congenital hemoglobinopathies

The use of fetal hematopoietic stem cells for in utero transplantation to create permanent hematochimerism represents a new concept in fetal therapy. In one fetus with alpha-thalassemia, one with sickle cell anemia, and one with beta-thalassemia, we have transplanted fetal liver cells obtained from legal abortions in gestational weeks 6-11. The fetus with alpha-thalassemia was transplanted twice during pregnancy, in the 15th (20.4 x 10(8) cells/kg) and in the 31st weeks of gestation (1.2 x 10(8) cells/kg), and is now two years of age. One fetus with sickle cell anemia received its transplant in the 13th week of gestation (16.7 x 10(8) cells/kg), and is now one year old. The fetus with beta-thalassemia was transplanted in 18th week (8.6 x 10(8) cells/kg), and is now three months old. Engraftment was evaluated by chromosomal analysis (sex chromosomes), red cell phenotyping, HLA class I and II typing, and PCR (polymerase chain reaction) for Y chromosome-specific sequences and DNA polymorphisms in cord and peripheral blood. The children with alpha- and beta-thalassemia underwent bone marrow aspirations at 3 and 7 months of age, respectively. In neither of these cases were we able to detect convincing evidence of stem cell engraftment. Thus, the administration of fetal stem cells to fetal recipients after the 12th week of gestation did not result in permanent hematochimerism. It remains to be determined whether the engraftment process can be promoted by earlier transplantations and/or higher cell doses.

Diagnosis of the Prader-Willi syndrome by proving the absence of the unmethylated PW71 DNA fragment

The Prader-Willi syndrome (PWS) is a genetic disorder which is difficult to diagnose from clinical symptoms in newborns and young children. However, it is known that in PWS a fragment within the q11-13 region of the paternally derived chromosome 15 is deleted. Recently it has been observed that the D15S63 (PW71) locus in chromosome 15q11-13 is methylated on the maternally derived chromosome, but unmethylated on the paternally derived chromosome. Based on this observation a rapid diagnostic test (the PW71 methylation test) using methylation-sensitive restriction enzymes has been developed for patients presumed to have PWS. We have studied 56 patients; 30 patients with classical features of PWS and 26 patients with only psychomotor retardation and obesity, referred to us from different part of Sweden. Twenty-nine of the 30 classical PWS patients were found to have an absence of the unmethylated paternally derived PW71(D15S63) locus in chromosome 15q11-13. None of the patients with only obesity and psychomotor retardation had this "absence" pattern on chromosome 15q11-13. Using the PW71 methylation test on patients with PWS, a concordance of 96% was found. The PW71 methylation test is presently the method of choice for rapid diagnostic testing of patients suspected of having PWS.

Parental exposure to hydrocarbons in Prader-Willi syndrome

The purpose of this study was to ascertain whether parental age and parental pre-conceptional exposure to various agents differentiated children with Prader-Willi syndrome (PWS) from obese children without PWS. Two groups of patients under 25 years of age were studied, 15 persons with PWS, and 13 with PWS-like symptoms. In the PWS group deletions were detected on chromosome 15q11-q13 and parents were older. The parents' occupational and recreational exposure to environmental toxins was recorded and correlated to the clinical diagnosis, genetics and behaviour characteristics. Paternal exposure to gasoline/petrol was significantly higher in the PWS group and is suggested as a possible important factor in the aetiology of PWS.

Four mutations in the porphobilinogen deaminase gene in patients with acute intermittent porphyria

We have detected four different mutations in the porphobilinogen deaminase (PBGD) gene in acute intermittent porphyria (AIP) families from England, Norway, and Sweden. A splicing mutation in the first position of intron 8 (Int8 + 1) was found in a family from England and a missense mutation in exon 12 (Glu250) was detected in a Norwegian family. Two mutations were identified in Swedish families, one splicing mutation in the first position of intron 3 (Int3 + 1) and one missense mutation in exon 8 (Pro119).

Constitutional and somatic mutations in the WT1 gene in Wilms' tumor patients

Wilms' tumor development, like most tumors, involves multiple genetic alterations affecting diverse genes. Only one of these has thus far been identified, the Wilms' tumor 1 (WT1) gene on 11p13, which functions as a tumor suppressor gene. We assessed the involvement of the WT1 gene constitutionally and somatically in 26 Wilms' tumor patients. Of these, the clinical data suggest a constitutional pre-disposition in 12 bilateral cases and 7 cases with early onset. We employed Southern blot analysis and PCR-based markers for analyses of somatic allelic losses in chromosome bands 11p13, 11p15 and 16q and screened for point mutations in exons 2-10 of the WT1 gene with denaturing gradient gel electrophoresis (DGGE). Of the 12 cases with multiple tumors, 1 exhibited a constitutional 11p13 deletion and a somatic stop mutation in exon 4 of the WT1 gene and 2 harbored constitutional mutations in the WT1 gene: a pre-mature stop codon in exon 6 in a boy with bilateral cryptorchidism and bilateral Wilms' tumors and an intragenic deletion in a girl with bilateral WT. Three additional bilateral tumors displayed WT1 rearrangements or allelic losses with 11p13 markers. Four of 7 patients with an early onset of unilateral tumor had losses of 11p13, though no WT1 mutations were detected. Two of the remaining cases that did not show any somatic or constitutional 11p13 alterations had Beckwith-Wiedemann syndrome, known to involve the 11p15 region.

Effects of the mutant von Willebrand factor gene in von Willebrand disease

Von Willebrand disease (vWD) is a common inherited bleeding disorder in humans, and can be divided into a mild (type 1) and severe (type 3) form. Previous linkage studies identified one subject with vWD type 1 who transmitted different alleles of the von Willebrand factor (vWF) gene to his two affected children, one having vWD type 3 and the other having type 1. By screening the promoter and coding sequence (52 exons) of the vWF gene, three missense mutations were detected in this family. The type 1 individual who transmitted different alleles of the gene to his two sick children carries two substitutions, one in exon 5 and the other in exon 18 on the respective alleles. The relationship between the genotype (mutations) and the phenotype in this family is complex. In order further to correlate the relationship in vWD type 1 individuals, fifty-five subjects who carry one null allele of the vWF gene were collected. All these subjects are from vWD type 3 families with known mutations. Biochemical data of these 55 subjects indicate that gene dosage and other factors, such as blood group, age, and environment factors, play a critical role in the development of the phenotype of the disease.

Absence of mutations in the WT1 gene in patients with XY gonadal dysgenesis

The WT1 gene is normally expressed during gonadal development and specific mutations in heterozygous form cause Drash syndrome, characterized by male pseudohermaphroditism and gonadal dysgenesis, renal failure and a predisposition for Wilms' tumour. These observations prompted us to test whether WT1 mutations are involved in isolated gonadal dysgenesis, being the most severe form of disturbance in gonadal differentiation. We studied 27 cases of 46,XY females with gonadal dysgenesis who had previously been screened for and found not to carry SRY gene mutations. We performed mutational screening of the WT1 gene with denaturing gradient gel electrophoresis. In one of these patients, a heterozygous point mutation in exon 8 was found. This mutation has previously been described in Drash syndrome and re-evaluation of the clinical data confirmed this diagnosis. Based on these results, we conclude that isolated gonadal dysgenesis is not caused by mutations in the WT1 gene.

Current status of genetic research in Rett syndrome

Rett syndrome is a progressive neurological development syndrome. Evidences for a genetic transmission, but no conclusive data which entirely support the X chromosome involvement, exist. The chromosomal region 11p14-pte has been investigated with polymorphic markers and genes in the region have been sequenced. No sequence divergencies were detected.

Diagnosis of acute intermittent porphyria in northern Sweden: an evaluation of mutation analysis and biochemical methods

OBJECTIVE

To validate the use of a recently observed guanine to adenine mutation in exon 10 in the porphobilinogen deaminase (PBGD) gene as a diagnostic marker of acute intermittent porphyria (AIP). To evaluate the efficiency of the traditional biochemical diagnostic methods.

DESIGN

Matched and blinded case-control study (1:4).

SETTING

A primary health care centre in Arjeplog, the National Porphyria Research Unit and a department of clinical genetics in Stockholm.

SUBJECTS

A total of 48/49 (98%) patients over the age of 15 years living in Arjeplog with AIP, diagnosed according to standard clinical and biochemical criteria. For each AIP patient, four controls were matched for age, sex and geographical area and 164/196 (86%) participated. In the validity study, 35 patients were selected as indisputable AIP gene carriers, according to strict biochemical criteria, and 92 matched controls were selected with strict exclusion criteria.

MAIN OUTCOME MEASURES

Validity, specificity and sensitivity of DNA diagnosis for this AIP mutation. Specificity and sensitivity of traditional biochemical methods.

RESULTS

Validity study: the mutation was found in all 35 individuals classified as carriers of AIP. None of the 92 controls had the mutation. Evaluation study: all 48 AIP gene carriers, diagnosed by traditional methods, had the mutation, as had one of the control persons. In an inconclusive group of five persons with heredity for AIP, two had a positive DNA test.

CONCLUSIONS

The PBGD mutation analysis was found to have full specificity and sensitivity and can be used as the sole diagnostic method in the family complex studied, representing the major AIP mutation in Sweden. The traditional diagnostic methods, used in optimal combinations, work in most cases, but they do not show high precision. However, they must be used when the specific mutation in the PBGD gene is not known.

Ancestral differences in the distribution of the delta 2642 glutamic acid polymorphism is associated with varying CAG repeat lengths on normal chromosomes: insights into the genetic evolution of Huntington disease

This study addresses genetic factors associated with normal variation of the CAG repeat in the Huntington disease (HD) gene. To achieve this, we have studied patterns of variation of three trinucleotide repeats in the HD gene including the CAG and adjacent CCG repeats as well as a GAG polymorphism at residue 2642 (delta 2642). We have previously demonstrated that variation in the CCG repeat is associated with variation of the CAG repeat length on normal chromosomes. Here we show that differences in the GAG trinucleotide polymorphism at residue 2642 is also significantly correlated with CAG size on normal chromosomes. The B allele which is associated with higher CAG repeat lengths on normal chromosomes is markedly enriched on affected chromosomes. Furthermore, this glutamic acid polymorphism shows significant variation in different ancestries and is absent in chromosomes of Japanese, Black and Chinese descent. Haplotype analysis of both the CCG and delta 2642 polymorphisms have indicated that both are independently associated with differences in CAG length on normal chromosomes. These findings lead to a model for the genetic evolution of new mutations for HD preferentially occurring on normal chromosomes with higher CAG repeat lengths and a CCG repeat length of seven and/or a deletion of the glutamic acid residue at delta 2642. This study also provides additional evidence for genetic contributions to demographic differences in prevalence rates for HD.

Rett syndrome: the bcl-2 gene--a mediator of neurotrophic mechanisms?

Rett syndrome affects young girls. The molecular basis is mysterious. One tenable hypothesis can be that the programmed early infantile death of neuronal cells is reprogrammed. The bcl-2 gene on chromosome 22 has been shown to play a role in the apoptosis process. No sequence abnormalities were detected in the bcl-2 gene of 6 patients with Rett syndrome which could explain the pathophysiology of Rett syndrome.

Imprinting mutations suggested by abnormal DNA methylation patterns in familial Angelman and Prader-Willi syndromes

The D15S9 and D15S63 loci in the Prader-Willi/Angelman syndrome region on chromosome 15 are subject to parent-of-origin-specific DNA methylation. We have found two Prader-Willi syndrome families in which the patients carry a maternal methylation imprint on the paternal chromosome. In one of these families, the patients have a small deletion encompassing the gene for the small nuclear ribonucleoprotein polypeptide N, which maps 130 kb telomeric to D15S63. Furthermore, we have identified a pair of nondeletion Angelman syndrome sibs and two isolated Angelman syndrome patients who carry a paternal methylation imprint on the maternal chromosome. These Angelman and Prader-Willi syndrome patients may have a defect in the imprinting process in 15q11-13. We propose a model in which a cis-acting mutation prevents the resetting of the imprinting signal in the germ line and thus disturbs the expression of imprinted genes in this region.

Characterization of the von Willebrand factor gene (VWF) in von Willebrand disease type III patients from 24 families of Swedish and Finnish origin

Twenty-four patients with von Willebrand disease type III were screened for mutations in the von Willebrand factor (VWF) gene using the PCR technique, followed by direct sequencing. More than 250 kb of genomic DNA were sequenced, including the promoter and coding regions (52 exons) of the VWF gene from 24 patients. In addition to the previously reported mutations of a single cytosine deletion in exon 18 and the nonsense mutations in exons 28, 32, and 45, nine new mutations were detected: two nonsense mutations in exons 15 and 16, one allele with a thymidine insertion in exon 14, one allele with a cytosine insertion in exon 28, one 20-bp deletion in exon 15, one mutation in the donor splice site of exon 43, and three missense mutations in exons 28, 49, and 51. Forty-two mutant chromosomes were identified (42/48); 11 probands are homozygous for the mutations, and 8 are compound heterozygous. In addition, a new subfamily of the Alu sequence in the promoter region and 10 new polymorphisms were identified.

Huntington disease without CAG expansion: phenocopies or errors in assignment?

Huntington disease (HD) has been shown to be associated with an expanded CAG repeat within a novel gene on 4p16.3 (IT15). A total of 30 of 1,022 affected persons (2.9% of our cohort) did not have an expanded CAG in the disease range. The reasons for not observing expansion in affected individuals are important for determining the sensitivity of using repeat length both for diagnosis of affected patients and for predictive testing programs and may have biological relevance for the understanding of the molecular mechanism underlying HD. Here we show that the majority (18) of the individuals with normal sized alleles represent misdiagnosis, sample mix-up, or clerical error. The remaining 12 patients represent possible phenocopies for HD. In at least four cases, family studies of these phenocopies excluded 4p16.3 as the region responsible for the phenotype. Mutations in the HD gene that are other than CAG expansion have not been excluded for the remaining eight cases; however, in as many as seven of these persons, retrospective review of these patients' clinical features identified characteristics not typical for HD. This study shows that on rare occasions mutations in other, as-yet-undefined genes can present with a clinical phenotype very similar to that of HD.

Characteristics of intergenerational contractions of the CTG repeat in myotonic dystrophy

In myotonic dystrophy (DM), the size of a CTG repeat in the DM kinase gene generally increases in successive generations with clinical evidence of anticipation. However, there have also been cases with an intergenerational contraction of the repeat. We examined 1,489 DM parent-offspring pairs, of which 95 (6.4%) showed such contractions in peripheral blood leukocytes (PBL). In 56 of the 95 pairs, clinical data allowed an analysis of their anticipation status. It is surprising that anticipation occurred in 27 (48%) of these 56 pairs, while none clearly showed a later onset of DM in the symptomatic offspring. The contraction occurred in 76 (10%) of 753 paternal transmissions and in 19 (3%) of 736 maternal transmissions. Anticipation was observed more frequently in maternal (85%) than in paternal (37%) transmissions (P < .001). The parental repeat size correlated with the size of intergenerational contraction (r2 = .50, P << .001), and the slope of linear regression was steeper in paternal (-.62) than in maternal (-.30) transmissions (P << .001). Sixteen DM parents had multiple DM offspring with the CTG repeat contractions. This frequency was higher than the frequency expected from the probability of the repeat contractions (6.4%) and the size of DM sib population (1.54 DM offspring per DM parent, in 968 DM parents). We conclude that (1) intergenerational contractions of the CTG repeat in leukocyte DNA frequently accompanies apparent anticipation, especially when DM is maternally transmitted, and (2) the paternal origin of the repeat and the presence of the repeat contraction in a sibling increase the probability of the CTG repeat contraction.

Complex genetic counseling and exclusion of Duchenne muscular dystrophy in a twin pregnancy after in vitro fertilization (IVF)

A twin pregnancy following in vitro fertilization-embryo transfer coincidentally at risk for the X-linked recessive Duchenne muscular dystrophy is described. First-trimester prenatal diagnosis by transabdominal chorionic villus samplings on the dichorionic placentae and molecular linkage analysis could exclude the disorder in both fetuses. Genetic counseling and prenatal diagnosis were particularly complex due to the twin pregnancy, the need for linkage analysis, and confined placental mosaicism 45,X/46XX in one of the fetuses. All parties should be aware that additional invasive diagnostic procedures in the second trimester might be required. It is proposed that, in similar situations, only one, arguably two, fertilized egg be transferred at a time to facilitate prenatal diagnosis and decision making for these rare couples. This problem, however, may be increasingly overcome by preimplantation diagnosis.

394delTT: a Nordic cystic fibrosis mutation

In a systematic screening for mutations in the gene encoding the cystic fibrosis transmembrane regulator among Danish cystic fibrosis (CF) patients, we identified a mutation in exon 3 (394delTT); this mutation was found to be relatively common in Denmark. We therefore screened for 394delTT in Sweden and Norway, where it turned out to be the second most frequent mutation, accounting for 4% of all CF mutations. It also occurs with a high frequency in Finland, but has not been found in larger surveys of mutations in the CFTR gene. Thus, 394delTT seems to be a specific Nordic CF mutation.