Early prenatal diagnosis of the ICF syndrome

The ICF syndrome (immunodeficiency, (para)centromeric instability and facial abnormalities) is a rare autosomal recessive disorder with characteristic cytogenetic aberrations of chromosomes 1, 9 and 16 in lymphocytes. Previously, only one case has been diagnosed prenatally in the second trimester of pregnancy by fetal blood sampling. We report the first early prenatal exclusion of the ICF syndrome by chorionic villous sampling (CVS) and linkage analysis in a family with a previous affected child. The fetus was heterozygous for marker D20S850 closely linked to the ICF locus. The family was counselled of a probability of over 90% that the fetus would be unaffected. Postnatal chromosome analysis on peripheral blood was normal and thus confirmed the prenatal diagnosis.

Variation of CTG-repeat number of the DMPK gene in muscle tissue

Myotonic dystrophy (DM) is associated with an unstable expansion of CTG repeats located in the 3' untranslated region of a protein kinase-encoding gene (DMPK) on chromosome 19 (19q13.3). The CTG repeat number varies between 5 and 37 in lymphocytes of normal individuals, whereas DM patients may have expansions from 50 to several thousand copies. Although the CTG expansions related to myotonic dystrophy (DM) are usually larger in muscle compared to peripheral blood, the variation in repeat number in non-dystrophic muscle is not known. In order to investigate if there is a variation, the CTG-repeat number was determined in percutaneous muscle biopsies obtained from 86 individuals without any muscle disorder or with a neuromuscular disorder without any clinical or histopathological signs of DM. The number of CTG repeats varied between 5 and 28, this being within the normal range reported for peripheral blood. A major sharp peak at n = 5 (27%) and a broader peak at n = 8-17 (56%) with peak values at n = 12 and 14 (11 and 14%, respectively) were observed. Alleles with 19 or more repeats amounted to 17% with a small peak at n = 20 and 21 (6 and 4%, respectively). It is concluded that the normal variation of CTG-repeat number in skeletal muscle is within the range found in peripheral blood, although there is a slight shift in the overall frequency distribution towards alleles with CTG repeat numbers in the higher range.

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.

Somatic mosaicism in sperm is associated with intergenerational (CAG)n changes in Huntington disease

We have analysed the CAG repeat in the Huntington disease (HD) gene in sperm and blood from 20 unrelated HD patients. Although the CAG repeat displayed significant mosaicism in sperm from all individuals, there were marked differences in the degree of repeat instability. Individuals who had either inherited or transmitted an expanded CAG repeat displayed the highest levels of repeat mosaicism, whereas individuals who had inherited or transmitted a contracted repeat had very limited CAG mosaicism in sperm. A strong association between intergenerational change in CAG allele size and the level of sperm repeat mosaicism was determined (P = 0.019). In contrast, neither blood CAG size nor repeat mosaicism in blood, were significantly associated with intergenerational CAG changes. These data suggest the presence of a cis-acting factor, separate from CAG size, that strongly influences the intergenerational behaviour of the CAG repeat. Additional studies are needed to determine whether analysis of CAG mosaicism in sperm is useful for assessing an individual's risk for transmitting large expansions or contractions to his offspring.

Geographical distribution of haplotypes in Swedish families with Huntington's disease

This study was planned to determine the number of origins of the mutation underlying Huntington's disease (HD) in Sweden. Haplotypes were constructed for 23 different HD families, using six different polymorphisms [(CCG)n, GT70, 674, BS1, E2 and 4.2], including two within the gene. In addition, extensive genealogical investigations were performed, and the geographical origin of the haplotypes was studied. Ten different haplotypes were observed suggesting multiple origins for the HD mutation in Sweden. Analysis of the two polymorphic markers within the HD gene (the CCG repeat and GT70) indicates that there are at least three origins for the HD mutation in Sweden. One of these haplotypes (7/A) accounts for 89% of the families, suggesting that the majority of the Swedish HD families are related through a single HD mutation of ancient origin. Furthermore, three of the families that were previously considered to be unrelated could be traced to a common ancestor in the 15th century, a finding that is consistent with this hypothesis.

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.

Larger expansions of the CTG repeat in muscle compared to lymphocytes from patients with myotonic dystrophy

The discovery that there is an expansion of a CTG repeat underlying myotonic dystrophy has led to new approaches in diagnosis and genetic counselling for this disorder. The size of the expansion correlates to a reasonable degree with the clinical symptoms within a given family. We report comparisons of the length of the expansion seen in lymphocytes and muscle samples from eight patients. In all cases the length of the expansion seen in DNA isolated from muscle was larger than that seen in lymphocytes from the same patient. There was no progression of the expansion over a period of 10-15 years in muscle samples from two of these patients even though there had been significant progression of the severity of symptoms during that time.