Single nucleotide polymorphisms of the factor IX gene for linkage analysis in the southern Chinese population.

Carrier detection and prenatal testing for haemophilia B in Oriental populations have been hampered by the lack of informative markers within the factor IX (FIX) gene. We detected a T/C nucleotide variation at nucleotide 32770 in the poly-A region of the FIX gene in the mother of a haemophilia B child. Analysis of 139 unrelated alleles revealed a heterozygosity rate of 0.193, thus offering an additional marker for linkage analysis. Together with two other polymorphic sites (5' MseI and 3' HhaI) found in Chinese and Thai populations, these polymorphisms were useful in 66% of the families studied.

A reverse dot-blot method for rapid detection of non-deletion alpha thalassaemia.

A reverse dot blot method based on membrane-bound allele-specific oligonucleotides as hybridization targets for amplified alpha-gene fragments has been developed for the rapid detection of four non-deletion alpha thalassaemia defects found in the Chinese. Since these non-deletion defects account for 22 8% of haemoglobin H disease, a sensitive, specific and rapid screening method should be of value.

Linkage of malignant hyperthermia and hyperkalemic periodic paralysis to the adult skeletal muscle sodium channel (SCN4A) gene in a large pedigree.

Hyperkalemic periodic paralysis (HPP) is caused by mutations of the adult skeletal muscle sodium channel (SCN4A) gene on chromosome 17. Malignant hyperthermia (MH) is a genetically heterogeneous autosomal-dominant disorder occurring in association with various neuromuscular diseases or without other apparent abnormalities. In some families, MH is associated with mutations of a calcium release channel (RYR1) gene on chromosome 19. In other families, linkage of this disorder to the SCN4A gene on chromosome 17 has been suggested. We report on linkage analysis in a family in which both HPP and MH are inherited as autosomal-dominant traits. Two polymorphisms within the SCN4A locus, an RFLP and a (C-A)n repeat, were typed on multiple family members. The findings were consistent with linkage of the polymorphic markers within the SCN4A gene to both HPP (Zmax = 6.79 at theta = 0.0) and MH (Zmax = 1.76 at theta = 0) in this family. Our data provide further evidence that MH is linked to the SCN4A locus in some families.

Parental origin of triploidy in human fetuses: evidence for genomic imprinting.

Two distinct phenotypes of triploid fetuses have been previously described and a correlation with parental origin of the triploidy has been suggested. We have studied the parental origin of the extra haploid set of chromosomes in nine triploid fetuses using analysis of DNA polymorphisms at a variety of loci. Maternal origin of the triploidy (digyny) was demonstrated in six fetuses with type II phenotype, paternal origin (diandry) in two cases with type I phenotype, and nonpaternity in one case. The predominance of digynic triploids in our study contrasts with the results reported in previous studies in which, through analysis of cytogenetic polymorphisms, paternal origin was found to account for the majority of triploid conceptuses. This difference may be accounted for by a combination of factors--the different methods of parental assignment used and analysis of a different subset of triploid conceptuses. The correlation between the observed phenotypes and the parental origin of triploidy may represent another example of imprinting in human development.

Uniparental disomy for chromosome 16 in humans.

The association between chromosomal mosaicism observed on chorionic villus sampling (CVS) and poor pregnancy outcome has been well documented. CVS mosaicism usually represents abnormal cell lines confined to the placenta and often involves chromosomal trisomy. Such confined placental mosaicism (CPM) may occur when there is complete dichotomy between a trisomic karyotype in the placenta and a normal diploid fetus or when both diploid and trisomic components are present within the placenta. Gestations involving pure or significant trisomy in placental lineages associated with a diploid fetal karyotype probably result from a trisomic zygote which has lost one copy of the trisomic chromosome in the embryonic progenitor cells during cleavage. Uniparental disomy would be expected to occur in one-third of such cases. Trisomy of chromosome 7, 9, 15, or 16 is most common among the gestations with these dichotomic CPMs. Nine pregnancies with trisomy 16 confined to the placenta were prenatally diagnosed. Pregnancy outcome, levels of trisomic cells in term placentas, and fetal uniparental disomy were studied. Intrauterine growth retardation (IUGR), low birthweight, or fetal death was observed in six of these pregnancies and correlated with high levels of trisomic cells in the term placentas. Four of the five cases of IUGR or fetal death showed fetal uniparental disomy for chromosome 16. One of the infants with maternal uniparental disomy 16 had a significant malformation (imperforate anus). All infants with normal intrauterine growth showed term placentas with low levels of trisomic cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular defects in haemophilia B: detection by direct restriction enzyme analysis.

The common restriction fragment length polymorphisms (RFLPs) associated with the FIX gene: 5' BamH I, Dde I, BamH I (2), Taq I and 3' Hha I were absent or of low incidence in Southern Chinese and are therefore not useful for linkage analysis. No deletion was detected amongst seven consecutive unrelated haemophilia B patients, but one had an insertion of a 15 kb Pvu II fragment containing exon d. Using an alternate strategy of polymerase chain reaction (PCR) amplification and direct sequencing, the molecular defect in the other six patients was defined. The four novel mutations characterized were: nucleotide (nt) 6410 G----C (Gly12----Ala); nt 31261 delta T (stop codon 31 bp downstream); nt 31260 C----G (Thr380----Ser) and nt 31122 C----A (Ala34----Asp). Two patients had the same mutation at nt 6365, G----A (Arg-4----Gln), identical to one previously described in other ethnic groups, suggesting that this is a hotspot for mutation. Each of the mutations was found to affect an enzyme recognition site and could thus be identified by direct visualization of abnormal restriction fragments in amplified genomic DNA. This allows rapid and accurate DNA diagnosis of haemophilia B in an ethnic group which otherwise shows little or no polymorphism for the common RFLP sites.