Genetic risk of Alzheimer's disease: advising relatives.

BACKGROUND: Clinicians are increasingly asked by relatives of patients with Alzheimer's disease to advise on their genetic risk of developing Alzheimer's disease in later life. Many clinicians find this a difficult question to answer. AIMS: To provide information for old age psychiatrists wishing to advise relatives of their risk of developing Alzheimer's disease. METHOD: A selective review of the key literature on the genetic epidemiology of Alzheimer's disease. RESULTS: Currently a DNA diagnosis is attainable in some 70% of families with autosomal dominant Alzheimer's disease. In first-degree relatives of most cases, risk is increased some three- or four-fold relative to controls, but only one-third of this is realised in the average life span. Apolipoprotein E genotyping cannot be used as a predictive test and confers only minimal diagnostic benefit. CONCLUSIONS: Pedigrees with familial Alzheimer's disease should be referred to a Regional Centre for Medical Genetics. Accurate risk prediction is not possible in the vast majority of pedigrees with Alzheimer's disease, although it is possible for the psychiatrist to give a rough estimate of the risk, which can reasonably the couched in reassuring terms.

No evidence that common allelic variation in the Amyloid Precursor Protein (APP) gene confers susceptibility to Alzheimer's disease.

In order to test the hypothesis that allelic variation within the Amyloid Precursor Protein (APP) gene influences susceptibility to common forms of Alzheimer's disease (AD) we screened the entire coding, promoter and 3' untranslated sequences of the APP gene for DNA variations in 30 unrelated patients and eight controls with probable AD by a combination of RT-PCR PCR and chemical cleavage mismatch analysis. Although we were unable to detect commonly occurring allelic variants, we were able to detect a novel mutation within the APP gene in one individual with late-onset AD. This mutation resulted in the substitution of a tryptophan residue for an arginine residue at codon 328 within exon 7 which encodes the so-called protease inhibitor domain of the 751 residue APP isoform. However, the pathological significance of this mutation is uncertain as neither this, nor any other mutation occurring within exon 7 of the APP gene was found in any of a further 102 AD patients and 86 age-matched controls. In conclusion, it is unlikely that susceptibility to AD results from commonly occurring allelic variants of the APP gene and it is even less probable that mutations within exon 7 of the APP gene are important risk factors for late-onset AD.

Family studies and prenatal diagnosis in severe von Willebrand disease by polymerase chain reaction amplification of a variable number tandem repeat region of the von Willebrand factor gene.

We have previously demonstrated within intron 40 of the von Willebrand factor (vWF) gene a region of ATCT repeats that was shown to vary in length between two different DNA clones from unrelated individuals. The polymerase chain reaction (PCR) was used to examine the variability in length of this variable number tandem repeat (VNTR) in 53 normal individuals, using primers to DNA sequence flanking the repeat region. Overall, eight different length allelic bands were seen. These were individually sequenced and shown to contain from 6 to 14 ATCT repeats (a nine-repeat band was not seen). Seventy-five percent of individuals were shown to be heterozygous for this vWF.VNTR, and family studies showed Mendelian inheritance with allelic frequencies from 1% (vWF.VNTR [8] and vWF.VNTR [14]) to 39% (vWF.VNTR [7]). In the family of a patient with type III severe von Willebrand disease (vWD), vWF.VNTR results mirrored the phenotypic data and results with previously reported intragenic vWF restriction fragment length polymorphisms (RFLP). The patient was shown to be a compound heterozygote. In a family with a child with severe type III vWD, prenatal diagnosis by vWF.VNTR analysis on DNA obtained by chorionic villus sampling at 10 weeks gestation during a subsequent pregnancy indicated a severely affected fetus. This diagnosis was confirmed by fetal blood sampling at 18 weeks.

A mutation adjacent to the beta cleavage site of factor IX (valine 182 to leucine) results in mild haemophilia Bm.

The genetic basis of a mild form of haemophilia Bm has been investigated. The patient under investigation has a mild bleeding disorder and has never experienced spontaneous bleeds. Factor IX coagulant activity (FIX:C) was 0.15 units/ml and factor IX antigen (FIX:Ag) 1.32 units/ml. The prothrombin time performed with an ox brain thromboplastin was 65 s (normal plasma 31 s). Studies of the abnormal factor IX protein in this patient showed a normal molecular weight and normal calcium binding properties. Activation of the mutant factor IX with factor XIa showed normal proteolytic cleavage. DNA sequence from the eight factor IX exons and flanking introns was amplified from this patient using the polymerase chain reaction. The amplified material was subjected to direct chain termination nucleotide sequencing. The only nucleotide sequence alteration found was a G----C transversion at nucleotide 20,524, changing the amino acid encoded at residue 182 from valine to leucine. This residue is one amino acid removed from the beta cleavage site of factor IX. This residue is highly conserved in other vitamin K dependent serine proteases and we propose that its alteration in this patient is responsible for his mild haemophilic phenotype, and for the abnormal interaction of this factor IX protein with the extrinsic system of coagulation.

Severe type III von Willebrand's disease caused by deletion of exon 42 of the von Willebrand factor gene: family studies that identify carriers of the condition and a compound heterozygous individual.

Southern blotting was performed with cDNA probes for the human von Willebrand factor (vWF) gene on six patients with severe type III von Willebrand's disease (vWD). A partial deletion in the 3' end of the vWF gene was demonstrated in one individual whose parents were related and who had an alloantibody inhibitor to vWF. A resulting novel 2.0-kilobase (kb) EcoRI fragment was used for carrier detection within the patient's family, and seven carriers of this recessive trait were identified. Of the six tested, five had normal or only slightly reduced levels of vWF antigen, but with generally higher levels of factor VIII. The sixth carrier had moderately severe vWD and it is proposed that this patient is heterozygous for the defective vWF gene and a second recessive vWF defect. The novel 2.0-kb EcoRI restriction fragment was cloned and sequenced, and compared with that of the corresponding normal 4.2-kb EcoRI fragment that includes exons 41 and 42 of the vWF gene. A deletion of 2,320 base pairs (bp) which included exon 42, was identified and a novel 182-bp insert was found between the breakpoints. This insert was detected by polymerase chain reaction amplification both in the patient's DNA and in his carrier relatives.

Factor IX Cardiff: a variant factor IX protein that shows abnormal activation is caused by an arginine to cysteine substitution at position 145.

Crude barium chloride eluates prepared from 12 unrelated patients with cross-reacting material positive (CRM+) haemophilia B were activated with celite eluate, the reaction products resolved after reduction by 13% SDS-PAGE, and factor IX antigenic material detected by probing with radiolabelled immunopurified rabbit anti-factor IX antiserum followed by autoradiography. Out of the 12, one sample showed faulty activation with the production of a stable reaction product with a MW compatible with that of a putative light chain-activation intermediate. In order to confirm this, two oligonucleotide primers that bracketed exon 6 of the factor IX gene were constructed and used to prime a polymerase chain reaction on DNA isolated from the patient's peripheral blood leucocytes. A single 489 nucleotide DNA fragment was obtained, gel purified, subcloned into M13, and DNA sequencing carried out on both strands. A single C to T transition was discovered that changed the Arg residue at position 145, the first residue of the first bond in the activation peptide, to a Cys, a result that confirmed the inferences drawn from the activation studies.

Defective propeptide processing and abnormal activation underlie the molecular pathology of factor IX Troed-y-Rhiw.

Affected members of a South Wales haemophilia B family (from Troed-y-Rhiw) were shown by Western blotting and immunoperoxidase detection to have a factor IX molecule of higher than normal molecular weight which also shows impaired calcium binding. Gene cloning and DNA sequencing revealed the same arginine to glutamine mutation at position -4 of the propeptide that has been found in two previously described factor IX variants which circulate with the propeptide still attached. The mutation also abolishes a HaeIII restriction enzyme recognition site. A potential carrier was shown to be normal both by Western blotting and DNA studies. The way in which the attached propeptide interferes with normal factor IX function was investigated by activation studies with crude normal and patient factor IX Troed-y-Rhiw preparations using Western blotting and detection with iodinated immunopurified polyclonal antifactor IX serum. We demonstrate that the -4 mutation appears to block cleavage between the Arg145-Ala146 peptide bond in the activation peptide, thus preventing the normal activation of factor IX Troed-y-Rhiw. A small amount of normally processed factor IX is produced, implying that the -4 mutation does not completely prevent propeptide cleavage, thus accounting for the low levels of factor IX activity measured in the plasma of affected family members.

Carrier detection in 50 haemophilia A kindred by means of three intragenic and two extragenic restriction fragment length polymorphisms.

Carrier detection was attempted in 50 haemophilia A kindred by means of restriction fragment length polymorphism (RFLP) analysis using two linked and three intragenic probes. The sample comprised 330 individuals including 70 haemophiliacs, 45 obligate carriers and 100 possible carriers. Non-related haplotypes contained within the sample group were used to tabulate the intragenic RFLP allele frequencies. The linkage disequilibrium existing between the XbaI and Bc1I RFLP was shown to be less than previously reported, such that 69% of women are informative for at least one of these two polymorphisms. 36 women were subsequently diagnosed as carriers and 37 as being normal: in 52 of these, diagnosis was based either on intragenic RFLPs or linked RFLP plus strong phenotypic data, and was considered to be unequivocal; in the remaining 21, diagnosis was based on linked RFLPs alone and was considered to be probabilistic, with a 5-10% possible error rate. 27 of the possible carriers, either from families with sporadic haemophilia or from families where missing members precluded haplotype analysis, remained unassigned. 72% of the obligate carriers and firmly diagnosed carriers were heterozygous and phase known for at least one intragenic RFLP, whereas 6% were uninformative for the RFLPs for which they were examined. In 54 informative meioses, three recombinations between the factor VIII locus and the DX13 and/or ST14 loci were observed, giving a recombination rate of 5.5%. 15 prenatal diagnoses were carried out. Of the 11 male fetuses, six were shown to be affected and five to be normal. In three of four prenatal diagnoses where only a linked RFLP was informative, the result was confirmed by fetoscopy, fetal blood sampling, and factor VIII assay.

Comparison of phenotypic assessment and the use of two restriction fragment length polymorphisms in the diagnosis of the carrier state in haemophilia B.

Carrier detection in haemophilia B has previously involved pedigree analysis and assessment of the coagulation phenotype. At best using these methods, carrier evaluation may be made with about 80% certainty (Orstavik et al, 1981). With isolation and cloning of the factor IX gene, DNA probes are now available to detect intragenic nucleotide changes. This study uses one such probe which detects restriction fragment length polymorphisms with the restriction endonucleases Taq 1 and Xmn 1. Seventy-eight individuals from eight haemophilia B kindred were initially evaluated for factor IXC and factor IXAg levels. DNA from these individuals was then digested with the two restriction enzymes and after Southern blotting, analysed for restriction fragment length polymorphisms (RFLPs) with the radiolabelled genomic probe. Four kindred proved informative with RFLP linkage assessment. In three families evaluation was possible with both Taq 1 and Xmn 1 polymorphic markers but in the fourth pedigree only Xmn 1 was informative. Using these techniques five potential carriers have been definitively assigned as either normal or carriers of the haemophilia B gene defect. Problems of phenotypic assessment are well illustrated in pedigree 4 where polymorphic linkage positively identified a carrier whose coagulation data were normal.