40440 Med Genet 1995;32:404-408 sponsible for HD has important implications one expanded allele were found in all HD for both research and clinical practice. How- patients. The comparison ofthe size variation LETTERS TO ever, most of the data relate to white pop- of triplet repeats in the control and HD chro- ulations. In this study we verify the presence mosomes between Chinese and whites is of the trinucleotide repeat elongation in Chi- shown in the table. Thirteen discrete allele THE EDITOR J Med Genet: first published as 10.1136/jmg.32.5.404 on 1 May 1995. Downloaded from nese patients with HD and establish the range sizes among the normal population, with a ofrepeat sizes in affected and normal subjects. heterozygosity frequency of 80-7%, and an The diagnosis of HD was based on clinical expanded range from 40 to 58 copies on HD and family histories, neurological ex- chromosomes, were found. The peaks of the aminations, and special investigations, such normal and HD ranges were well separated A comparison of the as electroencephalography and computerised at 17 and 43 repeats, respectively, and no Huntington's disease tomography. Genomic DNA samples were overlap of the two distributions was observed available from a total of 114 persons (51 (figure). Within our population, 65-7% ofthe associated trinucleotide normal controls and 63 members of 11 un- expanded alleles fell in the range of 40 to 45 repeat between Chinese related HD families, including 11 unrelated repeats, while only 17-1% consisted of more spouses who also served as controls). than 50 units. and white populations The size of the (CAG)n trinucleotide This is the first study examining the HD repeat was analysed by the polymerase chain mutation in a large Chinese population. The Huntington's disease (HD) affects one in reaction (PCR). The genomic primer se- range ofthe CAG repeat length of 159 control 10000 people in most white populations.12 quences used in our PCR assay for the chromosomes (124 from 51 normal subjects Its prevalence among the Chinese is not clear, (CAG)n repeat were HD-1: 5' ATG AAG and 11 unrelated spouses of the HD patients; but it is not expected to be much different GCC TTC GAG TCC CTC AAG TCC 35 from the affected or presymptomatic sub- from that of the white population. In 1993, TTC 3', and HD-3: 5' GGC GGT GGC jects, each having one chromosome that did a polymorphic (CAG)n trinucleotide repeat GGC TGT TGC TGC TGC TGC TGC not contain the expanded CAG repeat) was was identified3 in the 5' region of a new 3'.' The PCR assay was performed as de- nine to 29, with a median of 17 repeats transcript, IT1 5. The protein encoded by the scribed by Warner et al8 with minor mo- (figure, table). The normal allele distribution IT15 was named huntingtin.3 The ob- difications. of the polymorphic (CAG)n repeat in the servation that the length of the trinucleotide The PCR assay was highly specific and huntingtin gene among the Chinese popu- repeat was increased in virtually all HD chro- gave products in the 74 base pairs (bp) (nine lation is much tighter (with a statistically mosomes examined,46 as well as in families CAG repeats) to 134bp (29 CAG repeats) significant smaller standard deviation) than with a presumed new mutation,7 suggests the range from normal chromosomes, and prod- the distributions reported in the United mutational mechanism causing HD has been ucts of 167bp (40 repeats) and larger from States46 and Europe5 9-12 (table). The dis- identified. The discovery of the mutation re- HD chromosomes (figure). One normal and tribution looks more like the recently pub- lished African and Japanese distributions where there is a lower incidence of HD.4 The median of the size of the upper alleles representing the expanded HD chromosomes was 44 repeats, with a range of 40 to 58 (figure, table). All Chinese patients in whom HD was diagnosed were heterozygous, with one allele in the normal range and one ex- panded allele, indicating that expansion of this (CAG)n repeat is the major cause of Huntington's disease. A copy number of 40 or higher can result in the full phenotypic http://jmg.bmj.com/ manifestations of the disease. Thus, muta- 0- tional events leading to HD seem to be c 30 a) homogeneous worldwide.4'9-12 The finding that the molecular biology of HD is in- distinguishable in Chinese and white pop- LL ulations is of great interest. The idea that 20 worldwide HD evolved from a very limited number of European founders is no longer tenable.41314 This study provides evidence on September 28, 2021 by guest. Protected copyright. 10 that there is a basal rate of susceptibility to the HD mutation constantly adding new families. It will be very interesting to know whether new mutation rates differ between o races. 5 10 15 20 25 30 35 40 45 50 55 60 MacMillan et all' and Kremer et al4 re- Repeat units ported an estimated sensitivity of 98% and a specificity of 100% for the use of the number Distribution of absolute frequencies of trinucleotide repeat number in 159 independent control of CAG repeats to identify those with the chromosomes (shaded bars) and 35 HD chromosomes (open bars) among the Chinese. mutation for HD. The diagnostic value ofthe trinucleotide repeat expansion is apparently high within our population, with no overlap in Comparison of the distributions of triplet repeats in the control and HD chromosomes in the Chinese values between normal and affected persons. and white populations This molecular test can be used to differ- entiate from Boston Cardiff Vancouver Taipei HD other illnesses.'617 We have (Duyao et a16) (Snell et a15) (Kremer et al') (This study) not encountered any patient with the clinical phenotype of HD without the trinucleotide Normal controls 545 1160 600 159 expansion. Moreover, the ability to measure Range of repeat numbers 11-34 9-34 10-39 9-29 Mean 19-71 18 29 18-4 17-36* the size of the trinucleotide repeat in people Median 19 18 17 at risk for HD will revolutionise preclinical SD 3-21 3-13 1-97 testing and eliminate the need for linkage HD mutant genes 425 440 995 35 analysis. This will greatly facilitate Range of repeat numbers 37-86 16-70 36-121 40-58 coun- Mean 46-42 41-88 45-3 44-94t selling and makes the test available to ap- Median 45 44 44 plicants without living relatives, and without SD 6-68 6-67 4-15 undue involvement of other at risk sibs. We believe that the ethical and social * p<O-OOl for the comparison with the Boston group, and p<001 for the comparison with the Cardiff issues in- group, by Student's t test. volved will remain largely unchanged; they t p<0-01 for the comparison with the Cardiff group, by Student's t test. may well become more important as the Letters to the Editor 405 laboratory aspects are simplified. Accepted Rare variants of chromosome 9 (figs 1 and 2). Again, as in international guidelines and counselling pro- the AluI treatment, the result was the opposite tocols should be adhered to when using direct chromosome 9 with extra of that of Fernandez et al and showed that repeat size determination in doing pre- G positive band within the the extra band was not affected and remained symptomatic or prenatal testing.'8 condensed. The induction of J Med Genet: first published as 10.1136/jmg.32.5.404 on 1 May 1995. Downloaded from are 5-azacytidine qh region not alike (3-5 x 1i0- mol/1) during the last seven hours We would like to thank Dr Tsung-sheng Su for her of incubation apparently hypomethylates the helpful discussions on this manuscript. We are also The intriguing findings in a recent in- late classical satellite constitutive indebted to replicating Wen-yung Sheng, MPH, for her con- Fernandez et al' us tribution to the statistical analysis. Furthermore we vestigation by prompted heterochromatin regions of chromosomes want to thank all HD patients for providing samples to re-evaluate our previous case2 that had a 1, 9, 15, 16, and Y, and causes under- for scientific research. The work was supported similar extra G positive band within the 9qh condensation owing to its incorporation by grants from the National Science Council, the additional molecular this It is Republic of China (NSC 83-0412-B075-077-M02, region by techniques. during period.56 believed the un- NSC 84-233 1-B075-062-M02), and partly from the To our knowledge, these two studies, in ad- dercondensation of the heterochromatin that Yen Tjing-Ling Medical Foundation (CI-77-32, CI- dition to a third by Macera et al,' are the is caused by the induction of 5-azacytidine is 78-34, CI-81-11). only ones that have used various molecular possible owing to the relatively high variation techniques which showed satellite DNA loc- in the number of repeated units within the BING-WEN SOONG ations in this particular so-called rare variant classical satellite DNA region; generally this Department of Neurology, of chromosome 9. other which is not the case in the and beta satellite National Defense Medical Center and Many reports alpha National Yang-Ming University School of Medicine. have focused on this seeming paradox, stated DNA of chromosome 9.4 However, it has BING-WEN SOONG that the variant chromosome 9 appeared to be been shown that there are differences in the JIH-TSWU WANG similar by conventional banding condensation inhibition behaviour of cen- Department of Neurology, Neurological Institute, techniques.