J7 Med Genet 1994;31:595-601 595

Original articles J Med Genet: first published as 10.1136/jmg.31.8.595 on 1 August 1994. Downloaded from Anticipation resulting in elimination of the : a follow up study of one extended family

C E M de Die-Smulders, C J Howeler, J F Mirandolle, H G Brunner, V Hovers, H Bruggenwirth, H J M Smeets, J P M Geraedts

Abstract muscular manifestations, it is characterised by We have re-examined an extended myo- multiple systemic effects including cataract, tonic dystrophy (DM) family, previously mental retardation, cardiac involvement, and described in 1955, in order to study the testicular atrophy. Extreme variability is one of long term effects of anticipation in DM the hallmarks of the disease; clinical studies and in particular the implications for have led to the recognition of four disease types families affected by this disease. This fol- on the basis of age at onset and core symptoms: low up study provides data on 35 gene late onset (mild) type, adult onset (classical) carriers and 46 asymptomatic at risk type, childhood, and congenital type.'"3 family members in five generations. Anticipation, increasing severity and earlier Clinical anticipation, defined as the cas- age at onset in successive generations, has been cade ofmild, adult, childhood, or congen- observed in DM since the beginning of this ital disease in subsequent generations, century, but remained unexplained and contro- appeared to be a relentless process, oc- versial until recently."- With the discovery of curring in all affected branches of the an unstable CTG trinucleotide repeat in the 3' family. The cascade was found to proceed untranslated region of a protein kinase gene on asynchronously in the different branches, chromosome 19q, a biological explanation for mainly because of an unequal number of this phenomenon has emerged.>'4 The repeat generations with mild disease. The tran- length correlates with the severity of clinical sition from the mild to the adult type was symptoms and increases with transmission to associated with transmission through a subsequent generations.3 7 1516 However, most of http://jmg.bmj.com/ male parent. Stable transmission of the the dynamics of the unstable CTG trinucleo- asymptomatic/mild phenotype showed a tide repeat are not yet fully understood. female transmission bias. The exact implications of anticipation for We further examined the extent and DM families are still Department of unclear. As a rule the Clinical , causes of gene loss in this pedigree. Gene anticipation cascade proceeds and the pheno- University Hospital loss in the patient group was complete, type changes from mild type to adult type to Maastricht, PO Box owing to infertility of the male patients childhood/congenital type in subsequent gen- on September 26, 2021 by guest. Protected copyright. 1475, 6201 BL with Maastricht, The adult onset disease and the fact that erations. However, it is not known which events Netherlands mentally retarded patients did not pro- precede the cascade, which factors trigger C E M de Die-Smulders create. Out of the 46 at risk subjects in the the start of the cascade, and what happens after V Hovers two youngest generations, only one was the J P M Geraedts childhood/congenital onset generation. found to have a full mutation. This is the Fleischer4 argued that the disease would Department of only subject who may transmit the gene become extinct in families because of gene loss Neurology, University to the sixth Hospital Maastricht, generation. No protomu- in severely affected patients owing to celibacy, The Netherlands tation carriers were found in the fourth childless marriages, mental retardation, and C J Howeler and fifth generations. Therefore it is high infant mortality. Follow up studies to highly probable that the DM gene will be Department of determine whether the disease indeed disap- Neurology, de Wever eliminated from this pedigree within one pears from a given family have not been per- Hospital, Heerlen, generation. The high population fre- formed. We had the opportunity to re-examine The Netherlands quency of DM can at present not be a J F Mirandolle large DM family, the first three generations of explained by the contribution of asymp- which were described by de Jong"7 in 1955, and Department of tomatic cases in the younger generations so we could gain data on five generations. The Human Genetics, of known families, but is probably caused aim was to University Hospital study the consequences of anticipa- Nijmegen, The by the events in the ancestral genera- tion over more generations than can be studied Netherlands tions. in transverse studies. The inter- and intrage- H G Brunner nerational phenotypic variation was studied by H Briiggenwirth (J Med Genet H J M Smeets 1994;31:595-601) combining clinical and DNA data. The repro- ductive fitness of male and female patients was Correspondence to Dr de Die-Smulders. compared. To examine whether progressive Received 21 March 1994 Myotonic dystrophy (DM) is an autosomal severity leads to the extinction of the disease Accepted for publication dominant disorder with myotonia and muscular from a given pedigree, descendants of the third 18 April 1994 weakness as diagnostic features. Apart from the and fourth symptomatic generations were 596 de Die-Smulders, Howeler, Mirandolle, Brunner, Hovers, Bruggenwirth, Smeets, Geraedts studied, and the extent and causes of gene loss disease status of the intermediate family mem- in the youngest generations were evaluated. ber(s) was unknown. One at risk subject (V.20) was included in the group of gene carriers after DNA examination showed that she had inher- J Med Genet: first published as 10.1136/jmg.31.8.595 on 1 August 1994. Downloaded from Methods ited the expanded CTG repeat from her affec- FAMILY ted father. Eighteen out of the 36 gene carriers In 1955 de Jong'7 described an extended DM were no longer alive at the time of the study. family with three symptomatic generations (pedigree G). He gave detailed information on 50 family members, 16 of whom were desig- EXAMINATIONS nated as affected. Neurological examination and Careful histories from the patients or their slit lamp examination by an ophthalmologist family members or both were obtained to docu- were used as diagnostic methods. Electromyo- ment the age of onset and current clinical graphy was not available then. We carefully symptoms. When available, clinical records reviewed his data and only subjects showing were reviewed. Muscular weakness or myotonia definite signs of DM were classified as affected. or both, myotonic cataract, neonatal symptoms, We added data on the descendants of III.12, or mental retardation defined the onset of dis- who were examined by de Jong in 1958 (figure). ease. Neurological examination was performed II.2 and her descendants are not included in using a standardised form. Ophthalmological this follow up study, because she was not ex- assessments were carried out using a portable amined by de Jong. III.8, III.9, and III.10 and slit lamp. EMG examination was only per- their children were found to be normal in 1955. formed when the examination took place in They live abroad and were excluded from the hospital. present study. The living members of genera- tions IV and V were re-examined and so we were able to obtain data on five generations. CLINICAL CLASSIFICATION This follow up study was started with the To compare the disease types within and patients and at risk family members already between generations four clinical categories known to the Departments of Clinical Genetics were used, adapted from the classification of and Neurology of the University Hospital Harley et aP: mild, adult, childhood, and con- Maastricht and the Department of Neurology genital types. We added a category of additional of the De Wever Hospital in Heerlen, the later symptoms, because several of the present Netherlands. They were asked to approach patients have had symptomatic DM for over 40 their relatives. Most family members were years (table 1). Conclusions regarding the dis- visited and examined in their own homes, ease type were made on the basis of the results which turned out to have a significant advan- of our recent examination in combination with tage in terms of cooperation in the study. the data from medical records and de Jong's The total study comprises data on 81 family descriptions. members: initially there were 35 symptomatic http://jmg.bmj.com/ patients or obligate gene carriers (including de Jong's patients) and 46 at risk subjects. The at DNA ANALYSIS risk group comprised asymptomatic first degree Chromosomal DNA was isolated from peri- relatives of DM patients. Second and third pheral blood cells from 16 patients and 39 at degree relatives were defined as at risk if the risk subjects. Molecular analyses were 1 on September 26, 2021 by guest. Protected copyright.

11

III

IV

V 323 2.5 0.9

LI 0 Normal on clinical/DNA examination El ( Asymptomatic gene carrier !l O Mild type [] Adult type In Childhood type ** Congenital type E9 Not examined

Pedigree of the family. The numbers beneath the symbol denote the size (kb) of CTG repeat expansion by Southern blot analysis. Anticipation resulting in elimination of the myotonic dystrophy gene 597

Table 1 Disease types in DM related to age at onset and core symptoms Disease Age at Initial symptoms Additional later symptoms type onset (y) J Med Genet: first published as 10.1136/jmg.31.8.595 on 1 August 1994. Downloaded from Mild > 50 Cataract or iridescent Myotonia lens opacities Mild weakness Adult 10-50 Myotonia Progressive weakness Muscular weakness Inertia, hypersomnia Testicular atrophy Cataract Childhood 1-10 Neonatal symptoms absent Muscular weakness Speech/learning difficulties Varying degree of mental or mental retardation retardation No myotonia before school age (abdominal symptoms) Congenital < 1 Reduced fetal movements Initial improvement of weakness Polyhydramnios Psychomotor retardation Neonatal problems: hypotonia, No myotonia before school age facial diplegia, (abdominal symptoms) joint immoobility, respiratory insufficiency

performed according to methods described pre- showed that he carried the DM mutation, con- viously.912141819 Southern blot analysis was per- taining a 0 3 kb expansion. Electromyography, formed using restriction enzyme HindIII and that happened to be performed some years probe pGB2.6 respectively. In those samples previously, showed myotonic discharges. Based where smears were detected, the middle of the on these results he was classified in the mild smear was sized. Expansions <035 kb were group. This is in sharp contrast to his five also sized on polyacrylamide gels, after PCR affected sibs, who have (early) adult onset dis- amplification using primers flanking the CTG ease with expansions of 2 to 2-5 kb. The age of repeat.'4 All molecular results were expressed in the living patients in generation V is between 31 terms of kilobases of additional DNA. and 41 years. In this generation, four patients had congenital disease, two of whom died in the neonatal period, and two patients have child- Results hood onset disease. The 16 year old daughter of PEDIGREE IV.46 (V.20) was asymptomatic on clinical and The abbreviated pedigree is depicted in the slit lamp examination, but DNA studies figure; only gene carriers and their descendents showed that she has inherited the mutated and at risk persons are shown. The disease paternal chromosome, containing an expansion types are given in the pedigree. Generations II, of 0 9 kb. She is of normal intelligence and is III, and most members of generation IV were attending secondary school; we presume she

studied in 1955 or 1958. All subjects of genera- will develop (late) adult onset disease. http://jmg.bmj.com/ tions I, II, and III are dead. The common ancestor of generation II (L.1) was born in 1833 and died at the age of 84. According to his INTERGENERATIONAL DIFFERENCES IN children he never had visual or muscular com- PHENOTYPE plaints. Thus he was probably an asymptomatic In the different branches a variable number of gene carrier. In generation II, two subjects have generations with mild disease was observed. had cataract operations (II.3, II.4). When ex- The mild type was most frequently present in on September 26, 2021 by guest. Protected copyright. amined aged 80 and 76 years respectively, they two generations and the transition from the showed no muscular signs. Subject IL.1 died at mild to the classical type occurred from genera- the age of 26, a few days after the birth of her tion III to IV; 72% of the gene carriers in sixth child. In generation III, four patients had generation IV had the adult onset type. In one mild disease and two had the adult type, trans- branch this transition occurred one genera- mitted by their father. III.12 had no muscular tion earlier (offspring of II.3). In another case signs when examined at the age of 54. Slit lamp the transition was one generation later (trans- examination, however, was not performed. mission line II.4-III.12-IV.46-V.20). This Patients still alive in generation IV are now unequal number of generations with mild dis- aged 38 to 69 years. The most frequent type in ease in the different branches resulted in the generation IV is adult onset disease, with a few occurrence of the adult type in three different exceptions: one male is congenitally affected generations: III, IV, V (assuming V.20 will (IV.40) and three patients have the childhood have adult type disease). Also as a result of this form (IV.23, IV.29, IV.38); in all four cases the asynchronous anticipation a large intragenera- disease was transmitted by their affected tional variation in disease types was found, mother. These four patients with early onset becoming most obvious in generation IV, in disease are aged 46, 64, 47, and 56 years re- which patients with four different disease types spectively. They are still able to walk with are present. walking aids, despite having had symptoms for Disease types were compared in 32 parent- more than 40 years. They are moderately men- child pairs in generations II to V: 16 mother- tally retarded and not able to live indepen- child pairs and 16 father-child pairs. For this dently. One gene carrier in generation I.V analysis we combined the asymptomatic and (IV.46) has minor symptoms. This 52 year old mild patients and designated them as having the man was found to be normal on clinical and slit mild type, because inclusion in one group or the lamp examination. However, DNA analysis other strongly depends on the age at which a 598 de Die-Smulders, Howeler, Mirandolle, Brunner, Hovers, Bruggenwirth, Smeets, Geraedts

subject was examined. We also combined the Thirty eight of these 46 subjects were proven childhood and congenital onset groups, because not to be gene carriers; they showed no clinical the prognosis of these patients is essentially abnormalities and DNA analysis showed that determined by their mental retardation. Paren- they had inherited the normal allele from their J Med Genet: first published as 10.1136/jmg.31.8.595 on 1 August 1994. Downloaded from tal disease type was mild/asymptomatic in 24. affected (grand)parent. One at risk subject Six children of a parent with mild disease also (V.20) was found to be carrying the DM gene had mild disease; in five of these six stable with an expansion of 0 9 kb. Four subjects transmissions of the mild type the mother was (IV.8, IV.33, IV.36, V.11) refused to cooperate the transmitting parent. Sixteen children of a in the study. In three subjects (V.2, V.15, V.19) parent with mild disease had the adult type. In DNA analysis could not be performed. As they 15 of these 16 mild to adult transitions the gene were normal on clinical examination, including was transmitted by a male gene carrier. In two slit lamp examination and electromyography, cases maternal transmission of the mild type their residual risk was 8%.20 results in direct transition to childhood disease in her offspring (III.5-IV.23 and III.5-IV.29). III.5 was 50 years old when examined by de CAUSES OF GENE LOSS Jong in 1955. At that time her only symptoms We found three causes of gene loss: infertility, were a slightly myopathic face and iridescent infant mortality, and the fact that mentally lens opacities on slit lamp examination. Unfor- retarded patients did not procreate. The ferti- tunately DNA studies could not be performed lity of the patients, as defined by disease type in this woman. There were eight parent-child and by sex of transmitting parent, is given in pairs in which the parent had adult onset dis- table 3. A remarkable difference between the ease. The parents were all female. These sexes was observed in the adult onset group. women had no offspring with adult onset dis- Only one of the nine women with adult onset ease; all their children were normal or had DM was childless, while the other eight women childhood/congenital disease. None of the had a total of 19 children. These females are males with adult onset disease had offspring. now all middle aged and have completed their families. In contrast none of the six adult onset males has produced children; they are between CORRELATION OF DISEASE TYPE AND REPEAT SIZE 39 and 63 years of age and are all single, which DNA data were obtained from 17 DM gene makes it unlikely that they will produce off- carriers, with the following disease types: mild spring in the future. Childlessness of the male (1), adult (8), childhood (5), congenital (2), and patients with adult onset disease accounted for asymptomatic (1). The repeat sizes are given in 35% of the gene loss in generations IV and V. the pedigree (figure). Congenitally and child- Infertility of the female patients with adult hood affected patients showed the largest onset disease accounted for only 6% of gene bands. Patients with repeat sizes of approxim- loss. Patients with congenital or childhood dis- 2 5 kb were found in the adult and ease are all mentally retarded, unable to live ately group http://jmg.bmj.com/ in the childhood and congenital onset group. independently, and childless. They are beyond Vertical transmission of the CTG repeat could the age of 30 and are also unlikely to have only be studied in one father-daughter pair offspring in the future. Childlessness owing to (IV.46 and V.20); repeat lengths were 0 3 and mental retardation was responsible for 47% of 0 9 kb respectively. the total gene loss. Infant mortality in this family involved two children with congenital DM in generation V and accounted for 12% of AT RISK GROUP the gene loss. on September 26, 2021 by guest. Protected copyright. Forty six asymptomatic at risk persons were The effects of the different causes of gene loss examined. Their status is given in table 2. are also reflected in the number of patients in each generation, as given in table 4. The largest number of patients is found in the third symp- tomatic generation (generation IV). The first Table 2 Residual risk of being a gene carrier in at risk group (n = 46) decline in the number of gene carriers, after Risk generation IV, is mainly owing to infertility of Excluded Affected 50% 25% 8% the male patients with adult type DM. The second expected decline, after generation V, is No 38 1 1 3 3 because of the childlessness of childhood/con-

Table 3 Number of children of DM gene carriers aged over 20 years (n = 35) by disease type and sex of the transmitting parent Sex of parent Female Male Disease type of parent No patients No children No patients No children Asymptomatic 1 6 2 17 Mild 2 11 5 18 Adult 9 19 6 0 Childhood 3 0 2 0 Congenital 2 0 3 0 Total 17 36 18 35 Anticipation resulting in elimination of the myotonic dystrophy gene 599 Table 4 Number ofgene carriers by generation mild to the childhood type observed in this Generation family. Another exception was the asymptoma- tic male III.12 who had one son with mild I II III IV V VI disease as well as five children with adult onset J Med Genet: first published as 10.1136/jmg.31.8.595 on 1 August 1994. Downloaded from No of gene carriers 1 3 7 18 7 1 or 2 disease. It can be concluded from these obser- expected vations that gender is an important, but not the only, factor determining the start of the cas- genitally affected patients of both sexes. Only cade. one female gene carrier in generation V (V.20) In eight transmission lines the classical three will be able to transmit the gene to generation step anticipation cascade was observed with VI. If the anticipation proceeds in her descen- mild, adult, and childhood/congenital type in dants as in the rest of the family, her children subsequent generations. A two step cascade, may have childhood or congenital onset disease. present 13 times, occurred for two reasons. Therefore the gene may be expected to be Firstly because of the absence of the last step of eliminated from this family within one or two the cascade (childhood/congenital type). This generations. was the case in all six male patients with adult onset disease and was because of infertility. There were also five females with adult onset Discussion disease who had no offspring with childhood/ We re-examined an extended myotonic dys- congenital onset disease. Three of these women trophy family previously documented in 1955, had only normal children and two were single and analysed the effects of anticipation in a (so far). The second reason for a two step larger number of generations than can be cascade was the above mentioned direct conver- studied in transverse studies. We collected clin- sion of the mild type into the childhood type. ical information on five generations and DNA Our observations on anticipation in a five data on the fourth and fifth generations, in generation DM family are unique, as longitudi- order to examine the probability of transmis- nal studies of DM families have not been pre- sion of DM to the sixth generation. viously reported. Transverse studies have Anticipation can be defined at two levels. At shown anticipation in the clinical sense or at the the clinical level it is a cascade of mild, adult, DNA level or both in the great majority of and childhood or congenital disease types in parent-child pairs studied.462124-26 Asynchro- subsequent generations. At the molecular level nous anticipation occurred in some of the it can be defined as a stepwise increase of the reported pedigrees as ages ofonset ofpatients in CTG repeat number. A broad correlation the same generation differed consider- between clinical anticipation and DNA expan- ably.6212527 However, the course of the cascade sion has been shown by several authors.32123 and the ultimate effects of continuing anticipa- In this extended family clinical anticipation tion can be studied more completely in longitu- occurred in all 21 vertical transmission lines dinal studies. In the transverse study of Barcel6 descending from the common ancestor I. 1. No et al,24 the stable transmission of a protomu- http://jmg.bmj.com/ protomutations (50 to 80 repeats) were found in tation (+60 repeats) through four successive the asymptomatic members of the fourth and generations was described, where symptomatic fifth generations. Thus the presumed protomu- DM occurred in other branches of their family. tation of the common ancestor must have been The authors concluded that a protomutation unstable to such a degree that anticipation was a may be passed down relatively stably through relentless process in all affected branches of the many generations and may contribute to the family. genetic reservoir of DM in the population.24 on September 26, 2021 by guest. Protected copyright. However, the anticipation cascade developed Our study does not support this supposition. asynchronously in the different branches, We found clinical anticipation in all affected mainly depending on the number of genera- branches of the family. We therefore presume tions with mild disease. The start of the cas- that once anticipation starts in offspring of a cade, that is, the transition from mild to adult given gene carrier, anticipation will occur in all type disease was most frequently seen after two affected branches descending from this gene generations with mild disease. An earlier start, carrier within a limited number of generations. after only one generation with mild disease, was As our results are based on only one extended seen in one branch (descendants of II.3), while family, more follow up studies are needed to in another vertical transmission line (descen- confirm these findings. dants of II.4) anticipation was postponed, The gender differences we found with regard because three generations with asymptomatic/ to transmission are in agreement with the re- mild disease preceded the cascade. sults of others.232428 Our clinical observation The sex of the transmitting parent appeared that the transition from mild to adult type is to be the main influence on the transition from strongly associated with male transmission con- mild to adult type. Fifteen out of the 16 transi- firms previous data from molecular and genea- tions from mild to adult onset disease resulted logical studies that the probability of initial from male transmission. In contrast, in five out expansion of the CTG repeat is greater for of six stable transmissions of the mild type a paternally than for maternally transmitted small female was the transmitting parent. An excep- mutations.232428 As an exception we twice tion was a mildly affected woman (III.5) who observed a direct transition from the mild to the had two children with childhood onset disease childhood type DM after maternal transmis- as well as a daughter with adult onset disease. sion. Similar direct mild to childhood transi- This was the only direct transition from the tions have been described previously.21126 In all 600 de Die-Smulders, Howeler, Mirandolle, Brunner, Hovers, Bruggenwirth, Smeets, Geraedts cases the mother was the transmitting parent. predisposing properties for expansion.728 These This finding seems to contradict the hypothesis metastable protomutations may be passed down that maternally transmitted small mutations are through many generations, and in this way can more stable. One may speculate that as a rule spread through the population, before the acce- J Med Genet: first published as 10.1136/jmg.31.8.595 on 1 August 1994. Downloaded from small mutations remain relatively stable during lerated cascade of anticipation starts.728 Studies transmission in the female germline, but if a on linkage disequilibrium and extensive genea- small maternal mutation becomes unstable the logical studies, such as those in northern Que- risk of larger amplification, resulting in child- bec, support this hypothesis as they strongly hood or congenital disease in her offspring, is suggest that one, or at most a few, ancestral greater.'62126 An alternative explanation in the mutations have been responsible for most cases present case might be that a larger expansion, of the disorder."-" A third possible source of with a greater propensity for further expansion new DM families are the contracted repeats in in offspring, was present in III.5 and was known families. associated with relatively mild symptoms. Recently Ashizawa et al'6 found, in a large The absent paternal transmission of congen- multicentre study, that contraction of the CTG ital DM can be explained in this particular repeat, even to a normal length, occurred in family by decreased fertility of males with adult approximately 6% ofparent-child pairs studied onset disease. Another factor contributing to and was preferentially associated with male this phenomenon is the presumed selection transmission.'6 Surprisingly, clinical anticipa- against sperm bearing large mutations. 2329 tion occurred in 48% of these pairs. At present In conclusion, anticipation in this family was, it is unclear whether these contracted repeats once started, a relentless process occurring in all can be the starting point of a new anticipation affected branches. The start of the process was cascade.'637 Follow up studies of patients with asynchronous, because the transition from mild contracted repeats and of asymptomatic fami- to adult type occurred in different generations, lies with small mutations may give valuable and was mainly but not exclusively influenced information on the factors causing instability by the sex of the transmitting parent. The and hence the start of the anticipation cascade. classical three step cascade is often shortened to We wish to thank the members of the family for their kind two steps because of different reasons, the most cooperation, Dr J G Y de Jong, former neurologist in Heerlen, for providing his family data, and Mrs T Spaans for performing frequent being infertility of males with adult the genealogical studies. onset disease. 1 Harper PS. Myotonic dystrophy, 2nd ed. London: W B We further studied DM patients and their Saunders, 1989. descendants in the fourth and fifth symptomatic 2 Koch MC, Grimm T, Harley HG, Harper PS. Genetic risks to for children of women with myotonic dystrophy. Am J generations examine the probability of trans- Hum Genet 1991;48:1084-91. mission of the gene to the sixth generation. 3 Harley HG, Rundle SA, MacMillan JC, et al. Size of the unstable repeat CTG sequence in relation to phenotype Gene loss in the patient group was complete, and parental transmission in myotonic dystrophy. Am J owing to infertility of the male patients with Hum Genet 1993;52:1164-74. adult onset disease (35%) and childlessness of 4 Fleischer B. Uber myotonische dystrophie mit katarakt: eine heriditare, familiare degeneration. Arch Ophthalmol http://jmg.bmj.com/ the mentally retarded patients with childhood 1918;96:91-133. or 5 Penrose LS. The problem of anticipation in pedigrees of congenital disease (47%). In the nearly com- dystrophia myotonica. Ann Eugen 1948;14:125-32. pletely studied group of at risk asymptomatic 6 Howeler CJ, Busch HFM, Geraedts JPM, Niermeijer MF, we no a Staal A. Anticipation in myotonic dystrophy: fact or fic- family members, found carriers of tion? Brain 1989;112:779-97. protomutation. One at risk person (V.20) was 7 Harper PS, Harley HG, Reardon W, Shaw DJ. Anticipation to a in myotonic dystrophy: new light on an old problem. Am J found be carrier of full mutation and she is Hum Genet 1992;51:10-16. the only one who may transmit the gene to the 8 Aslanidis C, Jansen G, Amemiya C, et al. Cloning of the next We essential myotonic dystrophy region and mapping of the on September 26, 2021 by guest. Protected copyright. generation. therefore conclude that putative defect. Nature 1992;355:548-51. there will be complete loss of the DM gene 9 Brook JD, McCurrach ME, Harley HG, et al. Molecular from this within one or two basis of myotonic dystrophy: expansion of a trinucleotide family generations. (CTG) repeat at the 3' end of a transcript encoding a It has already been suggested by Fleischer4 protein kinase family member. Cell 1992;68:799-808. that of DM 10 Buxton J, Shelbourne P, Davies J, et al. Detection of an progressively increasing severity unstable fragment of DNA specific to individuals with leads to the elimination of the disease from a myotonic dystrophy. Nature 1992;355:547-8. follow studies to 11 Fu YH, Pizzuti A, Fenwick RG Jr, et al. An unstable triplet given pedigree. Systematic up repeat in a gene related to myotonic muscular dystrophy. prove this hypothesis have not been performed Science 1992;255:1256-8. before. On the other hand several authors 12 Harley HG, Brook JD, Rundle SA, et al. Expansion of an pre- unstable DNA region and phenotypic variation in myoto- sume that undetected cases may exist in the nic dystrophy. Nature 1992;355:545-6. of DM fami- 13 Jansen G, Mahadevan M, Amemiya C, et al. Characteriza- younger generations symptomatic tion of the myotonic dystrophy region predicts multiple lies and contribute to the maintenance of the protein isoform-encoding mRNAs. Nature Genet DM in It is 1992;1 :261-6. gene the population."2427"' import- 14 Mahadevan M, Tsilfidis C, Sabourin L, et al. Myotonic ant to stress that these minimal cases in younger dystrophy mutation: an unstable CTG repeat in the 3' of known families have not been untranslated region of the gene. Science 1992;255:1253-5. generations yet 15 Harley HG, Rundle SA, Reardon W, et al. Unstable DNA observed. If our results on elimination of the sequence in myotonic dystrophy. Lancet 1992;339: are confirmed in other follow studies 1125-8. gene up 16 Lavedan C, Hofmnann-Radvanyi H, Shelbourne P, et al. this implies that the gene loss from known DM Myotonic dystrophy: size- and sex-dependent dynamics of families is and that the CTG meiotic instability, and somatic mosaicism. Am J complete high popula- Hum Genet 1993;52:875-83. tion frequency of DM cannot be explained by 17 De Jong JGY. Dystrophia myotonica, paramyotonia and myo- the contribution of asymptomatic cases in the tonia congenita. Thesis, University of Utrecht, 1955. 18 Brunner HG, Nillisen W, van Oost BA, et al. Presymptoma- younger generations of known families. A more tic diagnosis of myotonic dystrophy. Y Med Genet likely source of new symptomatic DM families 1992;29:780-4. 19 Shelboume P, Davies J, Buxton J, et al. Direct diagnosis of is the class of (CTG)'>3' alleles, with an overall myotonic dystrophy with a disease-specific DNA marker. frequency of 10%, which probably possess N EnglY Med 1993;328:471-4. Anticipation resulting in elimination of the myotonic dystrophy gene 601

20 Brunner HG, Smeets HJM, Nillesen W, et al. Myotonic mosaicism in myotonic dystrophy patients: involvement of dystrophy. Predictive value of normal results on clinical mitotic events in (CTG)n repeat variation and selection examination. Brain 1991;114:2303-11. against extreme expansion in sperm. Am Hum Genet 21 Ashizawa T, Dubel JR, Dunne PW, et al. Anticipation in 1994;54:575-85

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