J Med Genet: first published as 10.1136/jmg.13.6.496 on 1 December 1976. Downloaded from Journal of Medical Genetics (1976). 13, 496-500. Dicentric X isochromosomes in man R. T. HOWELL, S. H. ROBERTS, and R. J. BEARD From the Child Health Laboratories, Department of Child Health, University Hospital of Wales, Heath Park, Cardiff Summary. Four cases of Turner's syndrome are presented in which an appa- rent X isochromosome i(Xq) has been found to possess two regions of centromeric heterochromatin. It is suggested that these chromosomes were isodicentric struc- tures capable offunctioning as monocentric elements as a result ofthe inactivation of one centromere. The prevalence of mosaicism is believed to be a consequence of the dicentric nature ofthese chromosomes, and it is considered possible that a high proportion of X isochromosomes are structurally dicentric. Banding patterns showed that the exchange site involved in the formation ofthe dicentric chromosome was different in at least three ofthe cases. The possibility that certain X isochromosomes Methods were dicentric was first postulated by de la Chapelle Chromosome preparations were made from peripheral et al (1966). In 2 of 5 cases investigated they noted blood lymphocyte cultures ofall 4 patients using standard that the isochromosome had an apparently elon- procedures. Preparations from skin fibroblast cultures an gated centromeric region. In 3 cases they found (Cases 1 and 2) and ovarian tissue culture (Case 3)copyright. abnormal anaphase configurations such as bridges, were also investigated. Slides were either stained with pseudochiasmata, and lagging chromosomes, indica- aceto-orcein in order to examine chromosome morpho- tive of the presence of a dicentric chromosome. logy and to obtain counts, Giemsa-banded using a tryp- When de la Chapelle and Stenstrand (1974) re- sin digestion technique modified from Seabright (1971), 2 or C-banded by a modification of the barium hydroxide investigated the isochromosomes in which the method described by Sumner et al (1971). elongated centromeric region had been found, they showed, by C-banding, the presence of 2 regions of centromeric heterochromatin, while in 2 other cases Clinical features http://jmg.bmj.com/ they showed the isochromosomes to be monocentric. The 4 patients all presented certain features typical of Yanagisawa and Yokoyama (1975) and Cohen et al Turner's syndrome, which are summarized in Table I. (1975) have reported apparently dicentric X iso- chromosomes with 2 C-bands, while Disteche et al (1972), Therman et al (1974), and de la Chapelle Cytogenetic studies and Stenstrand (1974) have described very long (a) Chromosome counts chromosomes consisting of 2 almost complete X Chromosome counts from orcein stained preparations chromosomes including 2 regions of centromeric of the 4 cases are shown in Table II. Mosaicism for the on September 25, 2021 by guest. Protected heterochromatin. abnormal chromosome was found in Cases 2, 3, and 4. In 4 to for In Case 2, the majority of cells in the blood cultures had females referred us chromosome studies a 45,X constitution, while the skin cultures consisted because they possessed features of Turner's syn- mostly of 46,X,dic(X)(pl 1) cells. In Case 3, roughly drome, an abnormal large metacentric chromosome equal proportions of 45,X and 46,X,dic(X)(pl 1) cells was found. In all of these cases, C-banding were observed in the blood cultures, but only 46,X,dic showed 2 distinct regions of centromeric hetero- (X)(pl 1) cells were found in the ovarian cultures. In chromatin, and trypsin-Giemsa banding suggested Case 4, complements of 47,X,dic(X)(pl1),dic(X)(p11) that the abnormal chromosome was structurally were found, in addition to 45,X and 46,X,dic(X)(pl 1). isodicentric, dic(X)(qter-*pll::p1 1->qter). (b) Structure of abnormal chromosomes Orcein-stained preparations. In all 4 cases, the Received 29 January 1976. presence of an X isochromosome was indicated in meta- 496 J Med Genet: first published as 10.1136/jmg.13.6.496 on 1 December 1976. Downloaded from Dicentric X isochromosomes in man 497 TABLE I SOME RELEVANT CLINICAL FEATURES OF THE FOUR CASES Age at Secondary Serum LH Case Reason for Investi- Height Weight Intelli- Webbing Sexual Amenor- and FSH Other No. Investigation gation (cm) (kg) gence of Neck Characters rhoea Gonads Levels Features (y) 1 Hypogonadism 17 140 60 Normnal Absent Absent Primary Not LH 30IU/1 Polydac- investi- FSH 80IU/1 tyly; gated horse- shoe kidney 2 Short stature; 12 132 35 Normal Present (Pre- (Pre- Not FSH 19.4 Pigmented increased pubertal) pubertal) investi- IU/1 naevi; carrying angle gated emotional 3 Short stature; 17 131 41 Normal Absent Absent Primary Streak LH 50IU/1 lack of ovaries FSH 801U/1 secondary vestigial sexual uterus characters 4 Primary 18 142 58 Normal Absent Slight Primary Streak Not investi- amenorrhoea breast ovaries gated develop- vestigial ment uterus after oestrogen therapy TABLE II CHROMOSOME COUNTS FROM THE 4 CASES Number of Mitoses Case No. Tissue < 45, random 45,X 46,X,dic(X)(pl 1) 47,X,dic(X)(pl 1), Total loss dic(X)(pll) copyright. [Blood 4 0 76 0 80} 92 Skin 2 0 10 0 12 Blood 3 66 11 0 801 2 ~112 lSkin 3 4 25 0 32) Blood 3 40 57 0 100) 3 ~~~~~~~~~~~~~~~~~~~128 tOvary 2 0 26 0 28) Blood 11 17 72 5 105 http://jmg.bmj.com/ phases having 46 chromosomes, because there appeared G-banded preparations. Examples of the abnor- to be 7 A group and 15 C group chromosomes. In mal X chromosome from the 4 cases after trypsin- Cases 1, 2, and 3, the abnormal X did not have the ap- Giemsa treatment are shown in Fig. 2, and diagrams of pearance of a dicentric chromosome. In Case 4, how- their banding pattems in Fig. 3. In Cases 1 and 2, the ever, it was recognizable in 36 out of 72 cells by its banding pattem was found to be asymmetrical about the dicentric morphology (Fig. 1A), or the presence of a centromeric constriction, thus showing that the abnormal secondary constriction (Fig. 1B). chromosomes were not classical isochromosomes. The shorter of the two arms was a normal long arm (Xq), on September 25, 2021 by guest. Protected C-banded preparations. Three C-banded abnor- while the longer consisted of a normal Xq with addi- mal X chromosomes from each of the cases are shown in tional material proximal to the centromere. This Fig. 2. Two regions of centromeric heterochromatin material included a prominent darkly staining band are clearly indicated in all of the examples. In Cases 1, which was interpreted as representing an interstitial 2, and 3, one C-band was at the centromeric constriction centromeric region because it corresponded in position and one was interstitial. The interstitial C-band was to the interstitial C-band, and was the same distance often separated into a pair of darkly staining spots, one from the landmark band Xq21 (Paris Conference, 1971; on each chromatid. In Case 4, the abnormal chromo- Fig. 3b) on the abnormal arm as was the normal centro- some sometimes had 2 centromeric constrictions, each mere from the landmark band Xq21 on the normal arm. with a C-band (Fig. 2 s and t). Fig. 2u shows 2 abnor- In Case 2, a narrow dark band was sometimes visible mal X chromosomes from a cell with 47 chromosomes. midway between the two centromeric bands (Fig. 2 j and The distance between the C-bands was consistent k). This band is believed to be equivalent to the fine within a case, but varied between cases. sub-band occasionally shown by our banding technique J Med Genet: first published as 10.1136/jmg.13.6.496 on 1 December 1976. Downloaded from 498 Howell, Roberts, and Beard arm of an X chromosome. Chromosomes of this kind may be referred to as isodicentric (Darlington and Wylie, 1953), and their most probable mode of origin is by an isochromatid break of the short arm of an X chromosome close to the centromere, with sister union of the chromatids proximal to the centromere. Their formation in other ways such 'W as translocation between two X chromosomes, or meiotic crossover within a paracentric inversion loop cannot, however, be discounted. It is prob- able that the other abnormal X chromosomes with 2 C-bands already reported (Disttche et al, 1972; 'z de la Chapelle and Stenstrand, 1974; Therman et al, 1974; Yanagisawa and Yokoyama, 1975; Cohen et al, 1975) were similarly isodicentric, but formed by breakage and sister union at different sites. A In the present cases, banding showed that dif- ferent break points were involved in the formation of the abnormal chromosomes. These are shown ~ ~~~~~~~~~~~~~~~~.@ in Fig. 3. In G-banded preparations of Case 2, the ,'; '... narrow dark band sometimes visible midway be- .. *...::::i .. tween the 2 centromeric bands (Fig. 2j and k; Fig. 3) indicated that the break point was further from the |~~~~~~~~~~~~~~~~~~~~~~~...., .'.... centromere than in the other cases where this band was absent. In Case 3, C-banding showed that the & break point was closer to the centromere than in the other cases. The length of the intercentric region .ww copyright. in Cases 1 and 4 was similar, indicating that, in these t 2 cases, the break points were in a similar position. B,M It is generally accepted that most dicentric chro- mosomes are unstable for two reasons. Firstly, the ,~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ rearrangement of chromosome material and subse- quent loss of the acentric fragment results in an imbalance which must be compatible with survival chromosome to (In the 4 FIG.