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Editor—Partial trisomy mosaicism describes the presence of a normal cell line together with an unbalanced translocation in a second cell line. Its incidence is not known. Only a few cases have been published,1 almost all with developmental delay and a pattern of dysmorphism. The presence of a normal cell line points towards postzygotic formation, but the origin and mechanism of formation have so far only been investigated in one case of partial trisomy 16p mosaicism and in another case of partial trisomy 21q mosaicism.2 3 In the former, a complex formation by trisomy first, translocation second, and uniparental disomy and partial trisomy third was inferred. In the latter, paternal meiotic origin of der(21;21)(q10;q10) mosaicism (46,XX/46,XX,der(21;21)(q10;q10),+21) in a girl with mild Down syndrome was described.
Here, we report on a 25 year old woman with mental retardation, dysmorphic features, partial trisomy 11q13→qter mosaicism (46,XX, der(19)t(11;19)(q13;p13.3)/46,XX), maternal uniparental isodisomy 11q13→qter in the normal cell line, and two maternal and one paternal segment(s) 11q13→qter in the abnrmal cell line.
The female patient is the second child of a healthy, unrelated, white couple. An older brother is healthy. At the proband's birth, her mother was 38 years old and her father was 39 years old. Following information, the parents opted against prenatal cytogenetic diagnosis. Delivery by caesarean section took place at 42 weeks of a normal gestation. Weight (2500 g) and length (48 cm) were below the 10th centile. A right inguinal hernia, ipsilateral pes equinovarus, and left hip dysplasia were surgically corrected. At the age of 6 years, height (1.7 m) was on the 10th centile, weight (22 kg) on the 75th centile, and occipitofrontal head circumference (OFC) (51 cm) on the 50th centile. At the last re-examination at the age of 25 years, height (1.50 m) was below the 3rd centile and OFC (55 cm) was between the 50th and 75th centile. Dysmorphic features included deep set eyes, downward slanting palpebral fissures, broad nasal root, large nares, flat and broad philtrum, diastema of the lower incisors, small and low set ears, contractures at both elbows with inability to supinate or pronate, ulnar deviation of both hands at the wrists, slender fingers, clinodactyly of fingers IV and V, and small feet with hypoplastic nails and partial 2/3 syndactyly (fig 1). Severe developmental delay was obvious.
Chromosome analysis was performed on GTG banded fibroblast and lymphocyte cell cultures at a level of about 550 bands according to standard procedures. Fluorescence in situ hybridisation (FISH) was done according to the manufacturer's instructions (Vysis Inc, Downers Grove, IL, USA).
For molecular investigations, DNA was extracted from blood and cultured fibroblasts of the patient and from blood of both parents. PCR amplification of highly polymorphic microsatellites (Research Genetics®, Huntsville, AL, USA) was carried out under standard condition. Bands were visualised by silver staining.
For a more detailed investigation of the origin of the normal chromosomes 11 and of the segment 11q13→qter translocated onto one chromosome 19, a new technical approach recently developed in our laboratory was applied.4 Briefly, the derivative chromosome 19 was microdissected from 10 metaphases according to standard procedures, the dissected chromosomes collected in a PCR tube containing 2 μl collection drop solution (10 mmol/l Tris/HCl, 10 mmol/l NaCl, 3 mg/ml proteinase K PCR grade), and incubated for two hours at 60°C. Subsequently, proteinase K was inactivated at 90°C for 10 minutes. Whole genome amplification was performed according to a slightly modified (no gelatine, 500 μmol/l dNTP, 1 mmol/l MgCl, 100 μmol/l totally degenerated, 15 nucleotide long primer) primer-extension-preamplification polymerase chain reaction (PEP-PCR) protocol.5 Finally, multiple highly polymorphic microsatellites were analysed by time release PCR (0.1 mmol/l dNTP, 0.24 μmol/l primers, 1.25 U AmpliTaqGold®, 1.5-2.5 mmol/l MgCl, using 5 μl aliquots of the preamplified DNA in a final volume of 50 μl in a Techne Progene® thermocyler for 56 cycles: 60-64°C for four minutes, 94°C for one minute), run on a 6% polyacrylamide gel, and bands visualised by silver staining.
Chromosome analysis from lymphocytes at the age of 6 years showed partial trisomy 11q mosaicism (46,XX,der(19)t(11;19)(q13;p13.3) /46,XX). Twenty years later, blood chromosome analysis showed a 46,XX karyotype in 40 metaphases. In fibroblasts, mosaicism was also present (46,XX,der(19)t(11;19) (q13;p13.3)de novo/46,XX). Involvement of chromosomes 11 and 19 in the rearrangement was confirmed by FISH with libraries of chromosomes 11 and 19 (Vysis®Inc, Downers Grove, IL, USA). FISH with a 19p subtelomeric probe (Vysis® Inc) showed signals on both chromosomes 19 in the normal cell line and on the normal and the derivative chromosome 19 in the abnormal cell line, and FISH with an “all telomeres” probe (Vysis® Inc) failed to show a telomere at the translocation breakpoint. Therefore, the breakpoint on 19p must be distal to the subtelomeric locus of this probe, and the rearrangement led to duplication of 11q13→qter without significant concomitant 19p deletion in the abnormal cell line. The karyotypes of both parents were normal.
Molecular investigations performed at the age of 25 years showed maternal uniparental isodisomy 11q13→qter in DNA from nucleated blood cells (table 1). In DNA from the patient's fibroblasts, paternal bands of weaker intensity of markers mapping to 11q13→qter were found (fig2). The breakpoint was determined between markers D11S916 and D11S527. In blood and fibroblasts, biparental inheritance of chromosome 19 markers was shown. The results of investigations with several microsatellites from various other chromosomes were in agreement with correct paternity (data not shown).
Following microdissection of the derivative chromosome 19 from 10 metaphases, primer-extension-preamplification, and subsequent microsatellite analysis, only a paternal allele and no maternal allele was found at markers D11S912 and D11S940 (fig 2). The additional weak signal in DNA from the patient's blood at the level of the paternal allele was interpreted as either an artefact or as representing low level mosaicism for the der(19) chromosome.
The unexpected molecular results obtained in the present case could theoretically be explained by three different mitotic and two meiotic mechanisms of formation.
(1) Each homologue of chromosome 11 was regularly duplicated in the late interphase of a normal mitosis (fig 3, left, line 2). Both paternal sister chromatids broke and simultaneously one chromatid segment 11q13→qter was translocated to one chromosome 19, while her sister chromatid segment was lost (line 3a). In the anaphase, one normal maternal chromatid and one deleted paternal chromatid segregated to each of the two daughter cells, as well as the derivative chromosome 19 to one of the daughter cells (line 3c) and the normal chromosome 19 to the other (line 3b). Finally, mitotic reduplication of the maternal segment 11q13→qter in both daughter cells (line 4a and b) resulted in maternal segmental uniparental isodisomy 11q13→qter in the normal (line 5a) as well as in the abnormal cell line (line 5b).
(2) Again, in a normal 46,XX zygote, inheritance of chromosome 11 was biparental and each homologue of chromosome 11 was duplicated in the late interphase of a regular mitosis (fig 3, right, line 2). Subsequently, after separation of the sister chromatids, a break occurred in only one paternal chromatid at 11q13, and the segment 11q13→qter was translocated to chromosome 19. The complete paternal chromatid segregated together with the derivative chromosome 19 regularly to one daughter cell (line 3e), while the isolated part of the 11q chromatid segregated with the other daughter cell together with two normal chromosomes 19 (line 3d). Finally, mitotic reduplication of the maternal segment 11q13→qter in the daughter cell containing the deleted 11 (line 4) resulted in maternal uniparental disomy (UPD) (11q13→qter) in the now (through correction) normal cell line (line 5c) and, in contrast to the first mechanism, in biparental inheritance of the whole chromosome 11 in the abnormal cell line (line 5d).
(3) Considering the third mechanism, two independent mitotic events must be postulated (fig 4). The first is characterised by an exchange between two non-sister chromatids (line 3a) resulting in two daughter cells with opposite segmental UPD (11q13→qter) (line 6a and b). The cell line with maternal UPD (11q13→qter) would persist as shown in blood (line 6a), whereas the cell line with paternal segmental UPD (11q13→qter) would not be viable (line 6b). In another cell with biparental inheritance of chromosome 11 (line 2b), a break in either one (line 3c) or in both (line 3b) chromatids would have occurred. In the latter case, one segment would have been translocated to one chromosome 19 (line 4), whereas the other would have been lost (line 6e). The cell line with del(11)(q13qter) would not have been viable (line 6d). In the other cell line, maternal segmental UPD (11q13→qter) (line 6c) would have been formed by reduplication during interphase (line 5). In the alternative case of breaking of only one chromatid (line 3c), one cell line with del(11)(q13qter) (lethal) (line 6g) and a second cell line with biparentally inherited chromosomes 11 and an additional translocation of the paternal segment 11q13-qter would have arisen (line 6f). This partial mechanism is less likely, because the molecular investigations showed a weaker intensity of the paternal versus the maternal alleles.
(4) A fourth mitotic mechanism (fig 5) would be characterised by a normal zygote and segmental mitotic reduplication on the maternal chromosome. Then, a crossing over between one paternal chromatid and the reduplicated segment as well as translocation of the paternal segment to 19p occurred. Segregation resulted in the karyotype described.
(5) A fifth, in part meiotic mechanism would require a balanced 11;19 translocation already present in the paternal gamete (fig 6, left, line 1a). Thereafter, again a mitotic reduplication of the maternal segment 11q13→qter must have happened (line 2) and, in addition, in the normal cell line the loss of the translocated paternal segment 11q13→qter (line 5a) or even the loss of the entire derivative chromosome 19 combined with mitotic reduplication of its normal homologue must be postulated. The latter was excluded by the biparental inheritance of chromosome 19 in blood and fibroblasts (data not shown).
(6) A sixth, again in part meiotic mechanism would require a trisomic zygote resulting from maternal non-disjunction (fig 6, right, line 1b). Mitotic crossing over between the paternal chromatid and one maternal chromatid (line 4b) would result in two different daughter cells (line 5e and f). One with biparental chromosomes 11 and translocation of the paternal segment 11q13→qter to one chromosome 19 (line 5e), and a second with exclusively maternal 11q13→qter material (line 5f). In addition, in both cell lines, one chromosome must have been lost, the chromosome with del(11) (q13→qter) in one (line 5d), and one complete maternal chromosome 11 in the other (line 5g). This mechanism is supported by the advanced maternal age of 38 years at delivery; on the other hand, complete isodisomy would better fit a postmeiotic formation.
In total, each mechanism requires a minimum of three subsequent or in part simultaneous events. With the presently available molecular investigations, it was not possible to define one single possible mechanism of formation. Isodisomy in the normal cell line could be considered to indicate mitotic formation. Apart from paternal UPD (11p15) in up to 20% of cases with Beckwith-Wiedemann syndrome,6 segmental UPD has rarely been reported.7-11 Similarly, paternal UPD of the whole chromosome 11 has been described only twice, in a fetus with severe intrauterine growth retardation, intestinal malrotation, and confined placental mosaicism,12 and in a mosaic state in a girl with Beckwith-Wiedemann syndrome.13 To the best of our knowledge, maternal segmental UPD of chromosome 11 has not been reported so far. Full trisomy 11 is not viable and non-mosaic duplication of 11q13→qter has been reported only rarely. The phenotype of the latter is characterised by cardiac anomalies, restricted elbow movements including supination and pronation, facial dysmorphism, small and low set ears, and mental retardation.1
The clinical consequences of the complex rearrangement found in our patient are difficult to evaluate. The phenotype strongly resembles several cases with duplication of 11q13→qter, and thus could be caused only by this. On the other hand, an additional effect of mosaicism for the loss of the normal homologue of an imprinted gene and/or of homozygosity for a mutated allele of a recessive gene cannot yet be fully excluded.
A 25 year old woman with mental retardation, dysmorphic features, and partial trisomy mosaicism 11q13→qter (karyotype 46,XX,der(19)t(11;19)(q13; p13.3)/46,XX) is reported.
Mosaicism for the abnormal karyotype was found in blood in childhood, whereas only the normal chromosomal complement was detectable at the age of 25 years, while in fibroblasts mosaicism was present.
Molecular investigations showed maternal uniparental isodisomy 11q13→qter in the normal cell line in DNA from blood cells and a weaker additional paternal allele by analysing DNA extracted from cultivated fibroblasts.
Using a new technical approach combining microdissection, primer-extension-preamplification PCR, and subsequent microsatellite analysis, we were able to show that the der(19) chromosome contains the paternal allele in the unbalanced cell line.
We are grateful to the family for their cooperation. The study was supported by the Swiss National Foundation, grant Nos 32-45604.95 and 32-56053.98.