A somatic cell hybrid panel to facilitate identification of DNA sequences in the vicinity of the incontinentia pigmenti locus (IP1)

Somatic cell hybrids that retain derivative X chromosomes from women with sporadic incontinentia pigmenti (IP1) and de novo X/autosomal translocations with consistent breakpoints at Xp11.21 were constructed. An assembled hybrid panel was used to physically map DNA sequences in relationship to the IP breakpoint. DSX14 was found to map to region Xp11.21----p11.1. Regional assignments of 19 X-chromosomal loci were reviewed.     

Translocation Down syndrome in Ohio 1970-1981: epidemiologic and cytogenetic factors and mutation rate estimates.

Sixteen hundred eighty-eight Down syndrome live births, including 65 (5.2%) translocations, were ascertained in Ohio between 1970 and 1981. Translocations of known origin were 24.4% maternal, 2.2% paternal, and 73.3% de novo. Translocation subtypes were 14/21 (45.7%), 15/21 (2.9%), 21/21 (40.0%), 21/22 (2.9%), and other (8.5%). Among 14/21 translocations, 33.3% were maternal in origin and 66.7% were de novo, while 100% of 21/21 translocations were de novo. No differences were found when the maternal- and paternal-age distributions of all translocations or various translocation subsets were compared with the live-birth control distributions. However, mean maternal and paternal ages of de novo translocations were significantly lower than that of the live-birth controls. Ohio data showed the average maternal age of de novo D/21 cases to be significantly lower than the control. Ages of both parents of de novo G/21 cases and paternal age of D/21 cases were not different from the control. De novo translocation mutation rate estimates were 0.8 X 10(-5) for 14/21, 1.2 X 10(-5) for 21/21, and 2.2 X 10(-5) overall. Ohio estimates (3.2 X 10(-5) for 1970-1972 and 1.4 X 10(-5) for 1973-1975) did not reflect the increase in mutation rate previously found in New York during 1973-1977.     

Translocation (X;13)(p11.21;q12.3) in a girl with incontinentia pigmenti and bilateral retinoblastoma

A de novo t(X;13)(p11.21;q12.3) translocation is described in an 19-month-old girl with incontinentia pigmenti (IP) and bilateral retinoblastoma. Based on previously reported two girls and this patient, each with a structural X chromosome abnormality and IP, it was assumed that the locus for IP is at Xp11.21. Q-banding analysis revealed that the translocated chromosomes were of paternal origin. The derivative X chromosome was late-replicating in 9% of cultured peripheral blood lymphocytes and in 1% of skin fibroblasts. The erythrocyte esterase D activity in the patient was normal. Several possibilities were considered for possible causative relationship between the X/13 translocation and the development of retinoblastoma. One possibility involved functional monosomy of 13q14 in a minority of retinoblasts due to the spreading of inactivation of the translocated X chromosome segment.     

Localization of DNA sequences to a region within Xp11.21 between incontinentia pigmenti (IP1) X-chromosomal translocation breakpoints.

Incontinentia pigmenti (IP) is an X-linked dominant disorder characterized by developmental anomalies of the tissues and organs derived from embryonic ectoderm and neuroectoderm. An IP locus, designated IP1, probably resides in Xp11.21, since five unrelated patients with nonfamilial IP have been identified who possess constitutional de novo reciprocal X;autosome translocations involving Xp11.21. We have used a series of somatic cell hybrids containing the rearranged chromosomes derived from three of the five IP1 patients, along with other hybrid cell lines, to map probes in the vicinity of the IP1 locus. Five anonymous DNA loci--DXS422, DXS14, DXS343, DXS429, and DXS370--have been mapped to a region within Xp11.21, between two IP1 X-chromosomal translocation breakpoints; the IP1 t(X;17) breakpoint is proximal (centromeric) to this region, and the IP1 t(X;13) and t(X;9) X-chromosomal breakpoints lie distal to it. While no IP1 translocation breakpoint has yet been identified by pulsed-field gel electrophoretic (PFGE) analysis, an overlap between three probes--p58-1, 7PSH3.5, and cpX210--has been detected, placing these probes within 125 kb. Four probes--p58-1, 7PSH3.5, cpX210, and 30CE2.8--have been helpful in constructing a 1,250-kb PFGE map of the region between the breakpoints; these results suggest that the IP1 X-chromosomal translocation breakpoints are separated by at least this distance. The combined somatic cell hybrid and PFGE analyses we report here favor the probe order DXS323-(IP1 t(X;13), IP1, t(X;9]-(DXS422, DXS14, DXS343, DXS429, DXS370)-(IP1 t(X;17), DXZ1). These sequences provide a starting point for identifying overlapping genomic sequences that span the IP1 translocation breakpoints; the availability of IP1 translocation breakpoints should now assist the cloning of this locus.     

Variegated silencing through epigenetic modifications of a large Xq region in a case of balanced X;2 translocation with Incontinentia Pigmenti-like phenotype

Molecular mechanisms underlying aberrant phenotypes in balanced X;autosome translocations are scarcely understood. We report the case of a de novo reciprocal balanced translocation X;2(q23;q33) presenting phenotypic alterations highly suggestive of Incontinentia Pigmenti (IP) syndrome, a genodermatosis with abnormal skin pigmentation and neurological failure, segregating as X-linked dominant disorder. Through molecular studies, we demonstrated that the altered phenotype could not be ascribed to chromosome microdeletions or to XIST-mediated inactivation of Xq24-qter. Interestingly, we found that the Xq24-qter region, which translocated downstream of the heterochromatic band 2q34, undergoes epigenetic silencing mediated by DNA methylation and histone alterations. Among the downregulated genes, we found the inhibitor of kappa light polypeptide gene enhancer in B cells, kinase gamma (IKBKG/NEMO), the causative gene of IP. We hypothesize that a mosaic functional nullisomy of the translocated genes, through a Position Effect Variegation-like heterochromatization, might be responsible for the proband's phenotypic anomalies. Partial silencing of IKBKG may be responsible for the skin anomalies observed, thereby mimicking the IP pathological condition. In addition to its clinical relevance, this paper addresses fundamental issues related to the chromatin status and nuclear localization of a human euchromatic region translocated proximally to heterochromatin. In conclusion, the study provides new insight into long-range gene silencing mechanisms and their direct impact in human disease.     

Variegated silencing through epigenetic modifications of a large Xq region in a case of balanced X;2 translocation with Incontinentia Pigmenti-like phenotype

Molecular mechanisms underlying aberrant phenotypes in balanced X;autosome translocations are scarcely understood. We report the case of a de novo reciprocal balanced translocation X;2(q23;q33) presenting phenotypic alterations highly suggestive of Incontinentia Pigmenti (IP) syndrome, a genodermatosis with abnormal skin pigmentation and neurological failure, segregating as X-linked dominant disorder. Through molecular studies, we demonstrated that the altered phenotype could not be ascribed to chromosome microdeletions or to XIST-mediated inactivation of Xq24-qter. Interestingly, we found that the Xq24-qter region, which translocated downstream of the heterochromatic band 2q34, undergoes epigenetic silencing mediated by DNA methylation and histone alterations. Among the downregulated genes, we found the inhibitor of kappa light polypeptide gene enhancer in B cells, kinase gamma (IKBKG/NEMO), the causative gene of IP. We hypothesize that a mosaic functional nullisomy of the translocated genes, through a Position Effect Variegation-like heterochromatization, might be responsible for the proband’s phenotypic anomalies. Partial silencing of IKBKG may be responsible for the skin anomalies observed, thereby mimicking the IP pathological condition. In addition to its clinical relevance, this paper addresses fundamental issues related to the chromatin status and nuclear localization of a human euchromatic region translocated proximally to heterochromatin. In conclusion, the study provides new insight into long-range gene silencing mechanisms and their direct impact in human disease.     

X-linked anhidrotic ectodermal dysplasia and de novo t(X;1) in a female

Summary A de novo translocation (X;1)(q13.1;p36.33) was found in a 2-year-old girl with typical clinical features of X-linked anhidrotic ectodermal dysplasia (EDA). The breakpoint at Xq13.1 is approximately the same as has been described in 2 other EDA females with X;autosome translocations.     

Disease associated balanced chromosome rearrangements (DBCR): report of two new cases

Disease associated balanced chromosome rearrangements (DBCR) causing truncation, deletion, inactivation or over-expression of specific genes are instrumental in identifying and cloning several disease genes and are estimated to be much more common than anticipated. In one survey, the minimal frequency of combined balanced de novo reciprocal translocations and inversions causing abnormal phenotype is estimated to be 0.17%, a sixfold increase compared to the general population suggesting a causative linkage between the abnormality and the observed phenotypic traits. Here, we report two new cases of apparently balanced de novo translocations resulting in developmental delay and dysmorphic features.     

Mll fusions generated by Cre-loxP-mediated de novo translocations can induce lineage reassignment in tumorigenesis

Chromosomal translocations are primary events in tumorigenesis. Those involving the mixed lineage leukaemia (MLL) gene are found in various guises and it is unclear whether MLL fusions can affect haematopoietic differentiation. We have used a model in which chromosomal translocations are generated in mice de novo by Cre-loxP-mediated recombination (translocator mice) to compare the functionally relevant haematopoietic cell contexts for Mll fusions, namely pluripotent stem cells, semicommitted progenitors or committed cells. Translocations between Mll and Enl or Af9 cause myeloid neoplasias, initiating in pluripotent stem cells or multipotent myeloid progenitors. However, while Mll-Enl translocations can also cause leukaemia from T-cell progenitors, no tumours arose with Mll-Af9 translocations in the T-cell compartment. Furthermore, Mll-Enl translocations in T-cell progenitors can cause lineage reassignment into myeloid tumours. Therefore, a permissive cellular environment is required for oncogenicity of Mll-associated translocations and Mll fusions can influence haematopoietic lineage commitment.     

Array-CGH characterization of a de novo t(X;Y)(p22;q11) in a female with short stature and mental retardation

We report the clinical and molecular investigations in a girl with 46,X,-X,+der(X)t(X;Y)(p22;q11) de novo karyotype who presented an intricate phenotype characterized by mental retardation and facial dysmorphisms in combination with short stature. The structure of the derivative X chromosome was studied using BAC array-CGH which disclosed the Xp22 breakpoint between the STS and the VCX3A gene and the presence of the Yq11.1qter chromosome. It is common that females with Xp;Yq translocations present only short stature and are normal in every other aspect. Thus, this would be the first case in which a girl with Xp;Yq translocation presents an unusual phenotype with intermediate male clinical features with Xp;Yq translocations. The risk of developing gonadoblastoma in females with Y chromosome material is also discussed and, to this effect, different explanations related to this apparent variation are also presented.     

Parental origin of de novo cytogenetically balanced reciprocal non-Robertsonian translocations

De novo cytogenetically balanced reciprocal non-Robertsonian translocations are rare findings in clinical cytogenetics and might be associated with an abnormal phenotype. Knowledge of the parental origin and mechanisms of formation is still limited. By microdissection of the derivative chromosomes and their normal homologs from metaphases followed by microsatellite-mediated marker analysis we identified 7 cases of paternal and 3 cases of maternal origin in a cohort of 10 patients with de novo cytogenetically balanced reciprocal non-Robertsonian translocations. Neither in the maternal nor in the paternal group of our study parental age seems to be increased. Together with the data from the literature our results confirm that the majority of de novo cytogenetically balanced reciprocal translocations are of paternal origin, but the preponderance does not appear to be as distinct as previously thought and the paternal age does not seem to be necessarily a major contributing factor.     

Two novel reciprocal translocations involving chromosome 6 in the domestic pig]

In the present study 2 new reciprocal translocations are described in two hypoprolific boars. The first one, a Gascon/Meishan boar, has produced a mean value of 7.37 +/- 0.69 piglets, vs 12.41 +/- 0.22 piglets per litter in contemporaries. The second one, a Pietrain/Large-White boar, has produced a mean of 5.42 +/- 0.69 vs 12.41 +/- 0.22 piglets per litter in contemporaries. Each of these animals carries a new chromosome translocation involving chromosomes 6, 8 and 15, respectively. The first translocation seems to be a de novo occurring abnormality. The economic consequences of these abnormalities are discussed. Carrier of the gene HAL, the abnormal chromosomes 6 involved in these translocations are important, and could useful markers in gene mapping and flow cytometry studies in pigs.     

De novo balanced chromosome rearrangements and extra marker chromosomes identified at prenatal diagnosis: clinical significance and distribution of breakpoints.

A questionnaire sent to major cytogenetics laboratories in the United States and Canada over a 10-year period collected data on the frequency and outcome of cases with either apparently balanced de novo rearrangements or de novo supernumerary marker chromosomes detected at amniocentesis. Of 377,357 reported amniocenteses, approximately 1/2,000 had a de novo reciprocal translocation, 1/9,000 a Robertsonian translocation, 1/10,000 a de novo inversion, and 1/2,500 an extra structurally abnormal chromosome of unidentifiable origin. The risk of a serious congenital anomaly was estimated to be 6.1% (n = 163) for de novo reciprocal translocations, 3.7% (n = 51) for Robertsonian translocations, and 9.4% (n = 32) for inversions. The combined risk for reciprocal translocations and inversions was 6.7% (95% confidence limits 3.1%-10.3%). The risk of abnormality for extra nonsatellited marker chromosomes was 14.7% (n = 68), and that for satellited marker chromosomes was 10.9% (n = 55). In non-Robertsonian rearrangements, distribution of breakpoints among chromosomes was not as would be expected strictly on the basis of length. Most breaks were stated to occur within G-negative bands, but there was little evidence of particular hot spots among these bands. Nevertheless, there did appear to be a correlation between those bands in which breakage was observed most often and those bands where common or rare fragile sites have been described.     

De novo appearance of a translocation t(5p; 2Iq), and its transmission in both balanced and unbalanced forms to the next generation.

A family is described in which a reciprocal translocation involving 5p and 21q appeared de novo in the chromosome complement of a woman who then transmitted it in both balanced and unbalanced form to her progeny. The proposita, a child with the cri du chat syndrome, had a deficiency for most of 5p, all of 21p, 21 centromere, and a small proximal segment of 21q. The reported cases of the cri du chat syndrome associated with translocations are reviewed and discussed in relation to this family.     

Incontinentia pigmenti and X-autosome translocations

Summary Incontinentia pigmenti (IP) is a rare X-linked disease with marked female-to-female transmission and a dominant pattern of inheritance. Reports of six unrelated females with IP and X-autosomal translocations, all with the X breakpoint at Xp11, and an additional report of a female with IP and a 45,X/46,X,r(X) karyotype suggests that this may be the locus for the IP gene. When four of these cases, including the r(X), were re-examined with a non-isotopic in situ hybridization technique and an X centromere-specific probe (pSV2X5), the Xp11 breakpoint was confirmed. However, results from a fifth reported case, t(X;17), showed that the X breakpoint was within the centromeric alphoid repetitive sequences recognized by the probe pSV2X5. As the clinical presentation of this patient was consitent with the IP phenotype and diagnosis, the centromeric position of the X-chromosome breakpoint raises several questions with respect to the homogeneity of the Xp11 locus for IP.     

Turner syndrome mosaicism: an unusual case with a de novo large dicentric marker chromosome: mos 45,X/46,X, ter rea(X;X)(p22.3;p22.3)

X/X translocations are quite rare in humans. The effect of this anomaly on the phenotype is variable and depends on the amount of deleted material and whether the chromosomes are joined by their long or short arms. We report an unusual case of Turner syndrome mosaicism in a 16-year-old girl, who was referred to our Institute for primary amenorrhoea associated with short stature. Endocrine evaluation revealed hypergonadotropic hypogonadism, which required a study of the karyotype. Cytogenetic analysis, performed on peripheral blood leucocytes, showed a mos 45,X/46,X,ter rea (X;X)(p22.3;p22.3) de novo karyotype. The prevalent cell line was 45,X (90% cells). A second cell line (10% cells) showed a very large marker chromosome, similar to a large metacentric chromosome. FISH (fluorescent in situ hybridisation) and molecular analysis revealed that the marker chromosome was dicentric and totally derived from the paternal X chromosome.     

Obligate short-arm exchange in de novo Robertsonian translocation formation influences placement of crossovers in chromosome 21 nondisjunction

Robertsonian translocations (ROBs) involving chromosome 21 are found in approximately 5% of patients with Down syndrome (DS). The most common nonhomologous ROB in DS is rob(14q21q). Aberrant recombination is associated with nondisjunction (NDJ) leading to trisomy 21. Haplotype analysis of 23 patients with DS and de novo rob(14q21q) showed that all translocations and all nondisjoined chromosomes 21 were maternally derived. Meiosis II NDJ occurred in 21 of 23 families. For these, a ROB DS chromosome 21 genetic map was constructed and compared to a normal female map and a published trisomy 21 map derived from meiosis II NDJ. The location of exchanges differed significantly from both maps, with a significant shift to a more distal interval in the ROB DS map. The shift may perturb segregation, leading to the meiosis II NDJ in this study, and is further evidence for crossover interference. More importantly, because the event in the short arms that forms the de novo ROB influences the placement of chiasmata in the long arm, it is most likely that the translocation formation occurs through a recombination pathway in meiosis. Additionally, we have demonstrated that events that occur in meiosis I can influence events, such as chromatid segregation in meiosis II, many decades later.     

Jumping translocations in spontaneous abortions

Chromosome translocations involving one donor chromosome and multiple recipient chromosomes have been referred to as jumping translocations (JTs). Acquired JTs are commonly observed in cancer patients, mainly involving chromosome 1. Constitutional forms of JTs mostly involve the acrocentric chromosomes and their satellites and have been reported in patients with clinical abnormalities. Recognizable phenotypes resulting from these events have included Down, Prader-Willi, and DiGeorge syndromes. The presence of JTs in spontaneous abortions has not been previously described. The breakpoints of all JTs occur in areas rich in repetitive DNA (telomeric, centromeric, and nucleolus organizing regions). We report two different unstable chromosome rearrangements in samples derived from spontaneous abortions. The first case involved a chromosome 15 donor. The recipient chromosomes were 1, 9, 15, and 21, and the respective breakpoints were in either the heterochromatic regions or the centromeres. FISH studies confirmed that the breakpoints of the jumping 15 rearrangement did not involve the Prader-Willi region but originated at the centromere or in the proximal short arm. A second case of instability was observed with a rearrangement resulting from a presumed de novo 8;21 translocation. Three JT cell lines were observed. They consisted of a deleted 8p chromosome, a dicentric 8;21 translocation, and an 8q isochromosome. The instability regions appeared to be at the pericentromeric region of chromosome 8 and the satellite region of chromosome 21. Both cases proved to be de novo events. The unstable nature of the JT resulting in chromosomal imbalance most likely contributed to the fetal loss. It appears that JT events may predispose to chromosomal imbalance via nondisjunction and chromosomal rearrangement and, therefore, may be an unrecognized cause of fetal loss.     

A molecular genetic approach to the identification of isochromosomes of chromosome 21

Summary The largest class of de novo chromosomal rearrangements in Down syndrome are rea(21q21q). Classically, these rearrangements have been termed Robertsonian translocations, implying an attachment of two different chromosome 21 homologues. Additionally, a Robertsonian translocation between two chromosomes 21 cannot be distinguished from an isochromosome composed of genetically identical arms by cytogenetic analyses. Therefore, we have used molecular techniques to differentiate between true Robertsonian translocations and isochromosomes. Samples were obtained from 12 probands, ascertained for de novo rearrangements between homologous chromosomes 21 [11 rea(21q21q) and 1 rea (21;21)(q22;q22)], their parents (n = 24) and available siblings (n = 7). The parental origins of the de novo rearrangements were assigned using molecular and cytogenetic analyses. Although not statistically significant, there was a two-fold increase in the number of paternally derived de novo rearrangements (n = 8) as compared with maternally derived rearrangements (n = 4). To distinguish between rob(21q21q) and i(21q), we used restriction fragment length polymorphisms (RFLPs) spanning the length of chromosome 21. Using all informative and partially informative RFLPs, we used the method of maximum likelihood to assign the most likely rearrangement definition (i or rob) and parental origin in each family. The maximum likelihood estimates indicated that all rearrangements tested (n = 8) were isochromosomes. C-banding revealed two centromeres in three cases indicating that a U-type exchange occurred between sister chromatids in these rearrangements. Our results suggest that the majority of de novo rea(21q21q) are isochromosomes derived from a single parental chromosome 21.     

The frequency and mutation rate of balanced autosomal rearrangements in man estimated from prenatal genetic studies for advanced maternal age.

The frequencies of balanced chromosome rearrangements were estimated from three series of advanced maternal-age prenatal genetic studies, and were compared to the frequencies that had been estimated from consecutive newborn surveys. In the maternal-age prenatal studies, the frequencies were: Robertsonian translocations, 0.11%; reciprocal translocations, 0.17%; and inversions, 0.12%. The total frequency of balanced rearrangements in the prenatal genetic studies performed with banding (0.40%, or 1 in 250) was twice that in the consecutive newborn surveys performed without banding (0.19%, or 1 in 526). The difference was limited to inversions and reciprocal translocations; the frequency of Robertsonian translocations was similar in the prenatal series and the newborn surveys. Both familial and de novo rearrangements were more common than anticipated. The de novo cases provided a mutation rate estimate of 4.3 per 10,000 gametes per generation (compared with 1.78 to 2.2 per 10,000 gametes in other surveys). These higher estimates may more reliably approximate the true mutation rate and frequencies of balanced rearrangements in the newborn population than do the newborn surveys.