A woman with multiple congenital anomalies, mental retardation and mosaicism for an unusual translocation chromosome t(6;19)

The study of genetic syndromes in a rural setting

The syndromal and genetic biology reported and reviewed herein can be studied, analyzed and reported by any "GP" with the required gifts, enthusiasm, drive, and ability to work with collaborators of goodwill at University centers near or far; and most importantly, to continue lifelong education and retraining. Beginning individually in rural Boulder, MT in 1947 it was possible to train in phenotype analysis with methods available to any GP, somewhat later to enlist collaborators at the Universities of Wisconsin and Washington, and finally to establish a genetic services program at a regional medical center (Shodair Children's Hospital in Helena) with fiscal support from the State Legislature amending and extending the prior Newborn Screening Act of Montana. With such financial stability it was possible to attract another physician, genetic counselors and a cytogeneticist to the Shodair Program. This genetic center now has expanded to a staff of 22 with advanced capabilities in cytogenetics, biochemistry and molecular biology (q.v. Elias in this issue). In these past 50 years then I have seen the Montana Genetics Program grow from humble rural beginnings to the amazing center it is now providing statewide outreach services, genetic education and the most advanced diagnostics and research. Now, it may not be inappropriate for me to recommend the Montana model for implementation in other genetically underserved regions throughout the United States.

Mixed gonadal dysgenesis in a patient with de novo tas(Y;19)(p11.3;q13.4) and 45,X mosaicism

We report a patient with a de novo telomeric association between chromosomes 19 and Y in conjunction with mixed gonadal dysgenesis. The patient was first admitted to the clinic because of abnormal external genitalia. Laparoscopic evaluation revealed (1) a rudimentary uterus, one fallopian tube, and a small gonad resembling an ovary on the right side, and (2) an immature fallopian tube, a vas deferens, and a gonad resembling a testis on the left side. Conventional cytogenetic analysis performed on cultivated peripheral blood cells, and tissue obtained from the phallus and a gonadal structure which resembled a testis revealed two different cell lines with the 46,X,tas (Y;19)(p11.3;q13.4) and 45,X karyotype. Y chromosome microdeletion analysis showed that the patient did not have any genomic deletions in the AZFa, b, c, or SRY regions on the long arm of the Y chromosome. This is the first report of a patient with mixed gonadal dysgenesis that is accompanied by a telomeric association between chromosomes 19 and Y with 45,X mosaicism.

Mosaic telomeric (2;14) association in a child with motor delay

In a 6-year-old girl referred because of mild motor delay and hyperextensible joints, chromosome analysis disclosed a derivative chromosome consisting of end-to-end fusion of chromosomes 2 and 14. Two cell lines existed in which this telomere association was present, one with a 45,XX,tas(2;14)(q37;p11) karyotype and one with a 45,XX,tas(2;14) (q37;q32) karyotype. The cell line with the telomeric fusion of 2q and 14p was present in 90% of the cells; a telomeric fusion of 2q and 14q was seen in the remaining 10% of the cells. In both association complexes, only the centromere of chromosome 14 was active. Fluorescence in situ hybridization with telomere and subtelomere probes disclosed no deletion of chromosomal material. Microsatellite analysis showed that the patient had a normal biparental contribution of chromosomes 14.

Direct karyotyping of unstimulated newborn blood: a rapid diagnostic method for the clinical management of critically ill newborns

Using spontaneously dividing nucleated erythrocytes present in newborn cord and peripheral blood, we performed direct karyotype analysis on a cohort of 162 infants suspected of chromosome abnormalities. A cytogenetic diagnosis was obtained in 149 cases (91.9%). In all cases conventional phytohaemagglutinin- (PHA)-stimulated cultures were used for comparison. Concordance between direct and stimulated karyotypes was seen in all but 5 cases. In these 5 cases, abnormalities were seen in the direct harvest while PHA-stimulated cultures showed normal results. Skin fibroblasts from 2 of these cases, available for follow-up, showed the abnormalities in a mosaic state. Our experience confirms that direct karyotyping of fetal and newborn blood is feasible, fast, and efficient and can provide accurate diagnosis of major chromosome abnormalities within 18-24 hours after obtaining the blood.

Cytogenetics of Bloom's syndrome

The quantitative aspects of Bloom's syndrome cytogenetics are reviewed. The most characteristic feature is an increased rate of homologous chromatid exchange, both sister chromatid exchange and mitotic crossing-over. Other phenomena are a tendency of somatic cells to fuse, an increased rate of chromosome breaks, often with sister chromatid reunion, formation of nonhomologous quadriradials, and occurrence of allocyclic and triradial chromosomes. Mitotic chiasmata are situated highly nonrandomly, preferably in Q-dark regions. Chromosomes containing chiasma "hot-spots" appear to contain more active genes than similarly sized control chromosomes. They also contain a high proportion of localized oncogenes. Bloom's syndrome homozygotes show a high incidence of cancer (1/4). This may depend on a) the high rate of homozygosity resulting from mitotic crossing-over, which would allow the expression of recessive cancer genes; b) unequal crossing-over would amplify these genes; c) chromosome structural changes that might transfer oncogenes to new locations and, thus, activate them; and d) immunodeficiency, which would promote malignant growth.

Familial transmission of a non-Robertsonian translocation dicentric

A second family showing transmission of a dicentric through three generations involving chromosomes 13 and 18 is presented. Features of non-Robertsonian dicentric chromosomes are presented and discussed.

Dicentric chromosomes and the inactivation of the centromere

The origin and behavior of human dicentric chromosomes are reviewed. Most dicentrics between two nonhomologous or two homologous chromosomes (isodicentrics), which are permanent members of a chromosome complement, probably originate from segregation of an adjacent quadriradial; such configurations are the result of a chromatid translocation between two nonhomologous chromosomes, or they represent an adjacent counterpart of a mitotic chiasma. The segregation of such a quadriradial may also give rise to a cell line monosomic for the chromosome concerned (e.g., a 45, X line). Contrary to the generally held opinion, isodicentrics rarely result from an isolocal break in two chromatids followed by rejoining of sister chromatids. In this case the daughter centromeres go to opposite poles in the next anaphase, and the resulting bridge breaks at a random point. This mechanism, therefore, leads to the formation of an isodicentric chromosome only if the two centromeres are close together, or if one centromere is immediately inactivated. Observations on the origin of dicentrics in Bloom syndrome support these conclusions. One centromere is permanently inactivated in most dicentric chromosomes, and even when the dicentric breaks into two chromosomes, the centromere is not reactivated. The appearance and behavior of the "acentric" X chromosomes show that their centromeres are similarly inactivated and not prematurely divided. Two Bloom syndrome lymphocytes, one with an extra chromosome 2 and the other with an extra chromosome 7, each having an inactivated centromere, show that this can also happen in monocentric autosomes.

Telomeric association of chromosomes in B-cell lymphoid leukemia

About 20% of leukemic bone marrow cells from each of two patients with B-cell lymphoid leukemias showed apparent translocations which appeared to be the result of telomeric association. In one patient, whole chromosomes were associated telomere to telomere in pairs; in the other patient, telomeres of whole chromosomes were associated with breakpoints located close to the proximal or distal ends of the heterochromatic band 1q12. Repeated base sequences, particularly (CA)n sequences, are believed to be the basis of telomere pairing, and likewise repeated base sequences of heterochromatin may explain the association of 1qh and telomeres. Telomeric association may be considered as a potential origin of new stable cytogenetic combinations that have a role in oncogene transposition and tumor etiology.

Cytogenetic studies of a patient with mosaicism of isochromosome 13q and a dicentric (Y;13) translocation showing differential centromeric activity

A case is presented in which both an isochromosome and a dicentric translocation with differential centromere activity are found in one individual. Three karyotypes are present: 46,XY, -13, + i(13q)/45,X, -13, + psu dic(13)t(13;Y)/45,X, -13, + psu dic(Y)t(Y;13). The isochromosome 13q is found in 23% of cells in blood and 5% in skin. The dicentric (Y;13) chromosome in all of the remaining cells displays differential centromeric activity; the ratio of cells with the active 13 centromere to the active Y centromere is about 3.5:1. The formation of the isochromosome 13q was a de novo gametic event. The translocation producing the dicentric occurred after fertilization with the breakpoints at band Yq12 and the juxta-centromeric region of the isochromosome 13. The finding of differential centromeric activity in this chromosome indicates that centromere inactivation is not always permanent in a dicentric translocation.

Single Cd band in dicentric translocations with one suppressed centromere

Six human dicentric translocations involving the presence of a suppressed centromere were studied: one case of t dic(9;22), one of t dic(13;18), one of t dic(14;15), and three of t dic(13;14). All exhibited chromatid separation at an acrocentric centromere and were demonstrated to have two regions of centromeric constitutive heterochromatin. The four that were available for retrospective study showed a single Cd band, with absence of the Cd band at the suppressed centromere.

18Q - syndrome resulting from a tdic(14p; 18q)

A case of 18q- syndrome due to a de novo tdic(14p;18q) is presented. The interest of this observation lies in the rarity of stable dicentric chromosomes arising from reciprocal translocations between autosomes.

A girl with mosaicism for a dicentric X chromosome (45,X/46,X,dic(X) (Xqter to p22::p22 to qter))

A 12-year-old girl was examined for growth retardation and a few very discrete dysmorphologic stigmata of Turner's syndrome; the genitalia were infantile yet both ovaries possessed functioning follicles. R- and C-banding techniques and Brdu treatment demonstrated a 45,X formula in 95% of lymphocytes, with 5% presenting a 46,X,dic(X) formula. Cytogenetic and clinical problems raised by this observation are discussed in relation to data from the literature.

Tissue limited mosaicism for unbalanced autosomal translocation in a child with congenital anomalies and mental retardation

We studied a patient with a sporadic mental retardation/multiple congenital anomalies syndrome. Chromosome analysis showed a 46,XX, inv(9)(p 11;q13) karyotype in all lymphocytes. Fibroblasts from two separate skin biopsies revealed a mosaic karyotype. Some 22.5% of fibroblasts had a karyotype like that of the lymphocytes, while 77.5% of fibroblasts had a karyotype 46,XX,inv(9)(p11;q13),der(12),t(12;?)(P13;?). The data in this case emphasize the drawbacks of confining cytogenetic analysis to lymphocytes.

Fusion of two apparently intact human X chromosomes

Cytological studies have been presented from a 15-year-old girl with short stature and failure of puberty. Buccal mucosa preparations revealed X-chromatin mass approximately double in size of that of a normal female. Leukocyte metaphases suggested a two cell line composition of the patient. One population of cells conformed with 45,X chromosome distribution. The chromosome complement of her other cell line had a modal number of 46. In this cell line a "C" chromosome was replaced by an exceptionally large submetacentric chromosome. This abnormal element exhibited late DNA replicating pattern. G-banding study revealed that the abnormal chromosome was produced as a result of fusion involving telomeric ends of long arms of 2 intact X chromosomes. This translocation X was bearing 2 C-banded areas; one around the centromere and the other at the distal end of the long arm. The distal C-band area did not show any evidence for centromeric function. It appears that a centromere becomes latent in the presence of another centromere in a translocation bearing 2 total chromosomes. Such a change of state in the additional centromere is vital for the stability of the translocation chromosome.

Biochemical and cytogenetic studies on the nucleolus organizing regions (NOR) of man. II. A family with the 15/21 translocation

The amount of ribosomal DNA (rDNA) was determined quantitatively by RNA-DNA hybridization in the genomes of a mother and her daughter, both with the karyotype 45,XX,t(15q21q). The saturation values found were 0.030% (mother), 0.023% (daughter), and 0.022% for the husband and father of the daughter. A detailed cytogenetic analysis of the short arms of the acrocentric chromosomes of these probands allowed the biochemical results to be interpreted in terms of the size of the individual set of nucleolus organizing regions (NORs) present in each proband. The correlation existing between the biochemical and the cytogenetic findings shows that the amount of rDNA in the human genome is not primarily a function of the number of acrocentric chromosomes, but depends on the individual combination of variant NORs occurring in the human genome.