Reciprocal translocation versus centric fusion between two No. 13 chromosomes. A case of 46,XX,-13,+t(13;13)(p12;q13) and a case of 46,XY,-13,+t(13;13)(p12;p12)

Genetics in pediatric ophthalmology

Genetic factors are becoming increasingly important causes of both congenital and acquired eye disease in the pediatric age group. Referral to an ophthalmologist is important both for global genetic disorders potentially affecting the eye and for eye disorders that may be diagnostic for genetic disease.

DNA repair defects and chromosome instability disorders

Xeroderma pigmentosum (XP), Fanconi anaemia (FA), ataxia telangiectasia (AT) and Bloom disease (BS) are four rare autosomal recessive disorders in which there is defective DNA repair and/or chromosome instability and proneness to malignancy. Between 80 and 90% of patients with XP have a defect, demonstrable at cell level, of excision of DNA lesions induced by ultraviolet rays, while the remainder have a cellular error of post-replication repair. XP cells are also deficient in repairing DNA damage caused by a variety of chemical mutagens. There are at least five different complementation groups of the first, or classical, type of XP (A to D, etc.) Apparently group C patients, as well as those with defective post-replication repair, do not show the progressive neurological illness found in a proportion of the other patients. AT is heterogeneous clinically and genetically. Clinically it presents with a progressive neurological illness, progressive telangiectases and a developmental disorder of the thymus. AT is characterized by sensitivity to X-rays and AT cells are unable to repair gamma-ray-induced damage to bases in the DNA. It appears that in many cases of the disorder a chromosomally marked cellular clone is found. In BS the main defect, which results in growth retardation, sun-induced lesions of the face and susceptibility to infection, appears to be a slow DNA chain maturation during DNA synthesis. An increase of sister chromatid exchanges is characteristically seen in the chromosomes of cultured BS cells. In FA, in which there is progressive pancytopenia with eventual bone marrow exhaustion and a tendency to haemorrhage and infection, the cellular defect seems to consist of faulty removal of repair of cross-links in the DNA. In this condition, as in BS and AT, various structural chromosome changes are detected in cultured cells. Patients with XP develop skin cancers in early life and often maligant melanomas. In the other three disorders, in which an immune deficiency is often present, leukaemia and related proliferative disorders are a frequent cause of death while other malignancies also occur. There is some evidence that points to an increased risk of malignancy in heterozygotes who carry the FA and AT genes.

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.

Mosaic 13 trisomy due to de novo 13/13 translocation with subsequent fission. Karyotype: 46,XX, - 13, + t(13;13)(p11;q11)/46,XX,del(13)(p11)

A severely retarded child with multiple malformations was found to present a mosaic karyotype 46,XX, - 13,+t(13;13)(p11;q11)/46,XX,del(13)(p11), which probably originated as the result of a de novo 13/13 translocation in a parental gamete, followed by postzygotic fission of the translocation chromosome.

The fate of DNA satellites I, II, III and ribosomal DNA in a familial dicentric chromosome 13:14

In a family with a stable dicentric 13:14 translocation chromosome, the distribution of DNA sequences complementary to satellite DNAs I, II and III and ribosomal RNA were studied. The translocation chromosome showed a loss of sequences complementary to all three satellite DNAs, located in the short arms of the acrocentric chromosomes, but slightly more of the sequences complementary to satellite I were retained than of the other two satellite DNAs. The fact that material was lost from all three satellites indicates that they are not present as single discrete blocks in these chromosomes, when we would expect to find the distal sequences lost and the proximal ones retained, but consist of interspersed blocks with each sequence represented by more than one, and probably several blocks. There was a total loss of ribosomal DNA from the nucleolar organiser regions of the chromosomes involved in the 13:14 translocation, but an interesting finding was the presence of extra ribosomal DNA and satellite DNAs I, II and III in one chromosome 22 which was found in seven out of nine individuals of the family with the 13:14 translocation, and in only one of five individuals without the translocation. There may be a compensatory mechanism present when certain sequences are elminated during chromosomal rearrangements. The relationship of such mechanisms to reproductive fitness is discussed.

Robertsonian fusion leading to the formation of stable dicentric chromosome in an ascites cell line of the mouse

The dicentric nature of a marker metacentric chromosome originated by robertsonian fusion has been established in the ascites cells of mouse sarcoma 180. C-banding analysis has revealed that the metacentric is actually a dicentric with 2 closely situated C-positive heterochromatic zones. The nature of the centromeres and the NF value of the cell indicate that this meta-dicentric marker has originated by breakage and fusion within each of the short arms of 2 acrocentric chromosomes.

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.

Structure and inheritance of some heterozygous Robertsonian translocation in man

Banding studies in 25 Robertsonian translocations showed that all could be interpreted as stable dicentrics. The mechanism for their stability is likely to be the proximity of their centromeres but centromeric suppression could also have a role. In many of these dicentric translocations, discontinuous centromeric suppression, as indicated by chromatid separation at one of the centromeric regions, was observed in C-banded preparations. A further observation of undefined relation to the first was that the ratio of the two constitutive centromeric heterochromatin (CCH) regions from the component chromosomes of the translocations was variable in the same translocation type, e.g. t(13;14). It is proposed that this ratio may influence the segregation ratio. Abnormal spermatogenesis is suggested as the likely mechanism for the difference in the proportion of aneuploid offspring in the progeny of maternal and paternal heterozygotes. Neither of the t dic(21;21)s could be interpreted as isochromosomes. It is proposed that Robertsonian fusion translocations be defined as stable, dicentric, whole-arm translocations, with both centromeres in a median position and resulting in the loss of a small acentric fragment during this formation. It is suggested that they occur at high frequency between telocentric or, as in man, certain acrocentric chromosomes because of some intrinsic property of those chromosomes not possessed by metacentric chromosomes and mediated by interphase association of centromeres.

Causes and consequences of Robertsonian exchange

At least two types of Robertsonian exchange are now known in the acrocentric chromosomes of man. Both types involve breakage in the arms adjacent to the centromere. Evidence is presented for a third type of exchange, one involving breakage within the centromere itself, in the grasshopper Percassa rugifrons. In this species, which is regularly homozygous for a single fusion metacentric, the eighteen rod autosomes have small but pronounced granules at the centric end of the chromosome. When C-banded these granules show differential Giemsa staining and appear to represent centromeric chromomeres; these chromomeres are lacking in the metacentric fusion product. Equivalent fusions may have occurred in some mammal species too and possible examples of this are discussed in sheep and mice. The Percassa fusion has led to a modification in both the frequency and the distribution of chiasmata as judged by a comparison of these properties in the metacentric relative to the two next smallest rod equivalents. Comparable modifications are known to occur in other naturally occurring fusions but these changes are certainly not automatic consequences of fusion since they are not shown in at least some newly produced fusion mutants.

Transmission of a t(13q22q) chromosome observed in three generations with segregation of the translocation D1-trisomy syndrome

A case of an inherited type of D/G translocation D1-trisomy syndrome was described. A female proposita who had the clinical signs of D1-trisomy syndrome was found to have a chromosome complement of 46,XX,--G,+t(DqGq). examination of Q- and G-stained karyotypes revealed that the chromosomes involved in the translocation were members of Nos. 13 and 22, or t(13q22q) with breaks at p12 of both chromosomes. C-stained figures also showed a large heterochromatin block in its centromeric region. The t(13q22q) chromosome was transmitted from the paternal grandmother of the proposita through at least three generations.

Dicentric X-isochromosome (Xqi dic) and pericentric inversion of No. 2 [inv(2) (p15 q21)] in a patient with gonadal dysgenesis

A patient with the clinical stigmata of gonadal dysgenesis is presented. Cytogenetic investigations revealed two distinct structural chromosome rearrangements. One of these, an isochromosome for the long arm of the X, proved to be a dicentric element following C-banding. The second abnormality, an inherited familial marker, was a pericentric inversion of No. 2 [(inv 2) (p15q21)].