Mitomycin C-induced mosaicism in C-band regions of human chromosomes 1, 9, 16, and Y

Balanced reciprocal whole arm translocation t(3;9): analysis by fluorescence in situ hybridisation

A patient with Turner phenotype was found to carry two de novo chromosome aberrations: a 45,X line and a whole arm reciprocal translocation t(3;9). Fluorescence in situ hybridisation on metaphase cells using alpha satellite DNA for chromosome 3 and beta satellite and 'classical' satellite DNA for chromosome 9 showed that the centromeric region of chromosome 3 was retained in the 3q9q translocation derivative, as was the secondary constriction heterochromatin of chromosome 9. No signals were observed in the 3p9p derivative with the three probes. This suggests that the breakpoints were on 3p11 and 9q11. The karyotype was 45,X,t(3;9)(3qter-->3p11::9q11-->9qter; 9qter-->9q11::3p11-->3pter).

Mosaicism in the C-banded region of chromosome 1 in cancer families

Chromosome 1 C-band length mosaicism was detected in lymphocytes from six tumor patients and one healthy subject belonging to three families with a high incidence of cancer. In all cases the variant cell population showed a decreased amount of C-heterochromatin in one chromosome, whereas the C-banded pattern of the homolog was identical to that of the nonvariant cell population. A family tendency to unequal mitotic crossing over, possibly leading to C-band heteromorphism, may explain the high frequency of detection of C-heterochromatin mosaicism in cancer family members (seven of 13 cases studied). The possible role of heterochromatin in inducing cancer has been widely discussed. The special feature of the acquired C-band variants vis-à-vis the inherited ones is that they mark intrinsic genetic instability that may result, through multiple mechanisms, in increased susceptibility to malignancy.