Mechanisms of the origin of a G-positive band within the secondary constriction region of human chromosome 9

Abstract

We report on a so-called rare variant where a G-positive band was sandwiched within the secondary constriction (qh) region of chromosome 9 and is apparently different from previous cases when characterized by the fluorescence in situ hybridization technique. The major differences included duplication of β-satellite and satellite III DNA sequences and bands 9q 13q21.1, without duplication or inversion of the alphoid sequences. Based on the reported cases, at least four types of variations can be accounted for. A variety of mechanisms have been proposed to describe the origin of a G-positive band within the 9qh region, which appears to be similar when studied by routine cytogenetic techniques but differs by molecular methods. It is hypothesized that the clinical consequences depend upon the size of the G-positive band(s) duplicated, and a genetic inactivation mechanism might have some sort of influence during the so-called heterochromatinization process. It appears that heterochromatin, once thought to be composed of junk DNA, may have some role after all in suppression of gene(s) and/or spreading of inactivation, if genes are embedded within the heterochromatic region. Apparently, the mixture of different types of DNA creating patches of genetic debris have become a fundamental hidden treasure, where genetically active chromatin could be inactivated without dire consequences. The variable nature of heterochromatin has resulted in cytogenetic heteromorphisms of a number of human chromosomes. Their characterization by molecular techniques is becoming imperative, because fetal wastage have occurred in many situations where variant chromosomes were wrongly identified as chromosomal abnormalities.

Homologous sequences at human chromosome 9 bands p12 and q13-21.1 are involved in different patterns of pericentric rearrangements

“…Verma et al 39 described an inverted duplication 9qh in a prenatal case. Inverted duplications of 9p10-q12 were also reported by Wang and Miller 25 and Macera et al (Type II) 5 using FISH with a pan-a-satellite probe and a chromosome 9-specific b-satellite DNA probe. The inversion type C described by Samonte et al 7 is similar to our 9qh+ with inversion type var3, while their types A, B and D were not detected in our study.…”

“…8 In summary, 12 heteromorphic patterns and the normal variant of the pericentric region of chromosome 9 are described. Previous studies attributed the extensive degree of variability in this region to unequal meiotic exchanges involving the various classes of repetitive sequences in the centromeric region 5,7,25 and to unequal exchanges involving stretches of homologous DNA sequences in the proximal short and long arms of chromosome 9. 15 The results of the present study would support such an assumption.…”

“…3 Since the introduction of fluorescence in situ hybridisation (FISH) techniques only a few systematic studies have been performed to classify the chromosome 9 heteromorphisms in more detail. In one study, four different types of inversion and duplication (ie 9qh+) were reported, 5 and overall, seven different types of rearrangement leading to inversions on chromosome 9 have been described. 1,6,7 However, neither the origin nor the clinical significance of these heteromorphisms is well understood.…”

Prenatal detection of de novo inversion of chromosome 9 with duplicated heterochromatic region and postnatal follow-up

“…Type A inversion consist of breakpoints within the α and β satellite DNA regions; type B consist of breakpoints the β satellite DNA region 9q13; type C involve breakage within the β and classical satellite DNA regions, and type D have breakpoints within the α and classical satellite DNA regions. Like these, obviously, reshuffling of satellite DNA sequences occurred, which has given rise to a variety of heteromorphisms whose clinical significance remains obscure (Samonte et al, 1996), few types of pericentric inversion have been observed due to the variable breakpoints within the qh regions, but their implications have also remained obscure (Verma et al, 1993;Macera et al, 1995).…”

Recurrent abortions and postnatal loses in two cases including G-negative band within Chromosome 1qh region

“…The variation of chromosomes with qh have been classified into two classes, termed "size" and "inversion" heteromorphisms (Verma et al, 1978). A third type of variation has surfaced wherein an additional G-positive or G-negative bands is sandwiched within the qh region of chromosome 9 and 1 (Macera et al, 1995;Verma et al, 1997). If Giemsa is used, the dark band is called G-band or G-positive band, and the light band is named G-negative band.…”