High resolution cytogenetics - uses and difficulties

Abstract

High resolution banding (HRB) in cytogenetics is essential in the diagnosis of micro-deletion syndromes. Individually these syndromes are rare, but collectively they accounted for 10% of cytogenetic referrals in post-natal cases to the Children’s Hospital, Sydney, over the last 12 months. The commonest are the Prader-Willi and Angelman’s Syndrome (Δ), in which the deletions are in the proximal long arm of chromosome 15. Others include Miller Diecker, Alagille, Smith Magenis, Langer Gedion Δ, Wilms tumour and Retinoblastoma, each with deletions at different loci. To produce HRB of metaphase chromosomes the following is required:- a) a block to the mitotic cycle is introduced at a known point in time before S phase; b) the block is released and the cell cycle is allowed to progress; c) a decondensing agent is added prior to metaphase arrest; and d) metaphase arrest is short. Banding is subsequently applied by the standard procedures. The chromosomes may then be scored according to the number of bands i.e. 550-1000 band stage of resolution. This type of cytogenetic analysis is considerably labour intensive. Difficulties encountered are mostly concerned with interpreting differences in length and banding pattern between homologous regions of chromosomes. The ideogram (ISCN 1978)) is only a guide as differential decondensation occurs. Furthermore certain chromosome regions are relatively resistant to these techniques e.g. the Prader-Willi Chromosome Region (PWCR) 15q11-13, because of the DNA composition of repeat sequence heterochromatin found here. Objective standards must be applied to account for the inherent mitotic variability between homologues. The best resolution achieved overall needs to be considered as well as the band stage of resolution for the particular chromosomal region of interest. High resolution banding (HRB) in cytogenetics is essential in the diagnosis of micro-deletion syndromes. Individually these syndromes are rare, but collectively they accounted for 10% of cytogenetic referrals in post-natal cases to the Children’s Hospital, Sydney, over the last 12 months. The commonest are the Prader-Willi and Angelman’s Syndrome (Δ), in which the deletions are in the proximal long arm of chromosome 15. Others include Miller Diecker, Alagille, Smith Magenis, Langer Gedion Δ, Wilms tumour and Retinoblastoma, each with deletions at different loci. To produce HRB of metaphase chromosomes the following is required:- a) a block to the mitotic cycle is introduced at a known point in time before S phase; b) the block is released and the cell cycle is allowed to progress; c) a decondensing agent is added prior to metaphase arrest; and d) metaphase arrest is short. Banding is subsequently applied by the standard procedures. The chromosomes may then be scored according to the number of bands i.e. 550-1000 band stage of resolution. This type of cytogenetic analysis is considerably labour intensive. Difficulties encountered are mostly concerned with interpreting differences in length and banding pattern between homologous regions of chromosomes. The ideogram (ISCN 1978)) is only a guide as differential decondensation occurs. Furthermore certain chromosome regions are relatively resistant to these techniques e.g. the Prader-Willi Chromosome Region (PWCR) 15q11-13, because of the DNA composition of repeat sequence heterochromatin found here. Objective standards must be applied to account for the inherent mitotic variability between homologues. The best resolution achieved overall needs to be considered as well as the band stage of resolution for the particular chromosomal region of interest.