First report on a 20qh+ heteromorphism characterized by molecular cytogenetics as amplification of D20Z1 sequences
Background: Chromosomal heteromorphisms (CHMs) in human are still understudied. This is partially due to the fact that heterochromatic regions are not well documented in human genome browsers, even though data would be available. Another peculiarity associated with CHMs is that they are routinely seen in cytogenetic analyses, however, considered as being without any clinical meaning. Nonetheless, CHMs are more frequently observed in infertility patients, suggesting very well a potential impact for human genome function. Aim of the study: Since literature is still lacking the exact characterization or complete documentation of numerous CHMs, we aimed to characterize CHMs on chromosome 20 and to characterize whether these result from an amplification of D20Z1 or D20Z2 or both. Materials and methods: An infertile adult and a fetus have been studied by fluorescence in situ hybridization (FISH) with D20Z2 (specific for 20p11.1) and D20Z1 (specific for 20q11.1) probes. Results: Here we prove for the first time that the two different types of CHMs in chromosome 20 are indeed caused by amplification of two different stretches of the alphoid DNA: 20ph+ variants are normally due to enlarged D20Z2-stretches while 20qh+ variants are caused by enlargement of D20Z1-stretches. Conclusions: We have characterized 20qh+ CHM by means of FISH for the first time showing it to result from amplification of D20Z1. This is another puzzle piece for exact characterization and complete documentation of CHMs in human.