Microdeletion syndromes

A chromosome deletion is an abnormality where some of the chromosome is missing and this can cause congenital abnormalities such as delay in developmental milestones in children or mental retardation, individuals with unusual (dysmorphic) physical features, or a combination of both. Some very very small deletions, referred to as ‘microdeletions’, cannot be seen by routine chromosome analysis using a microscope and the chromosomes look otherwise normal. Such microdeletions can be the cause of specific microdeletion syndromes.

Since microdeletions are difficult to seen using a normal microscope we use a special technique called fluorescent in situ hybridisation (FISH). FISH is a technique where fluorescent coloured markers (probes) are allowed to attach to parts of the chromosomes, or genes.

These images show an individual affected by a microdeletion disorder called Smith Magenis syndrome (SMS).   The image above shows the chromosomes and you can just make out that one short arm of one chromosome is smaller than the other and shows a deletion.   The image below shows a probe set for SMS. The SMS region (red signal) is missing from one chromosome 17.

    

Some of the other more common microdeletion syndromes that we test for include Prader Willi Syndrome, Angleman syndrome, Miller-Dieker syndrome, Williams syndrome, Wof-Hirschorn disease and Cri-du-chat syndrome.