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Clinical Diagnosis



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How is chromosomal mosaicism diagnosed?

Chromosomal mosaicism can be diagnosed in three ways:

  1. during prenatal diagnosis
  2. in an individual's blood sample or skin biopsy
  3. during preimplantation diagnosis

We have organized this section by timing of the diagnosis, which sometimes gives a clue as to how the mosaicism may impact the health of the affected individual.  Most of the concerns with chromosomal mosaicism arise when it is identified at prenatal diagnosis. Thus, most comments in this section relate to diagnosis of chromosomal mosaicism either prenatally or in early life.

Prenatal Diagnosis

  Chorionic villus sampling



   Confined mosaicism

   Uniparental disomy

Diagnosis in blood

Preimplantation diagnosis

    How does chromosomal mosaicism affect the health of a developing baby or grown adult?

It seems likely that everyone contains some small number of cells in their body which are chromosomally abnormal

So, when does chromosomal mosaicism matter?

When chromosomal mosaicism arises during development, pregnancy outcome depends on which tissue, and how much of that tissue is abnormal.  In theory, cases with a relatively high proportion of trisomic cells are more likely to be associated with an abnormal outcome than those with a low proportion of trisomic cells.  That is, if a majority of the cells are abnormal then human development is likely to be abnormal.  If only a tiny fraction of some tissue were involved, the aneuploidy would likely have little effect on growth and development.  Perhaps many people carry a tiny and completely unimportant abnormal cell line somewhere in their body.  However, a very minor degree of mosaicism could still be important if a crucial tissue carries the abnormal cells.  For example, an abnormal chromosome change confined to one part of the brain could theoretically impair neurological function (Gardner & Sutherland, 1996).

As a general principle, an individual with a chromosome abnormality in only some of their tissues is likely to have less severe but qualitatively similar clinical features to that of someone with the non-mosaic form of the same chromosome abnormality.  For example mosaic Down syndrome can be associated with a less characteristic facial appearance and milder mental impairment than the those with typical trisomy 21.  Some chromosome changes can only exist in a mosaic form, because in a non-mosaic form they are lethal.  Sometimes if the distribution of the aneuploid cell line is asymetric, the body shape or appearance may be asymmetric.  Generally it is the cells that are aneuploid that are smaller and less developed (Gardner & Sutherland, 1996).


It is worth noting though that chromosomally abnormal cells may also arise with age and contribute to such health problems as the occurrence of cancer.  However, most age-related chromosome changes are likely either eliminated due to poor cell growth or have no obvious harmful effect.  For example, 45, X0 cells are increasingly common in female blood cells as they age, but appear to have no harmful effect.



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