Autosomal Aneuploidy – Cytogenetic Abnormalities

Cytogenetic abnormalities are what I have learnt during my internship in Hospital Kuala Lumpur. Today, I’m going to introduce Autosomal Aneuploidy in this Cytogenetics and Cancer Research blog.

In Cytogenetics, the term aneuploidy refers to cytogenetic abnormalities in which all or part of one or more chromosomes is added or deleted. Autosomal aneuploidy is the abnormality that does not involve the sex chromosomes. Sometimes, the abnormalities can be either numerical or structural. Normally we only have pair of chromosomes which are structurally similar. Other than that, it can be recognized as abnormal. Those cytogenetic abnormalities can be present only in some cells which we called mosaicism or in all cells.

Meiotic Nondisjunction Causes Autosomal Aneuploidy

The origin of autosomal aneuploidy is because of meiotic nondisjunction. The meiotic nondisjunction is random for all autosomes except for chromosome 21. Chromosome 21 has shown the highest frequency of autosomal aneuploidy.

According to cytogenetic studies, the incidence of autosomal aneuploidy in spontaneous abortuses (die before birth) is much higher than incidences in newborns. So, what is the case for aneuploidy actually observed in spontaneous abortuses or liveborns? All trisomies for all autosomes have been reported in spontaneous abortuses. The fetal only can survive if and only if the trisomies are in mosaic form. However, there are still many exceptions for the trisomies 13, 18 and 21. Some of the foetus still can survive even though the trisomies 13, 18 or 21 are in nonmosaic form.

Why the frequencies of trisomy for each chromosome might be similar at the time of conception but differ greatly among abortuses and liveborns especially for trisomy 21? It can be explained by the devastating effect of chromosomal imbalance. Most of the autosomal aneuploidies are very deleterious and lethal in the pre-embryonic stage. As a result, those abnormalities are unrecognized and, therefore, unstudied spontaneous abortions.

Furthermore, the lethality of a particular autosomal aneuploidy is related to the gene content of the particular chromosome. Aneuploidies for the gene rich chromosomes are less likely to survive. However, the less gene rich chromosomes like chromosome 13, 18 and 21 are more likely to survive to term.

Anyway, we will just focus on those observed in liveborns for the autosomal aneuploidy in Cytogenetics. I will talk more details about the monosomies and trisomies in my future post. Stay tuned!