The cells in the body of a healthy person all have the same DNA, right? Not really! It has been known for quite some time now that there are genetic differences between cells within one person. The expression to describe these between-cell differences is “somatic mosaicism“, because cells can represent a mosaic of genetic profiles, even within a single organ. During embryonic development, all cells are derived from one fertilized egg and ought to be genetically identical. However, during every cell division errors and differences during DNA replication can occur and this can lead to genetic differences between cells. This process not only occurs during embryonic development but continues after birth.
As we age, our cells are exposed to numerous factors such as radiation, chemicals or other stressors which can causes genetic alterations, ranging from single nucleotide mutations to duplications and deletions of large chunks of DNA. Some mutations are known to cause cancer by making a single cell grow rapidly, but not all mutations lead to cancer. Many spontaneous mutations can either result in the death of a cell or do not even impact its function in any significant manner. DNA copy number variations (CNVs) is an expression used to describe a variable copy number of larger DNA segments of one kilobase (kb). Most recent studies on CNVs have compared CNVs between people, i.e. how many CNVs does person A have when compared to person B. It turns out that there may be quite a bit of genetic diversity between people that had previously been overlooked.
A new paper published in the journal Nature takes this one step further. It not only shows that there are significant CNVs between people, but even within a single person. In the study “Somatic copy number mosaicism in human skin revealed by induced pluripotent stem cells“, Alexej Abyzov and colleagues found significant CNVs in induced pluripotent stem cells (iPSCs) that they had generated from the adult skin cells of human subjects. Importantly, most of these CNVs were not the result of reprogramming adult skin cells to the stem cell state. They were already present in the skin fibroblasts obtained from the human subjects. Most analyses of CNVs are performed on whole tissues or biopsies, but not on single cells, which is why so little is known about between cell CNV differences. However, when iPSCs are generated from skin fibroblast cells, they are often derived from a single cell. This enables the evaluation of genetic diversity between cells.
Abyzov and colleagues estimate that 30% of adult skin fibroblasts carry large CNVs. This estimate is based on a very small number of fibroblast samples. It is not clear whether other cells such as neurons or heart cells also have similar CNVs and whether the 30% estimate would hold up in a larger sample. Their work leads to the intriguing question: What percentage of neighboring cells in a single heart, brain or kidney are actually genetically identical? Cell types, such as heart cells or adult neurons cannot be clonally expanded so it may be difficult to determine the genetic diversity within a heart or a brain using the methods employed by Abyzov and colleagues.
What are the implications of this work? On a practical level, this study suggests that it may be important to derive multiple iPSC clones from a subject’s or patient’s skin cells, if one wants to use the iPSCs for disease modeling. This will help control for the genetic diversity that exists among the skin cells. However, a much more profound implication of this work is that we have to think about between-cell diversity within a single organ. We need to develop better tools for how to analyze genetic diversity between individual cells, and more importantly, we have to understand how this genetic diversity impacts health and disease.