34 Introduction to Autism Spectrum Disorders
bymutagenic agents such as electromagnetic radiation, oxygen free radicals,
and endogenous transposons (segments of DNA capable of moving around
spontenously and independently). By contrast, cells elsewhere in the body can
experience mutation during the process of DNA replication [106].
Among the most critical and intriguing aspects of this creative and ground
breaking research is the creation of nested lineage trees, which permits dating
relative to landmarks of development and brings greater understanding of the
cerebral cortex’s architecture that arrives from multiple progenitor cells.
Somatic mutations found in the human brain constitute a solid and continuing
documentation of the genealogy of neuronal life, from early development
throughout the life of an adult person (i.e., mitosis and thereafter in the post‐
mitotic phase).Is Finding Mutations the Path to Discovering the Genesis of ASD?
Since the beginning of contemporary science, scientists have utilized the
mouse and numerous other small mammalian animal models to reproduce
various theories of autism. However, it should be noted that the brain of Homo
sapiens is unbelievably more complex and large. It has over 100 billion neu
rons and 0.15 quadrillion synapses. If one is looking for genetic mutations and
comparing research on humans with mouse animal models, then one has to
realize that these comparisons are far from accurate. For example, the average
adult human weighs 62 kg, is made up of 3.7 trillion cells (3.7 × 1013 cells), and
lives to about 70 years of age. On the other hand, the weight, number of cells,
and lifespan of an average mouse would be dramatically less: 20 g, or about
1/3,100 of an adult human’s weight; roughly 1.2×10^10 cells, and a lifespan of
3 years. The human genome has about 3 billion base pairs; these reside in the
23 chromosome pairs that are in the nucleus of each cell. Every chromosome
is home to between hundreds and thousands of genes; these contain the
instructions needed to produce proteins. Comprised of 23 chromosome pairs,
the mouse genome contains approximately 2.7 billion base pairs, about 15%
less than that of the human genome. Using a conservative estimate of the
mutation rate, per cell per generation, of 5×10−7, then the generation of
3.7×10^13 cells would lead to more mutations, by several logarithms, in humans
compared with mice. Similar reasoning may be applied to a comparison of the
human and mouse brains. This is very simplistic, however, since the human
brain is far more complex than that of a mouse, and the inference of findings
from mouse studies to human studies is, at best, a study in gross approxima
tions. This is not to imply that animal models lack usefulness in suggesting
broader implications; it is to say that in the case of autism research, rodent or
other small animal models are highly inadequate. It should also be noted that