Genetic
basics to help understand retinitis pigmentosa (RP)
Retinitis pigmentosa
as a genetic disease
The following
general discussion of genetics is not intended to replace proper genetic
counseling by a medical professional rather it is an overview of the
mechanisms involved in autosomal and sex linked diseases, specifically
retinitis pigmentosa.
The starting
point of any discussion on genetics is the gene. Genes are responsible
for determining the form and function of every living organism. It is
this inherited unit that is passed from parent to offspring.
Well - this
isn't entirely correct. Rather it is chromosomes that are passed from
one generation to the next while genes happen to be located on these
chromosomes. Genes are composed of DNA (deoxyribonucleic acid) - the
basic building block of living things. DNA looks somewhat like a long
bead like strand that exists in pairs.
The human
genome consists of 23 chromosome pairs. Each chromosome in turn contains
unique DNA that when damaged through mutations or errors incorporated
into the strand results in the improper functioning of cells. This may
lead to the death of tissues, organs or even the body . However, because
there are 2 copies of each chromosome and therefore each gene - an altered
gene does not necessarily mean that this mutant gene will be the one
expressed by the individual as we will see shortly.
What
this means to Retinitis pigmentosa
There are
four basic types of inheritance in retinitis pigmentosa: autosomal recessive,
autosomal dominant, sex linked and digenic. But first an understanding
of how chromosomes are passed from parent to offspring.
Back to the
46 chromosomes (23 chromosome pairs) - 23 of which come from dad (sperm)
and 23 of which come from mom (egg) during fertilization. 22 of these
23 chromosomes are referred to as autosomal chromosomes and are identical
in both males and females. The only differences between the two chromosomes
of an autosomal chromosome pair are variances in the DNA that is responsible
for character traits such as eye or hair color.
The 23rd
chromosome pair differs from the autosomes in that it marks the physical
and metabolical variances between males and females. These chromosomes
are designated X and Y and determine the sex of a baby, a fact that
gives them their name - sex chromosomes.
So, returning
to fertilization, when mom provides 22 autosomal chromosomes (not pairs)
and an X chromosome and dad provides 22 chromosomes (not pairs) and
either an X or Y chromosome, there is a total complement of 23 chromosome
pairs. A sex chromosome combination of XX results in a girl while an
XY combination results in a boy. This will become important in our discussion
of sex linked retinitis pigmentosa.
Every
child that is born receives 23 chromosomes (not pairs) from mom (maternal
chromosomes) and 23 chromosomes (not pairs) from dad (paternal chromosomes).
This means that each chromosome of a chromosome pair has a 50 % chance
of being passed onto a child in the next generation. Any genes located
on a given chromosome, whether normal or altered, become part of a child's
genome. The genes from one parent will interact with the maternal or
paternal genes from the child's other parent according to the set of
instructions set out by the DNA encoded within them.
With retinitis pigmentosa, which already has more than 7 genes identified
with it, scientists have determined four hereditary patterns: autosomal
recessive, autosomal
dominant, sex
linked or digenic
degenerative retinal diseases.