"Classical " Mendelian
Genetics versus
"Reverse" Molecular
Genetics:
Genetics and Molecular
Biology of Alkaptonuria,
an inborn error of metabolism
In the first classical genetics approach to a human trait, the
physician Archibald Garrod in 1902 observed "Black Urine Disease"
(Alkaptonuria, AKU) in his patients (Step 1). The urine and diapers of infants
with Alkaptonuria darken upon exposure to air,
and adults show darkening of the cartilage in the ears and
nose. Chemical analysis identified a high level of a
substance called alkapton
in their urine (Step 2). From the pattern of
inheritance (pedigree)
observed in families under his care (two unaffected parents,
and unaffected and affected children in an approximate 3:1
ratio) (Step
3), Garrod deduced that the condition was inherited as a recessive trait, following
the reasoning of Gregor
Mendel whose work in 1867 had recently been
rediscovered, and was widely discussed. Following Mendelian Rules,
the birth of an alkaptonuric child to two unaffected parents
suggests that she had two recessive alleles (aa)
at some Gene for the trait. The parents must both
then be Aa, and do not show the condition because A
is dominant to a. The other, unaffected
children are either AA or Aa (shown as A-).
Garrod further suggested that the condition was due to the absence
of an enzyme to metabolize (break down) alkapton.
Subsequent biochemical analysis showed that alkapton (now
called Homogentisic Acid) is metabolized by Homogentisic
Acid Oxidase (HGO) to Maleylacetoacetic Acid (Step 4). Thus, genetic
analysis of crosses in a pedigree allows inference of the
existence and recessive nature of a gene for the trait
Alkaptonuria. The physical nature of the gene was at that
time entirely unknown.
Molecular
Genetics in the 21st century proceeds
from knowledge of the 3,200 Mbp human genome,
completed in 2003 (Step 1). Based on knowledge
of the amino acid sequences of HGO in other
organisms, bioinformatic analysis of all 23 pairs of
chromosomes mapped the gene for Homogentisic Acid Oxidase
(HGO) to Band 2 on the long (q) arm of
Chromosome 3 (3q2) (Step 2).
Detailed sequence analysis of this region identified an HGO
gene locus with 14 expressed exons and 13
intervening introns (Step 3). DNA sequence
analysis of multiple individuals shows a larger number of Single
Nucleotide Polymorphism (SNP) variants in particular exons.
Two of these SNPs are predicted to cause amino acid
substitutions in the protein products of Exons
10 & 12. These are respectively a
change of Pro to
Ser at residue 230 (P230S), and
substitution of Glu for Val
at residue 300 (V300G)
(Step 4). A child with Alkaptonuria is born to
unaffected parents: DNA
sequencing shows that the parents have the two
different allelic variants (Step 5), each in
combination with an alternative "+" allele.
DNA sequence analysis shows that the unaffected
siblings have the three possible combinations of the "+",
P230S, and V300G alleles.
The affected child has inherited both the P230S and
V300G alleles, both of which are non-functional. HOMEWORK: what SNPs are
responsible for the two amino acid substitutions?
Garrod's analysis in the
early 20th century is an example of Classical
or Mendelian Genetics: given observable phenotypic
variation, he inferred the genotypic nature
of inheritance from an analysis of pedigrees. This
differs slightly from what Mendel did, which was to
arrange controlled crosses and measured the
observed outcomes. This is how the
science of Genetics was understood for more than
50 years, which only in the molecular era was sometimes
described as "Forward Genetics".
Molecular Biology in the late 20th and early 21st
centuries is sometimes called "Reverse Genetics",
because detailed knowledge of the molecular genotype predicts
how it produces a disease phenotype arises in certain pedigrees.
Also, formulation of the Central Dogma as "DNA
makes RNA makes Protein",
and
meant to imply forward information transfer, when
understood as moving from an enzyme defect to the
underlying DNA variant, implies a "reversal"
of molecular logic.
For
the advanced student: Mendel's
experiments carefully isolated and brought together
exactly two allelic variants of each gene, A
& a. Garrod's interpretation was that any
individual with Alkaptonuria combined two copies
of the same "a", such that the
individual was an aa homozygote. This tacitly
assumes that there are only two alleles, the
"normal" A allele carried by most
people, and the "disease" a allele
found in affected persons. This became the standard
interpretation: most individuals were homozygous AA
for a standard "wild type" allele A,
whereas a minority were homozygous aa for a "disease"
allele a. Early molecular analysis began to
show instead that genetic defects might occur at
several places in the DNA of any gene. The
combination of two different alleles a'a could
produce a compound heterozygote, with
the genetic phenotype of an aa homozygote. True
homozygosity would require that an individual
inherited an allele identical by descent (autozygous)
from a more or less distant ancestor. One result of the
Human Genome Project is to demonstrate extensive
heterogeneity among the alleles associated with any
particular genetic condition, such that compound heterozygosity
is far more frequent than previously expected.
Step 3 of the analysis
presented above (observation of a 3:1 ratio of
unaffected to affected children) is an exaggeration.
Garrod's actual data departed from 3:1 due to ascertainment bias.
Investigate and explain.
Archibald Garrod (ca. 1908)
Figure © 2016 by Steven M Carr,
after ©2002 by Griffiths et al.; All text
material ©2024 by Steven M Carr