Cardiovascular Journal of Africa: Vol 29 No 2 (March/April 2018)

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CARDIOVASCULAR JOURNAL OF AFRICA • Volume 29, No 2, March/April 2018 126 AFRICA
positive or directional selection could not have acted identically
on all lineages, and therefore would result in a different rate of
accumulation of variants on haplogroup lineages, thus affecting
our ability to time divergence events by counting the mutational
events between lineages. On the other hand, it is possible that
negative or purifying selection could act evenly across lineages
and not impact on our use of mtDNA as a molecular clock;
the reliability of mtDNA as a molecular clock has been widely
discussed.110
Because of the central role that mitochondria play in cell
signalling and apoptosis, mitochondria have been implicated
in several age-related diseases, including Parkinson’s disease,
Alzheimer’s disease, multiple sclerosis and psoriasis.101,111,112 CVDs
are also classified as late-onset diseases and mitochondria
have also been implicated in CVDs. Consequently, haplogroup
association studies on CVD phenotypes are plentiful, but, as will
be revealed, also prone to pitfalls.
Crispim et al.113 reported an association of European
haplogroup cluster J/T with insulin resistance and type 2 diabetes
in a Caucasian-Brazilian cohort. On the other hand, Li et al.114
found no association between mtDNA variation and risk for
developing diabetes, while Chinnery et al.115 found no association
with type 2 diabetes and major European haplogroups in a large
study using 897 cases and 1 010 controls. Rather, Achilli et al.116
found that the risk for developing specific types of diabetes
complications (disease outcome) was significantly associated
with different mitochondrial haplogroups.
Several mtDNA population variants in cytochrome c oxidase
and NADH dehydrogenase subunit genes have been associated
with body mass index (BMI) in adults.117 In a very large study
using a second cohort, Chinnery et al.118 found no significant
associations between mtDNA haplogroups and ischaemic heart
disease, hypertension, diabetes or the metabolic syndrome, but
did find a significant association of sub-haplogroup K with risk
of cerebral ischaemic vascular effects.
Therefore, while some studies investigating phenotypes
included in CVDs have reported results that support a role for
mtDNA in CVD,116,117,119,120 there are also conflicting reports.115,118,121
This is not only common in CVD-related literature, but all areas
where haplogroup association studies have been applied. This is
an indication of the many difficulties that need to be overcome
when considering mtDNA variation in the context of disease.122
The unique way in which mtDNA is inherited (lack of
bi-parental recombination), which results in the emergence of
numerous unique haplogroups, contributes to the complexity
encountered when investigating mtDNA involvement in disease.
Non-biological factors such as differences in approach to
statistical analysis;123 difficulty in proper case and control
matching; small effective population size, which results in a
higher likelihood of population stratification; and insufficient
cohort size,122 further undermine the consistency of these studies.
Meta-analysis of data generated by several studies with
overlapping phenotypes can be employed to overcome sample
size difficulties, but these bring along challenges of their own,
as independent studies have different goals/methods, and do not
necessarily generate directly comparable datasets.101 So, while
haplogroup association studies might have fulfilled an important
role in the ongoing pursuit of the involvement of mtDNA
variation in disease, it is now well recognised that the field needs
to consider alternative models.
Rare mtDNA population variants
It has been shown that negative or purifying selection plays a
significant role in mtDNA evolution, with deleterious variants
being removed from the population over time,107 and that the
power of selection has been equally effective in all human
lineages.124 Consequently, rare mtDNA population variants are
more likely to be mildly deleterious than common variants, as
selection has had less time to remove them from the populations.
Indeed, rare mtDNA variants have been linked to changes in
CVDs and risk factors.
In a study by Govindaraj et al.,120 complete mtDNA analysis
revealed 10 non-synonymous variants present in hypertrophic
cardiomyopathy patients, but not present in controls or on
databases. Seven of these variants were classified as likely
‘pathogenic’, using several online scoring tools such as
PolyPhen-2, PMUT and PROVEAN, and were therefore thought
to be involved in the development of cardiomyopathy. Rare
variants m.5913G > A and m.3316G>A have both been suggested
to be associated with increased fasting blood glucose levels, while
m.5913G>A was shown to also be associated with increased
blood pressure in a selected Framingham heart study subset, all
of whom were of European descent.7
In addition, several rare mtDNA variants, such as
m.3316G>A,7,125 have been implicated in diabetes mellitus, of
which an up-to-date list can be found on
Another possibility is that the effect of an accumulation of mildly
deleterious variants may only become clinically significant once
a population is challenged by a rapid change of confounding
factors, such as diet or other environmental factors (toxins).126,127
In conclusion, several approaches are currently in use for
investigating the role of mtDNA in common complex disease.
mtDNA copy number is an emerging approach that might
become more prevalent in studies concerning CVDs as well. In
terms of mtDNA variants, rare population variants have been
linked to several disease phenotypes, including CVD-related
diseases such as cardiomyopathy and diabetes mellitus, and
might be found to be associated with other CVDs or risk factors
such as hypertension.
Rare population variants are more likely to be mildly
deleterious,124 but might not have a high enough impact on their
own to alter disease onset; rather, these variants might be more
likely to alter disease progression or outcomes. For common
population variants, several haplogroup association studies have
been done in CVDs, but have also been marred by the challenges
these types of studies face.122 It seems then that an alternative
approach to investigating the role of mtDNA variation in disease
is needed when investigating common complex disease.
Alternative approach for investigating mtDNA
involvement in disease: the adjusted mutation-
al load hypothesis
An alternative approach, the mutational load hypothesis was put
forward in Elson et al.111 Mutational load refers to the synergistic
effect of several changes in, for example, a specific gene or
functionally related set of genes. It does not look for associations
with a specific variant but rather a summative effect. While
some mtDNA variants might be of little effect on their own, an
increased mutational load might be associated with increased
risk for a certain disease. mtDNA mutational loads can then