Human Reproduction, Vol. 15, (Suppl. 2), pp. 44-56, 2000
Organismal effects of mitochondrial dysfunction
Robert K.Naviaux1 and Karen A.McGowan
Mitochondrial and Metabolic Disease Center, University of California,
San Diego, School of Medicine 200 West Arbor Drive, San Diego,
CA 92103-8467, USA
'To whom correspondence should be addressed at: Mitochondrial and Metabolic
Disease Center, University of California, San Diego, School of Medicine
200 West Arbor Drive, San Diego, CA 92103-8467, USA.
E-mail: [email protected]
Mitochondrial disease can lead to clinical
abnormalities in any organ system. Both
inherited and spontaneous disorders are
known. The spontaneous forms can occur
as a mitochondrial DNA (mtDNA) mutation
early in embryogenesis or, later in life, as
somatic mutations that accumulate with
age. The inherited forms may arise from
any of >100 characterized mutations in
mtDNA or from >200 nuclear gene defects
that affect proteins required for mitochondrial function. Most dividing cells survive
and interact normally despite their mitochondrial defects. Thus post-mitotic,
terminally differentiated cells are preferentially affected in mitochondrial disease.
This review emphasizes cellular metabolic
co-operation and the structural and biochemical diversity of mitochondria as the
framework for understanding the clinical
spectrum of mitochondrial disease. The
principles of the mitochondrial clinical
assessment scale I (MCAS-I) are presented
to assist in the development of diagnostic
spectra of mitochondrial disease.
Keywords: cellular metabolic co-operation/
diagnosis/mitochondrial clinical assessment
scale/mitochondrial disease/mitochondrial function
44
Introduction
No man, tissue, cell, organelle, or gene is an island,
entire of itself; each is a piece of the whole, a part of
the galloping beast and a single voice in a living choir.
An evolutionary biochemist's translation
(with apologies to John Donne)
from Devotions upon Emergent Occasions, 1624
The organismal effects of mitochondrial dysfunction spring from ruptures in an ancient
alliance. Mitochondria were once free-living
bacteria that merged their metabolic
machinery with archaebacteria to produce a
co-operative metabolism that became the first
eukaryotic cell (Margulis, 1970, 1992; Gray
et al, 1999). Today, nearly 2.5 billion years
after that alliance was forged, all multicellular
animal life is strictly dependent on the maintenance of a fluid dialectic between mitochondria and host cell. This dialectic regulates the
function of mitochondria in ways that enable
cells to become metabolically specialized and
interdependent. Co-operative metabolism is
the foundation for tissue differentiation.
Adjoining cells are locked in symbiosis when
each produces at least one product the other
needs to survive. Metabolic co-operation
underlies both the radiation and diversity of
animal body plans during evolutionary time,
and during the development of embryo to
adult.
© European Society of Human Reproduction & Embryology
Organismal effects of mitochondria! disease
Fatty Acids,
Branched Chain Amino Acids,
Lysinc,Tryptophan,Choline
Figure 1. The mitochondrial electron transport chain used for ATP synthesis. Complex I = NADH-CoQ oxidoreductase.
Complex II = succinate-CoQ oxidoreductase. Complex III = cytochrome C-CoQ oxidoreductase. Complex IV = cytochrome
C oxidase (COX). Complex V = ATP synthase; ANT = adenine nucleotide transporter; DHO-QO = dihydroorotate-CoQ
oxidoreductase (required for de-novo pyrimidine synthesis); ETF-QO = electron transfer flavoprotein-CoQ oxidoreductase;
CAD = carbamyl phosphate synthetase Il/aspartate transcarbamylase/carbamyl aspartate dihydroorotase; UMPS = uridine
monophosphate synthetase.
Mitochondrial specialization is a natural
consequence of tissue-specific transcription.
Mitochondrial biogenesis requires the expression of -3000 genes (Wallace, 1992). Only 37
RNA and peptide transcripts are encoded by
human mitochondrial DNA (mtDNA). This is
<2% (37 of 3000 = 1.2%) of the genetic
information required. In the genetic economy
of the endosymbiont, >98% of the genes
needed for mitochondrial biogenesis must be
transcribed in the host cell nucleus and the
products must be imported into the organelles.
The biochemical palette used by mitochondria
is exceedingly rich, yet largely unemphasized
in traditional didactic presentations of mitochondrial function. To understand the scope
of mitochondrial disease, one needs to push
beyond the 'energy factory' stereotype and the
chauvinism of single mitochondrial electron
transport chains. Indeed, there is a fertile
pluralism of electron transport chains in mitochondria. Only one of these chains stores
energy as a proton chemiosmotic gradient
that leads to ATP synthesis via oxidative
phosphorylation (Figure 1). The others also
consume oxygen and make water, but they
differ from the oxidative phosphorylation
sequence in that they use electrons to form
and break chemical bonds in the biosynthesis
of numerous cell-specific products. Figure 2
illustrates one example: the synthesis of steroid
hormones (see also Donohoue et al, 1995).
ATP is just one of a myriad of products that
specialized cells require their mitochondria to
assemble. Table I is an incomplete but useful
summary of some of the tissue-specific biosynthetic functions of mitochondria. Some of
these are linked to electron transport. Others
are not. Structural and biochemical diversity,
tissue-specificity, and developmental control
of mitochondrial functions are the keys to
understanding the dizzying array of symptoms
characteristic of inherited mitochondrial disease and many of the more complex disorders
of ageing.
Solid state biochemistry
Mitochondria are solid state bioreactors in
which biochemical transformations are conducted in and on surfaces. The geometric
packing of proteins within the mitochondrial
45
R.Naviaux and K.McGowan
Cholesterol
Pregnenolone
Figure 2. Mitochondrial electron transport chain used for steroid hormone synthesis in the adrenal cortex, ovaries, and testis.
Table I. Tissue-specific biosynthetic functions of mitochondria
Cell type
Function
Mitochondrial enzyme
Deficiency disease
Perivenous hepatocytes,
retinal cone cells
Periarteriolar hepatocytes,
intestinal epithelium
Proerythroblasts, hepatocytes
Pyrroline-5-carboxylate,
(P5C) and glutamine
Urea cycle
Gyrate atrophy, cataracts,
blindness
Hyperammonaemia,
encephalopathy
X-linked sideroblastic anaemia
Zona glomerulosa
Zona fasciculata
Aldosterone
Cortisol
Ornithine amino transferase
(OAT) (2.6.1.13)
Ornithine transcarbamylase (OTC)
(2.1.3.3)
Amino levulinic acid synthase
(2.3.1.37)
CYP11B2 (Mitocytochrome P450)
CYP11B1 (Mitocytochrome P450)
Zona reticularis, (adrenal
cortex), Leydig, theca,
placenta
Hepatocytes, Renal proximal
tubule
Hepatocytes, macrophages
Sex steroids
CYP11A1 (mitochondrial
cytochrome P450scc)
Salt wasting
Congenital adrenal hyperplasia
with virilization
Congenital adrenal hyperplasia
with feminization
Vitamin D
Liver P450 25/27-oc-hydroxylase,
Kidney 1-a-hydroxylase
Mitochondrial NO synthase
Hypocalcaemia, rickets, muscle
weakness
?Impaired immunity, apoptosis
Dihydroorotate dehydrogenase
(1.3.99.11)
Pyrimidine deficiency
Hepatocytes, kidney
Haeme
Nitric oxide (NO),
respiratory control
Uridine
matrix, and in and on mitochondrial membranes, is highly ordered. Protein concentrations in the matrix approach 500 mg/ml;
reflecting a 50% hydration state (similar to
the protein packing observed in a crystal of
trypsin). Like the ordered arrangement of
American football players at the scrimmage
line before the ball is snapped, mitochondrial
proteins must be assembled in the correct
relative positions in order to receive reactants
and convey products to the next enzyme in the
46
metabolic chain (Srere, 1985, 1987). Nature
dispensed with solution biochemistry as catalytically cumbersome >3.8 billion years ago,
with the emergence of the first bacterial cell.
Reliance on Brownian motion in three-dimensional space was, we believe, unacceptably
slow and provided limited opportunities for
tight control of metabolic flux. Growth and
differentiation could not evolve until surface
catalysis emerged and an autopoietic metabolism could ignite.
Organismal effects of mitochondria! disease
The tight geometric packing of mitochondrial proteins creates a de-facto mechanism
for fine tuning the catalytic efficiency of
individual organelles. Volume regulation of
surface catalysis is an important mechanism
of biochemical control. Imagine the inner
surface of a balloon decorated with small
styrofoam spheres of all shapes, sizes, and
colours. Under native conditions, all the
spheres are optimally distributed and just
touching. As the balloon is blown up, its
volume increases and the spheres move away
from one another. The proteins in swollen
mitochondria are likewise mutually separated.
Like damping rods in a nuclear fission reactor,
water molecules fill the gaps created by the
separation of previously adjoining proteins.
This slows the flux of reactants and products in
the affected metabolic chains. Mitochondrial
swelling can take place over minutes to hours
in response to a variety of environmental
stimuli. Under certain circumstances, this can
lead to a loss of mitochondrial transmembrane
potential, cytochrome C release to the cytoplasm, and programmed cell death (Kluck
et al, 1997). Over-expression of certain
proteins, e.g. bcl-xL, can prevent this swelling
and prevent apoptosis in certain cell types
(Vander Heiden et al, 1997).
Imagine the time it takes a drop (50 JLLI) of
red oil, which has a density equal to that of
water, to reach the opposite side of a swimming
pool (2X10X25 m; 5X10 5 1) by a random
walk in three-dimensional space. The volume
ratio of the dye to the pool is l:1010. This is
about the same volume ratio of -100 proteins
with hydration spheres of 5 nm, magically
conducting themselves through the cell membrane, past actin filaments, intermediate filaments, microtubules, rough endoplasmic
reticulum (ER), smooth ER, and the Golgi, to
the eccentrically located nucleus of a 50 |im
diameter hepatocyte. Even with signal transduction and amplification via second messengers, this is a kinetically herculean task. Now
imagine the same drop of red dye conducted
on a one-dimensional, hydrophobic filament
that is continuously moving toward the other
side of the pool. Max Delbriick developed a
mathematical demonstration of how dimensional reduction in biological systems along
sheets (two-dimensions) and rods (one-dimension) could accelerate signal transduction by
several orders of magnitude (Delbriick, 1968).
Evolution made this discovery when life first
began: the chemistry of living cells is conducted in sheets and rods (membranes and the
cytoskeleton), not in solution.
Cellular tensegrity and mechanotransduction are concepts that help to explain biochemical function in terms of cell shape (Ingber,
1993; Wang et al, 1993). Mitochondria are
intimately associated with all elements of the
cytoskeleton at different times throughout the
life of different cell types in different tissues
(Soltys and Gupta, 1992; Rappaport et al,
1998). Because biochemical resources are not
evenly distributed within the cell, organellar
position and juxtaposition with the Golgi, ER,
lysosomes, peroxisomes, and the nucleus have
dramatic effects on mitochondrial function.
Subcellular redistribution of mitochondria
controls the flow of substrates and products
through the organelles according to cellular
need. Phenomena such as axoplasmic transport
of mitochondria to nerve terminals (Okada
et al, 1995) and the perinuclear accumulation
of mitochondria after heat shock (Collier et al,
1993) help illustrate differences in mitochondrial function that occur with differences in
subcellular location. Changes in mitochondrial
respiration accompany these simple changes
in address (or the subcellular address of neighbours) of mitochondria within the cell (Saks
et al, 1995). The proteins responsible for the
redistribution of mitochondria within mammalian cells are related to the kinesins,
intermediate filament-, microtubule- and actinassociated proteins that have been, and are
47
R.Naviaux and K.McGowan
Figure 3. A typical mitochondrion from a pancreatic islet
cell. Mitochondria are, however, not monomorphic little
'sausages' with lammelar cristae, but occur in many sizes and
shapes, with cristae that take many different patterns.
being, identified in simple cell model systems
(Yaffe, 1999).
Structural diversity of mitochondria
Mitochondria (Figure 3) occur in many sizes
and shapes, with inner mitochondrial membranes (cristae) that take many different patterns, depending on the membrane-bound
metabolism called for. As mentioned earlier,
98% of mitochondrial function (in terms of
gene number) is determined by the genetic
program of the host cell via cell-specific
nuclear transcription. Only 2% is determined
by mtDNA. There are roughly 250 different
cell types in the human body plan (Kauffman,
1991), each with its own unique pattern of gene
expression, regulated by nuclear transcription.
Correspondingly, there are probably 250
unique types of mitochondria, each tailored by
unique patterns of nuclear gene transcription to
perform suites of metabolic functions that are
regulated both developmentally and tissuespecifically. Function follows form.
Tissue-specific and developmental
regulation of mitochondria
mtDNA is amplified in some, but not all,
tissues after birth. In a study that compared
48
the mtDNA content with developmental age
in a number of human tissues, Heerdt and
Augenlicht (1990) showed that by 22-32
weeks gestational age the mtDNA content of
marrow, peripheral blood leukocytes, spleen,
and adrenal glands had already achieved the
adult level. However, there was a 2-3-fold
amplification of mtDNA between 32 weeks
gestation and the adult in liver, kidney and
muscle. The brain is likely to show similar
mtDNA amplification after birth, although it
was not specifically addressed in this study.
Other workers have also observed the agedependent amplification of skeletal muscle
mtDNA (Poulton et ai, 1995). There is a
rapid, 3-5-fold increase in the second and
third trimesters of pregnancy, then a more
gradual, additional 3-fold amplification which
may not be complete until around the time
of puberty.
Liver mitochondrial respiratory capacities
increase two-fold in the first 2 h after birth in
the rat, achieving 50% of the adult value
(Valcarce, 1988). This is accompanied by a
marked reduction in mitochondrial volume and
an increase in available adenine nucleotides, in
keeping with the concept of volume regulation
of surface catalysis discussed above. Pyruvate
oxidation in skeletal muscle of the rat increases
more slowly and does not achieve the adult
value until about the time of reproductive
maturity (Sperl et al, 1992). This is consistent
with the notion that maximum respiratory
potential in skeletal muscle is not achieved
until the adult level of mtDNA is reached,
around the time of puberty.
Biotic potential - why cells growing
in vitro are different
Why are cells growing in culture different to
cells in tissues? Perhaps the most fundamental
differences are cellular address (location) and
selection. Cells growing in culture have continuously changing neighbours, whereas cells
Organismal effects of mitochondria! disease
in tissues have stable neighbours. Growing
Central
Vein
cells must each remain biochemically selfHepatic
reliant and homogeneous in order to provide
Artery
the self-renewing resource that is so experimentally powerful. Growing cells with these
attributes undergo intense Darwinian selection
Region of
in the laboratory. Differentiated cells that exit
Mitochondrial
Region of Mitochondrial
OTC Expression
the cell cycle, or senesce as they exceed the
OAT Expression
Hayflick limit, are difficult to work with. They Figure 4. Distribution of mitochondrial proteins along
are continuously being diluted by the biotic oxygen gradients in a lobule of the liver. Mitochondrial
ornithine aminotransferase (OAT) is expressed in the oxygenpotential of their dividing neighbours.
poor region surrounding the central vein. Mitochondrial
Stationary cells in tissues, on the other hand, ornithine transcarbamylase (OTC) is expressed in the more
are differentiated and naturally heterogeneous. oxygen-rich region surrounding the hepatic artery.
Metabolic co-operation emerges spontaneously as cells reduce the number of genes
How do cells of the growing embryo
expressed. Only communities of cells with
complementary patterns of nutritional needs develop into tissues? A fundamental answer
and export products can survive in steady state to this question has been the focus of intense
throughout the life of the organism. About and richly productive research for over a
0.1-1% of hepatocytes make albumin: the century. However, a numerical framework can
great majority of liver cells do not. This pattern be sketched that will help to understand why
of heterogeneous expression of differentiated, tissues appear to be made of cells that are
non-housekeeping gene products is character- metabolically interdependent. In genetic
istic of every tissue in the body (and cannot terms, the cells of terminally differentiated
be replicated easily in cell or tissue culture). tissues are complementary auxotrophs, unable
Differential expression of proteins within liver to synthesize the full suite of molecules
mitochondria also occurs. Only the mitochon- required for survival. In contrast, the mRNA
dria in the relatively oxygen-poor, perivenous repertoire stored and expressed by the fertilhepatocytes contain ornithine aminotransfer- ized sea urchin oocyte approaches 40 000
ase (OAT), whereas only the mitochondria genes, virtually the full repertoire of genes
in the oxygen-rich, periarteriolar hepatocytes encoded by the nuclear DNA of this uniquely
express the urea cycle enzyme ornithine trans- versatile and experimentally useful organism
carbamylase (Figure 4) (Valle and Simell, (Galau etal, 1976; Hough-Evans etal, 1977).
1995). A similar differential phenomenon can
The oocyte expresses the highest fraction
be seen in skeletal muscle. A lactate importer of nuclear genes of any cell type. It also has
is expressed on the surface of aerobic, type the largest number of mitochondria of any cell.
I skeletal muscle fibres and high levels of The mammalian oocyte contains -200 000
succinate dehydrogenase (SDH) are found mitochondria (Piko and Matsumoto, 1976),
in their mitochondria (Garcia et al, 1994). with -1-2 copies of mtDNA per organelle
Adjoining, glycolytic, type II muscle fibres do (Michaels etal., 1982). Somatic cells typically
not express the lactate importer and express contain 1000-2000 mitochondria and 3-10
low levels of mitochondrial SDH. Like all copies of mtDNA per organelle (Michaels
tissues, muscle is a mosaic of functionally et al, 1982; Piko and Taylor, 1987). This is
interdependent cells. Brain is the most com- just 1% of the level in primary oocytes.
plex mosaic of any tissue.
Therefore, the process of embryogenesis pro49
R.Naviaux and K.McGowan
duces cells that express RNA from progressively smaller subsets of their nuclear DNA,
and contain progressively fewer mitochondria
and mtDNA, although the copy number of
mtDNA per organelle increases 3-5-fold with
differentiation after gastrulation. In the human,
expression of nearly 100 000 mRNAs in the
oocyte must be pruned to the 10 000-15 000
mRNAs characteristic of terminally differentiated cells. As cells differentiate, therefore,
genes are inexorably shut down, and pluripotentiality is progressively restricted.
As genes are shut down, cells become
auxotrophs for certain nutrients. They can
no longer sustain the reducing intracellular
environment in which most of the free electrons are routed to the biosynthesis of polymers
(polypeptides, membrane lipids, glycogen,
DNA, etc) that were needed to increase biomass during mitosis. Differentiating cells exit
the cell cycle and shift from a perpetual growth
(reducing) metabolism to a maintenance (oxidizing) metabolism. In the stationary phase of
life, cell biomass is no longer increasing. In
order to maintain a fixed biomass, electrons
that were once liberated for polymer synthesis
are shifted away from bond-formation. These
electrons are used to reduce oxygen to water
in the mitochondrial electron transport chain.
Combustion becomes more complete, and
energy production is more efficient. The price
of efficiency is metabolic co-operation (comparable to the insight Adam Smith made in
his Wealth of Nations). In a system made up
of parts, there is no qualitative advantage to the
assembly of identical subunits of generalists: it
only pays to co-operate among specialists.
Industrial and biochemical assembly lines
have much in common. Generalists, independence, growth and specialists, co-operation, and
production are two sides of the same coin.
The primacy of diagnosis
In medicine, accurate diagnosis is the essential
first step in the care of the patient. Prognosis
50
Table II. Diagnostic work-up of mitochondrial disease
Basic studies
Blood for mtDNA (PCR and Southern)
Blood and CSF for lactate and pyruvate, or MR Spectroscopy
Urine organic acids (by GC/MS)
Plasma and urine amino acids
Blood and urine carnitine
Brain MR imaging
Muscle biopsy
Neuropathology and electron microscopy
Mitochondrial electron transport studies
Fresh (coupled) mitochondrial polarography
Muscle mtDNA (PCR, Southern, Depletion)
mtDNA = mitochondrial DNA; PCR = polymerase chain
reaction; CSF = cerebrospinal fluid; MR = magnetic
resonance; GC/MS = gas chromatography mass
spectrometry.
cannot be assigned, therapies cannot be effectively tailored, and accurate genetic counselling cannot be offered until a pathogenetic
diagnosis is established. The complexity of
mitochondrial disease particularly mandates a
methodical approach to diagnosis. Table II
reports a 7-point work-up for mitochondrial
disease. It provides the foundation for diagnosis of a number of established mitochondrial disorders and provides clues for further,
more comprehensive studies of disorders that
are not yet fully characterized.
Organismal effects of
mitochondrial dysfunction
The English neurologist, John Hughlings Jackson, taught his students, 'The study of the
causes of things must begin with the study of
things caused'. We developed the mitochondrial clinical assessment scale (MCAS) to
catalogue the patterns of clinical and biochemical features caused by mitochondrial disease
(a full description of this scale and its implementation will be described elsewhere)
(R.K.Naviaux and K.A.McGowan, unpublished data). The mitochondrial clinical assessment scale (MCAS) is an organ-system survey
Organismal effects of mitochondrial disease
Table III. Mitochondrial clinical assessment scale (MCAS)-I
demographics of 40 patients with lactic acidaemia and
mitochondrial disease
Patients
Diagnosis
10
5
5
2
3
1
1
1
1
1
10
MELAS A3243G
Complex I
COX
NARP/LS T8993G
PDHL
KSS
PMPS
mtDNA depletion
3-HIBA
EMA-V
Unknown (25%)
MELAS = mitochondrial encephalomyopathy, lactic
acidaemia and stroke-like episodes; COX = cytochrome C
oxidase deficiency; NARP/LS = neurogenic muscular
weakness, ataxia, retinitis pigmentosa/maternally inherited
Leigh syndrome; PDH = pyruvate dehydrogenase deficiency
and Leigh syndrome; KSS = Kearns—Sayre syndrome;
PMPS = Pearson Marrow Pancreas syndrome; 3-HIBA = 3hydroxyisobutyric aciduria; EMA = ethylmalonic aciduria
with vasculopathy.
of the pattern and severity of disease resulting
from mitochondrial dysfunction. The MCASI had 23 organ systems divided into 55 categories, arranged roughly in anatomical order,
from head to toe. Each of the categories had
a three point grading scale: 0 = no abnormality
was detected; 1 = moderate dysfunction; 2 =
abnormalities that were not corrected by treatment. The maximum possible MCAS-I score
was 110 (55 categories X2 points/category).
Healthy subjects have a score of 0.
Over 300 inpatients were referred and evaluated for possible mitochondrial disease from
1994 to 1998 at the Mitochondrial and Metabolic Disease Center, University of California,
San Diego School of Medicine. All of these
patients received a comprehensive, elective,
4-day metabolic evaluation in the hospital. Of
these patients, 40 were found to have forms of
mitochondrial disease that led to unstimulated
blood or cerebrospinal fluid lactic acid levels
>3 mmol/1; half of these patients with lactic
acidaemia were under the age of 5 years. The
Table IV. Rank order of systems affected affected in 40
patients with mitochondrial disease and lactic acidaemia
Rank
System affected
Patients
affected
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
Metabolic/biochemical
Brain
Endocrine
Skeletal muscle
Eyes
Growth/nutrition/anorexia
Recurrent Infection
Kidneys
Genetics/affected relatives
Heart
Hearing
Bones/joints
Skin/hair/nails
Blood/bone marrow
Swallowing/secretions/reflux
Peripheral nerves
Bowel
Liver
Lungs
Gall bladder
Dysmorphism
Urinary bladder/neurogenic
Exocrine pancreas
40
39
33
32
30
28
25
23
23
18
16
16
15
15
14
10
10
10
8
6
3
2
1
median age was 7 years (range 0.5-53). Table
III summarizes the mitochondrial diagnoses
discovered in this group. Each of these 40
patients was scored using the MCAS-I. The
scores were normally distributed with a mean
of 32.7 and a SD of 7.0 (range 21-50). MCASI scores for patients aged >5 years were
not significantly different from the scores of
patients aged <5 years. Analysis of these data
permitted us to identify and rank-order the
most frequently affected organ systems and
laboratory abnormalities. Table IV summarizes these data.
The traditional standards for making a diagnosis of mitochondrial disease lie with biochemical, enzymological and molecular
studies (Walker et al., 1996; Bernier et al.,
1998) and we employed these. Thus, by
definition, 40 of 40 patients in our series had
abnormalities in one or more of these studies
(Table IV). The most commonly affected organ
51
R.Naviaux and K.McGowan
was the brain. About 75% of patients had
some degree of endocrine dysfunction, the
most common manifestation of which was
subclinical insulin resistance, seen as an exaggerated hyperglycaemic and hyperinsulinaemic response to a standardized glucose load.
Only two of our 10 patients with the MELAS
(mitochondrial encephalomyopathy, lactic acidaemia and stroke-like episodes) (A3243G)
mutation had frank diabetes mellitus; both
type I and type II diabetes were observed with
this mutation.
Some of the organ systems that were less
frequently observed to be abnormal in our
series were simply the result of lack of ascertainment. For example, we evaluated only one
child with Pearson Marrow Pancreas syndrome (PMPS) (Table III). This was the only
patient with evidence of exocrine pancreas
deficiency. This example serves as a warning,
that a simple tally of organ systems affected
in a population of mitochondrial disease is
only the first step toward understanding 'the
study of things caused', as John Hughlings
Jackson put it. To begin a study 'of the causes
of things' one needs to organize the patterns
of abnormalities associated with specific mitochondrial disease. We call these patterns the
diagnostic spectra of mitochondrial disease.
The infection connection
Of particular note is an observation that does
not receive sufficient emphasis in the literature
of mitochondrial disease: the frequency of
recurrent infections. Two thirds of the patients
in our series (25 out of 40) had histories of
recurrent upper respiratory or genito-urinary
tract infections. The importance of infection
in mitochondrial disease should not be underestimated. The majority of clinically important
neurodegenerative episodes associated with
mitochondrial diseases occur during or in the
week following an otherwise trivial infection,
e.g. otitis media, sinusitis, gastroenteritis, or
52
coryza. The mechanism for neurodegeneration
in this period is not related to catabolic stress:
patients are often recovering from their infections and have good nutrient intakes when
ataxia, dysphagia, aphasia, or encephalopathy
supervene. The tight coupling between mitochondrial failure and infection could relate to
the involvement of host defence cytokines and
mitochondria in apoptosis (Green and Reed,
1998), but the precise mechanisms await further investigation.
Developing diagnostic spectra of
mitochondrial disease
By displaying the results of the MCAS-I as a
bar graph with 55 categories, it became apparent that each patient's unique pattern of clinical
and laboratory abnormalities took on the quality of a spectrum with 55 channels. This was
reminiscent of chemical ion spectr/a observed
in the analysis of organic acids by mass
spectrometry. We arrayed the MCAS-I spectra
of patients with the same molecular diagnosis
and summed them to produce a composite
spectrum that revealed the similarities and
differences among the diseases. This is illustrated in Figure 5 for MELAS. Figure 6
compares the composite MCAS-I spectra of
10 patients with MELAS, five with COX
(cytochrome C oxidase) deficiency and Leigh
syndrome, and five with Complex I deficiency
and Leigh syndrome. The numerical codes
that corresponded to features observed in 80100% of the patients with each diagnosis were
listed as 'MCAS-I spectra' on the right of
Figure 6. Exclusion criteria also proved useful
and were listed with the numerical codes. An
important conclusion from this study was that
there is considerable overlap in the phenotypes
of mitochondrial disease. Even so, discriminant patterns emerged that were useful in developing both quantitative and qualitative
instruments for characterizing mitochondrial
disease.
Organismal effects of mitochondrial disease
MELAS Patients
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Composite MELAS Spectrum
ill
lln
Figure 5. Mitochondrial clinical assessment scale-I (MCAS-I) analysis of MELAS (A3243G). Patients with this mutation in
mtDNA were evaluated and scored using the MCAS-I form. The results of 10 patients (aged 5-53 years, median 16) are
displayed. A total of 55 characters were scored for each patient and arrayed on the jc-axis to yield a spectrum. Spectra were
summed to produce the composite MELAS spectrum shown at the bottom.
Figures 5 and 6 are meant to convey the
principles behind the MCAS-I analysis without belabouring the details of each organ
system analysed. The scoring of a single
patient takes ~5 min, once all the relevant
clinical and biochemical data are assembled.
Our hope is that interest in the development
of clinically useful diagnostic spectra of mitochondrial disease will foster the use of the
MCAS-II in medical centres around the world
that care for patients these disorders. By combining the clinical experiences of many
centres, it will be possible to identify distinct-
ive patterns that exist. These will guide both
clinicians and scientists. Clinicians will have
a strong, quantitative resource to guide them
in diagnosis and prognosis. Scientists will
develop a more complete knowledge of the
manifestations of mitochondrial disease, many
of which are completely unpredicted by current knowledge of mitochondrial biology. This
will help drive new investigations into the role
of mitochondria in evolution, development,
ageing, and disease. The MCAS-II scoring
forms are available upon request from the
authors.
53
R.Naviaux and K.McGowan
MCAS-I Spectra
MELAS Composite--10 Patients
MELAS:
1-6-10-15-25/26-34-37-38-46-54
and not 18-19-52-55
nil n n
10
20
30
40
50
COX/Leigh Composite--5 Patients
COX/Leigh:
1-2-3-10-11-29-33-37-38-42-46-51
and not 25/26-34
10
20
30
40
50
Complex I/Leigh Composite--5 Patients
Complex I/Leigh:
1.2-3-4-7-10-11-33-37-38
and not 8-12-54
1
10
Inlllln
20
30
40
50
Figure 6. Developing diagnostic spectra of mitochondrial disease. Composite MCAS-I spectra are illustrated on the left for
three mitochondrial diseases: MELAS (A3243G), cytochrome C oxidase (COX) deficiency with Leigh syndrome, and Complex
1 deficiency with Leigh syndrome. On the right are numerical codes that corresponded to the clinical or biochemical
characteristics that were observed in 80-100% of patients with the corresponding disorders. Exclusion criteria corresponded to
characteristics that were observed in 0% of the patients studied with the disorder.
Conclusions
Mitochondrial biology has a rich and noble
history dating back to 1888, when Kolliker
first teased the organelles from insect flight
muscle (Lehninger, 1965). Exactly a century
later the discovery of the first mtDNA
mutations in human disease was made (Holt
et al, 1988; Wallace et al, 1988; Zeviani
et al, 1988) and helped renew interest in
the biology of these fascinating organelles.
Mitochondrial medicine is now one of the
fastest growing fields in all of human biology.
It is poised to become a bridging discipline that
in the coming years will illuminate previously
non-apparent connections between subspecialties of medicine and biology.
This review has emphasized the tissuespecific differences in mitochondrial form and
function. Disturbances in mitochondrial func54
tion lead to selective abnormalities in organ
systems that are explainable in retrospect, but
which cannot be predicted on the basis of
current paradigms. Every year sees the publication of new observations in mitochondrial
replication and biology that defy the explanatory power of current theory. This represents
a growing tide of anomaly in mitochondrial
biology. When accepted theory lacks predictive power, and the catalogue of anomalies
grows beyond theory's embrace, then fundamental surprises and novel insights are inevitable.
Acknowledgements
This work was assisted by grants from the Lennox
Foundation to RKN, and a National Institutes of Health
training grant to KAM.
Organismal effects of mitochondria! disease
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