Encyclopedia of Health and Medicine

M–N
mitochondrial disorders Inherited mutations in
mitochondrial genes that result in functional disturbances
in various body systems. Mitochondria
are structures within the cell that generate the
energy, in the form of adenosine triphosphate
(ATP), the cell requires to function. A cell may
contain dozens of mitochondria. Each mitochondrion
contains the specific genetic material (MITO-
CHONDRIAL DNA [MTDNA]) to encode the enzymes
(specialized proteins) that regulate the biochemical
reactions within the mitochondrion that generate
ATP. The only function of mtDNA is to
regulate these processes of energy production.
Each mitochondrion contains multiple copies of
its DNA. Mutations typically affect some but not
all DNA copies, so mitochondrial function continues
though may be impaired whenever the
mutated mitochondrial GENE sends incorrect code.
Only the ovum contains mitochondria that pass
on to the ZYGOTE at CONCEPTION. SPERM cells contain
few mitochondria, and these are in the sperm
cell’s tail, which breaks away as soon as the sperm
penetrates the ovum. As the zygote continues to
divide, it may perpetuate errors in mtDNA that are
widespread or pervasive.
Mitochondrial disorders include myositis, some
types of CARDIOMYOPATHY, some types of MYOPATHY,
and carnitine deficiency syndrome. Often, symptoms
are multisystem and inconsistent with the
conventional presentations of the health conditions
they suggest. MUSCLE and NERVE cells have
particularly high energy needs, so mitochondrial
disorders often manifest symptoms such as weakness
and poor muscle tone (hypotonia).
Because mitochondrial disorders are rare and
their symptoms are confusing, the diagnostic path
may lead to numerous dead ends. Though this
process rules out other diagnoses, it is a frustrating
134
experience for those patients looking for answers
for their symptoms. There are no definitive diagnostic
tests for mitochondrial disorders, though
muscle biopsy often can provide strong evidence
supporting diagnosis once doctors rule out other
conditions and disorders. Treatment targets managing
symptoms and preventing common complications
such as DEHYDRATION. Some doctors advocate
COENZYME Q10 supplementation for people who
have mitochondrial disorders, which appears to
improve the efficiency of cellular METABOLISM as
well as protect cells from oxidative damage. People
who have mitochondrial disorders should include
GENETIC COUNSELING in their FAMILY PLANNING efforts.
See also CELL STRUCTURE AND FUNCTION; CHROMO-
SOME DISORDERS; GENETIC DISORDERS; MUTATION; REPLI-
CATION ERROR.
mosaicism A chromosomal disorder in which
some cells are normal and some cells contain the
chromosomal abnormalities of the disorder, in
contrast to a complete distribution of the abnormal
chromosomes throughout all cells. The distribution
of abnormal cells in mosaicism is usually
random and unpredictable. Mosaicism most commonly
occurs in AUTOSOMAL TRISOMY, in which
there is an additional copy of one CHROMOSOME
that appears in some cells and not in others. The
result generally is a milder presentation of symptoms
when only some cells express the abnormality
(mosaic disorder) than occurs when all cells
express the abnormality (complete disorder). People
who have a mosaic expression of the autosomal
trisomy disorder DOWN SYNDROME, for
example, typically have milder symptoms than
people who have a complete expression.
Mosaicism may also affect genetic expressions
other than health disorders.