M–N

mitochondrial disorders Inherited mutations in
mitochondrial genes that result in functional dis-
turbances 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 mitochon-
drion contains the specific genetic material (MITO-
CHONDRIAL DNA [MTDNA]) to encode the enzymes
(specialized proteins) that regulate the biochemi-
cal reactions within the mitochondrion that gen-
erate 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 contin-
ues though may be impaired whenever the
mutated mitochondrial GENEsends incorrect code.
Only the ovum contains mitochondria that pass
on to the ZYGOTEat CONCEPTION. SPERMcells 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, symp-
toms are multisystem and inconsistent with the
conventional presentations of the health condi-
tions they suggest. MUSCLEand NERVEcells have
particularly high energy needs, so mitochondrial
disorders often manifest symptoms such as weak-
ness 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

experience for those patients looking for answers

for their symptoms. There are no definitive diag-

nostic tests for mitochondrial disorders, though

muscle biopsy often can provide strong evidence

supporting diagnosis once doctors rule out other

conditions and disorders. Treatment targets manag-

ing symptoms and preventing common complica-

tions such as DEHYDRATION. Some doctors advocate

COENZYME Q 10 supplementation for people who

have mitochondrial disorders, which appears to

improve the efficiency of cellular METABOLISMas

well as protect cells from oxidative damage. People

who have mitochondrial disorders should include

GENETIC COUNSELINGin their FAMILY PLANNINGefforts.

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 abnor-

mal chromosomes throughout all cells. The distri-

bution of abnormal cells in mosaicism is usually

random and unpredictable. Mosaicism most com-

monly 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 symp-

toms when only some cells express the abnormal-

ity (mosaic disorder) than occurs when all cells

express the abnormality (complete disorder). Peo-

ple who have a mosaic expression of the autoso-

mal 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.

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