Genetic Disorders 2 Flashcards
Lysosomal Storage Diseases
Lysosomes are key components of the “intracellular digestive
tract.” They contain a battery of hydrolytic enzymes, which have
two special properties. First, they function in the acidic milieu of
the lysosomes. Second, these enzymes constitute a special
category of secretory proteins that are destined not for the
extracellular fluids but for intracellular organelles. This latter
characteristic requires special processing within the Golgi
apparatus
Similar to all other secretory proteins, lysosomal enzymes (or
acid hydrolases, as they are sometimes called) are synthesized
in the
ER
The phosphorylated mannose
residues serve as an
“address label” that is recognized by
specific receptors found on the inner surface of the Golgi
membrane.
n inherited deficiency of a
functional lysosomal enzyme gives rise to two pathologic consequences (
Catabolism of the substrate of the missing enzyme remains
incomplete, leading to the accumulation of the partially
degraded insoluble metabolite within the lysosomes.
Since lysosomal function is also essential for autophagy,
impaired autophagy
, leading to the accumulation of the partially
degraded insoluble metabolite within the lysosomes. This is
called
“primary accumulation”.
impaired autophagy gives rise to
“secondary accumulation” of
autophagic substrates such as polyubiquinated proteins and old
and effete mitochondria. The absence of this quality control
mechanism causes accumulation of dysfunctional mitochondria
with poor calcium buffering capacity and altered membrane
potentials. This can trigger generation of free radicals and
apoptosis.
Tay-Sachs Disease (GM2
Gangliosidosis:
Hexosaminidase α-Subunit Deficiency)
GM2 gangliosidoses are a group of three lysosomal storage
diseases caused by an inability to catabolize GM2 gan‐
gliosides. Degradation of GM2 gangliosides requires three
polypeptides encoded by three distinct genes
The phenotypic
effects of mutations affecting these genes are fairly similar,
because they result from accumulation of GM2 gangliosides. The
underlying enzyme defect, however, is different for each
the most common form of GM2 gangliosidosis,
results from mutations in the α-subunit locus on chromosome 15
that cause a severe deficiency of hexosaminidase A.
Tay Sachs
This disease
is especially prevalent among
Jews, particularly among those of
Eastern European (Ashkenazic) origin, in whom a carrier rate of
1 in 30 has been reported.
ganglioside
glycosphingolipid ith one or mote sialic acids on the sugar chain
Complex lipids in the brain
cell-cell recognition, adhesion, transductiom
degraded by ceramides and sequential removal of sugar units in oligosaccharide
The hexosaminidase A is absent from virtually all the
tissues, so GM2 ganglioside accumulates in many
tissues
heart, liver, spleen, nervous system), but
the involvement of neurons in the central and
autonomic nervous systems and retina dominates
the clinical picture
histologic examination
neurons are ballooned with cytoplasmic vacuoles,
each representing a markedly distended lysosome
filled with gangliosides (Fig. 5-11A). Stains for fat
such as oil red O and Sudan black B are positive. With
the electron microscope, several types of cytoplasmic
inclusions can be visualized, the most prominent
being whorled configurations within lysosomes
composed of onion-skin layers of membranes (Fig. 5-
11B).
A cherry-red spot thus appears in the
macula, representing accentuation of the normal
color of the macular choroid contrasted with the pallor
produced by the swollen ganglion cells in the
remainder of the retina (Chapter 29). This finding is
characteristic of Tay-Sachs disease and other storage
disorders affecting the neurons.
Clinical Features of TS
The affected infants appear normal at birth but begin to
manifest signs and symptoms at about age 6 months. There is
relentless motor and mental deterioration, beginning with motor
incoordination, mental obtundation leading to muscular
flaccidity, blindness, and increasing dementia.
cherry-red spot appears in the macula of the
eye in almost all patients. Over the span of 1 or 2 years a
complete vegetative state is reached, followed by death at age 2
to 3 years. More than 100 mutations have been described in the
α-subunit gene; most affect protein folding. Such misfolded
proteins trigger the “unfolded protein” response (Chapter 1)
leading to apoptosis
Niemann-Pick Disease Types A and B
Niemann-Pick disease types A and B are two related
disorders that are characterized by lysosomal
accumulation of sphingomyelin due to an inherited
deficiency of sphingomyelinase.
Type A
severe infantile
form with extensive neurologic involvement, marked visceral
accumulations of sphingomyelin, and progressive wasting and
early death within the first 3 years of life
Type B
disease patients have organomegaly but generally no central
nervous system involvement. They usually survive into
adulthood. As with Tay-Sachs disease, Niemann-Pick disease
types A and B are common in Ashkenazi Jews. The gene for acid
sphingomyelinase maps to chromosome 11p15.4 and is one of
the imprinted genes that is preferentially expressed from the
maternal chromosome as a result of epigenetic silencing of the
paternal gene
Although, this disease is
typically inherited as an autosomal recessive,
those
heterozygotes who inherit the mutant allele from the mother can
develop Nieman Pick Disease. More than 100 mutations have
been found in the acid sphingomyelinase gene and there seems
to be a correlation between the type of mutation, the severity of
enzyme deficiency, and the phenotype.
Morphology of NEIP
In the classic infantile type A variant, a missense
mutation causes almost complete deficiency of
sphingomyelinase. Sphingomyelin is a ubiquitous
component of cellular (including organellar)
membranes, and so the enzyme deficiency blocks
degradation of the lipid, resulting in its progressive
accumulation within lysosomes, particularly within
cells of the mononuclear phagocyte system. Affected
cells become enlarged, sometimes to 90 µm in
diameter, due to the distention of lysosomes with
sphingomyelin and cholesterol
Innumerable small
vacuoles of relatively uniform size are created
imparting foaminess to the cytoplasm
In frozen sections of fresh tissue
the vacuoles stain for
fat. Electron microscopy confirms that the vacuoles
are engorged secondary lysosomes that often contain
membranous cytoplasmic bodies resembling
concentric lamellated myelin figures, sometimes
called “zebra” bodies
The lipid-laden phagocytic foam cells are
widely
distributed in the spleen, liver, lymph nodes, bone
marrow, tonsils, gastrointestinal tract, and lungs. The
involvement of the spleen generally produces
massive enlargement, sometimes to ten times its
normal weight, but the hepatomegaly is usually not
quite so striking.
In the brain
the gyri are shrunken and the
sulci widened. The neuronal involvement is diffuse,
affecting all parts of the nervous system. Vacuolation
and ballooning of neurons constitute the dominant
histologic change, which in time leads to cell death
and loss of brain substance. A retinal cherry-red
spot similar to that seen in Tay-Sachs disease is
present in about one third to one half of affected
individuals.
Clinical manifestations in type A disease
may be present at
birth and almost invariably become evident by age 6 months.
Infants typically have a protuberant abdomen because of the
hepatosplenomegaly. Once the manifestations appear, they are
followed by progressive failure to thrive, vomiting, fever, and
generalized lymphadenopathy as well as progressive
deterioration of psychomotor function. Death comes, usually
within the first or second year of life