Page 859 - Hematology_ Basic Principles and Practice ( PDFDrive )

742 Part VI Non-Malignant Leukocytes

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dysfunction and cell death. In fact, an approved therapy for NPD mutations from the paternal chromosome. This also suggests that
type C, Miglustat (see later), is based on the principle of reducing some type A and B NPD carrier individuals with maternally derived
ganglioside storage in the brain rather than correcting the primary mutations might exhibit clinical or laboratory manifestations of the
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cholesterol transport defect. Similarly, in NPD types A and B, the disorder, and there is at least one report documenting very low serum
sphingolipid sphingomyelin is the primary accumulating substrate, high-density lipoprotein levels in such carrier individuals. 20
although cholesterol storage also is a major contributory factor. Diagnostic assays for patients suspected of having LSDs generally
Indeed, for any individual LSD the pattern of macromolecule rely on the measurement of specific enzymatic activities in isolated
accumulation may be extremely heterogeneous and, importantly, also leukocytes, cultured fibroblasts, or transformed lymphoblasts. For
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may be tissue and cell specific. Although the mechanism(s) leading some disorders, carrier identification and prenatal diagnosis are avail-
to this heterogenous storage pattern is not always known, it presum- able as well. However, because detection of the individual enzyme
ably relates to a global dysfunction of the lysosomal system, providing activities is often complex (e.g., uses nonnatural substrates, detergents,
a connection between seemingly distinct metabolic pathways. and other specific assay conditions), it is recommended that the
As the macromolecules accumulate, the lysosomes become dis- enzymatic confirmation of suspected cases be carried out in special-
tended and destabilized, and may eventually fail to carry out their ized laboratories experienced in these methods. In addition, because
normal functions related to phagocytosis and autophagocytosis. In in most LSDs leukocytes and skin fibroblasts are not the clinically
turn, this may result in cell dysfunction, senescence, and/or death. relevant cell types, these assay methods are at best indirect measures
In addition, because of the progressive cell and organ disease that of the defective lysosomal protein’s function at the pathologic sites.
occurs in the LSDs, inflammatory pathways are frequently activated For this reason, predicting the clinical outcome from these in vitro
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in an attempt to repair the damage. However, because the inflamma- measurements is generally not reliable. For example, cells from
tory cells themselves are dysfunctional, the diseases progress and the patients with the infantile, neurologic form of ASM-deficient NPD
inflammatory changes may become chronic. Inflammation is there- (type A) and the later-onset, nonneurologic form (type B) often have
fore also an important contributory factor to the pathogenesis of similar residual enzymatic activities, although their clinical course is
LSDs. For example, in the MPS diseases GAG storage is known to markedly different. This likely reflects the function of the individual
activate the Toll-like receptor 4 signaling pathway, leading to the mutant ASM polypeptides in the brain. Numerous other examples
release of tumor necrosis factor-α (TNF-α) and other inflammatory exist for other LSDs as well, posing a unique challenge for predicting
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cytokines. TNF-α, in turn, causes elevation of the toxic lipid phenotypic outcomes in newly diagnosed patients.
ceramide within cartilage cells (chondrocytes), contributing to cell As already noted, many genetic abnormalities have been identified
death. Treatment of animal models of MPS with anti-TNF-α drugs for most of the individual LSDs. For most diseases, multiple muta-
alone has resulted in the reduction of chondrocyte death and sub- tions have been found, and the majority of these are unique (i.e.,
stantial clinical benefits, demonstrating the importance of inflamma- private) to individual families. However, for some LSDs there are
tion in these diseases. 15,16 specific populations that may have recurrent mutations caused by
founder effects and/or consanguinity, facilitating the use of DNA-
GENETICS AND DIAGNOSIS OF LYSOSOMAL based screening methods for the detection of the LSD. This has been
most effectively translated into clinical use in the Ashkenazi Jewish
STORAGE DISEASES population, in which relatively small number(s) of mutations account
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for several LSDs. This has led to the establishment of a DNA-based
All LSDs except for two, Fabry disease and MPS type II (Hunter “Jewish Genetic Disease” screening panel and the population-based
disease), are inherited as autosomal recessive traits. Fabry and Hunter identification of carrier individuals for the same disorder. Such
diseases are inherited as X-linked recessive traits. Most mutations individuals are referred for genetic counseling to assist with family
causing individual LSDs result in single amino acid changes in the planning and pregnancy outcome choices. The implementation of
enzyme’s polypeptide chain, resulting in absent or defective function. such screening has led to a dramatic reduction in the incidence of
The genes encoding most lysosomal proteins have been cloned, and some diseases within this population (e.g., infantile Tay–Sachs
there is no obvious clustering of these genes within the genome. In disease), and will likely lead to the prevention of other disorders as
some cases, nonfunctional pseudogenes also have been described, well. The rapid evolution of cost-effective, high-throughput sequenc-
which may or may not be transcribed or translated into a nonfunc- ing methods is also likely to open other populations and disorders to
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tional protein. Although there is no clustering of the lysosomal genes, these DNA-based screening approaches. However, it is important
most exhibit coordinated transcriptional behavior and are regulated to note that the functional consequences of most DNA abnormalities
by TFEB. 4,17 In the LSDs TFEB is often translocated from the on the protein function have not been confirmed, and thus DNA-
cytoplasm to the nucleus to “turn on” expression of other lysosomal based methods alone should not be used to predict clinical outcomes
proteins and enhance lysosomal biogenesis. This is a mechanism by in patients unless the biochemical consequences of these abnormali-
which cells try to compensate for lysosomal dysfunction. However, ties are fully established. In general, confirmation of a suspected LSD
because the newly formed lysosomes in these diseases will retain the case should be accomplished by both enzymatic and DNA-based
same primary metabolic defect, this may lead to amplification of the studies, and in only rare cases are these laboratory tests useful to
disease pathology. predict clinical outcomes.
The mutated proteins in LSD patients may be stable and delivered
to lysosomes (albeit with reduced catalytic function), or may be
unstable with only partial or absent delivery to lysosomes. The effects THERAPY OF LYSOSOMAL STORAGE DISEASES:
of the individual mutations may also be cell and tissue specific AN OVERVIEW
depending on the normal expression pattern of the enzymes. In
general, heterozygous “carriers” of single mutations in a lysosomal Since the first recognition that LSDs resulted from the defective
gene do not develop clinical symptoms of the disorder, except in the function of lysosomal proteins, the concept of simply “replacing” the
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X-linked disorders. For example, in Fabry disease X-inactivation missing protein in individual patients was put forward. This concept
patterns can lead to clusters of cells without enzyme activity, and was further strengthened in the 1970s by the identification of the
female individuals carrying one α-galactosidase A mutation may M6P-targeting system for lysosomal enzymes and the finding that the
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develop disease-related pathology. In addition, one lysosomal gene, secreted forms of many lysosomal enzymes could be rapidly internal-
SMPD1 encoding acid sphingomyelinase (ASM), is known to be ized, or “taken up”, by cell surface M6P receptors and delivered into
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“paternally imprinted” (i.e., preferentially expressed from the mater- the lysosomal system. Today, the principle underlying the treatment
nal chromosome), suggesting that type A and B NPD individuals of most LSDs remains the replacement of the missing or defective
who inherit “severe” SMPD1 mutations on the maternal chromosome protein. This can be accomplished by stem cell transplantation,
may be more severely affected than those who inherit the same protein (enzyme) replacement therapy, or gene therapy. In some cases,