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Summary second exam Medical Biochemistry and Pathophysiology
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Summary second exam of the course Medical Biochemistry and Pathophysiology; Pathophysiology, in the track Patient-centered Research
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PathophysiologyLecture 29 – Isoprenoid Biosynthesis Defects
Inherited metabolic orders → an enzyme can’t function properly and thus there is less product
➔ Diagnostic options:
o Metabolite analysis in plasma, urine, tissue
o Enzyme measurements in cells, tissues
o Mutation analysis in DNA
Research on inherited metabolic disorders
➔ Identification of novel defects → molecular and biochemical basis of
novel defects → development of diagnostic methods →
understanding the pathophysiology and molecular mechanisms →
development of therapeutic options
Cholesterol → important component of cellular membranes and myelin
➔ Also precursor for oxysterols, steroid hormones and bile acids
➔ Acquired by de novo synthesis → isoprenoid biosynthesis, or via the
uptake of LDL particles
➔ Plays an important role in human embryogenesis and development
Isoprenoid biosynthetic pathway → produces numerous biomolecules (isoprenoids)
➔ Isoprenoids function in a variety of important cellular processes including cell growth, differentiation,
protein glycosylation, signal transduction pathways and mitochondrial electron transport
Clinical aspects of cholesterol biosynthesis defects → multiple congenital anomalies (incl. organs), skeletal
abnormalities, dysmorphism, skin abnormalities, developmental retardation, psychomotor retardation
Of the eight different inherited disorders currently linked to specific enzyme defects in cholesterol/isoprenoid
biosynthesis, six specifically affect the synthesis of cholesterol, whereas two defects, both due to a deficiency
of the enzyme mevalonate kinase (MK), in principle affect the synthesis of all isoprenoids
➔ Mevalonic Aciduria (MA) and Hyper-Immunoglobulinaemia-D and Periodic Fever Syndrome (HIDS)
Classic MA and the more benign HIDS → represent the severe and mild clinical and biochemical ends of the
mevalonate kinase (MK) deficiency continuum and are both autosomal recessive
➔ Characterized by episodes of high fever that last 3–5 days, recur in average every 4–6 weeks, usually
start in the first year of life and may be provoked by vaccinations, physical and emotional stress, and
minor trauma. The episodes are associated with abdominal pain, vomiting and diarrhea, (cervical)
lymphadenopathy, hepatosplenomegaly, arthralgia (joint pains), and skin rash
o During fever → granulocytosis, monocytosis, elevated IL-1β, IL-6 and TNF-α
➔ Patients with the MA presentation may also present with congenital defects such as mental retardation,
ataxia, cerebellar atrophy, hypotonia, severe failure to thrive and dysmorphic features, which in the
most severely affected patients may lead to death in early infancy
HIDS → autoinflammatory disease
➔ Characterized by lifelong recurring, seemingly spontaneous episodes of fever and inflammation (no
antibodies as opposed to autoimmune diseases)
➔ Due to dysregulation of innate immune system
➔ Often associated with increased / prolonged production of the proinflammatory cytokine IL-1β
➔ Inherited: Familial Mediterranean Fever (FMF), TNF-Receptor Associated Periodic Fever Syndrome
(TRAPS), Cryopyrin Associated Periodic Fever Syndrome (CAPS), Hyper-IgD and Periodic Fever syndrome
(HIDS)
,Both MA and HIDS are caused by a deficiency of MK → distinguished by the residual MK activities in their cells
➔ In white blood cells or cultured primary skin fibroblasts of MA patients the activity of MK is usually below
detection level, whereas in cells of HIDS patients a residual MK activity of up to 10% of the activities in
cells of healthy controls is found
➔ High and moderately elevated levels of mevalonic acid can be detected in plasma and urine of patients
with MA and HIDS, respectively
HIDS → due to temperature-sensitive mutation
Most patients with the HIDS presentation are compound heterozygotes for the V377I MVK allele, which is found
exclusively in HIDS patients, and a second mutant allele, which can be found also in MA patients
➔ The V377I allele encodes an active MK enzyme, the correct assembly/maturation of which is
temperature-dependent and thus responsible for the observed residual MK enzyme activity associated
with the HIDS phenotype
Between fever episodes the MK activity is low, but
during fever the activity is even lower (due to the
temperature-sensitive mutation)
MK functions relatively early in the biosynthetic pathway, the
synthesis of all isoprenoids will be affected to a certain extent →
most of the characteristic clinical manifestations are thought to
be due to (temporary) shortage of nonsterol isoprenoid end
products. It may well be possible, however, that in severe MA
cases a relative shortage of sterol isoprenoids during embryonic
development leads to some of the clinical problems
No efficacious treatment available for MK deficiency
Clinical improvement because of treatment with corticosteroid,
colchicine, or cyclosporin has been reported, but in majority of
patients these treatments do not have beneficial effects. In a
small group of HIDS patients, simvastatin treatment had positive effect on the number of days of illness, but
treatment with similar statins in MA patients led to worsening of clinical symptoms. Also, treatment of few HIDS
patients with etanercept, a soluble p75 TNF alpha receptor-Fc fusion protein, or anakinra, led to reduction of
frequency and severity of symptoms, but have not been tested in larger groups of patients. The clinical
symptoms in HIDS tend to become less frequent and less severe with age
,Geranylgeranyl-PP is limiting in MKD-HIDS
Rho is bound to geranylgeranyl in controls
➔ Thus can be activated for signal
transduction cascade
Rho is not bound to geranylgeranyl in
patients with HIDS
➔ No signal transduction
Options for manipulating isoprenoid biosynthesis:
All manipulations result in increased synthesis of GG-PP in vitro (plus lowered HMG-CoA reductase activities and
Il-1β secretion)
Summary:
➔ The non-sterol isoprenoid biosynthesis pathway plays an important role in inflammation and innate
immune regulation
➔ MK deficiency renders MKD-HIDS patients more susceptible for developing a massive inflammatory
response to ‘minor’ precipitating events
➔ Inflammatory phenotype in MKD-HIDS due to a temporary deficit of geranylgeranyl-PP compromising
protein geranylgeranylation leading to ectopic activation of GTPases involved in immune regulation
➔ Ectopic activation of other GTPases may explain additional symptoms in MKD
➔ Targeting of isoprenoid biosynthesis pathway may render MKD-HIDS patients less susceptible to
development of inflammatory episodes
, Lecture 30 – Fatty Acid Oxidation Disorders
VLCAD deficiency → autosomal recessive
inheritance, ACADVL gene
➔ Liver → hypoglycemia (<2.5 mmol/l)
➔ Heart → cardiomyopathy and/or
arrhythmia
➔ Muscle → myopathy (CK > 250 U/L,
normal 70-170), and at least two of
the following: myoglobinuria, myalgia,
exercise intolerance, muscle
weakness, and/or frequent fatigue
Pathophysiological mechanisms:
➔ Energy shortage
➔ Accumulation of acylcarnitines
Symptoms → often triggered during catabolism (fasting, activity, infection) can be fatal
➔ Outcome very variable. Patients with mild vs. severe phenotype
Inclusion in newborn screening (hielprik) in 2007
➔ Pros:
o Prevent early-life complications
o We know all Dutch patients and can use their cells
➔ Cons:
o Cannot prevent late-life complications
o How to predict future disease?
FAO flux is variable between patients and can be increased when
culturing at 30ºC
➔ FAO flux correlates with clinical severity → the less the
flux, the more severe the disease is
Resveratrol increases FAO flux in lcFAO patient fibroblasts, and bezafibrate has similar effects
➔ But can similar beneficial effects be achieved in relevant cell types?
hiPS-cardiomyocytes as a model for arrhythmias in VLCAD deficiency
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