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NHLBI WORKING GROUP ON THE ROLE OF
CHOLESTEROL AND LIPIDS IN EMBRYONIC DEVELOPMENT AND CONGENITAL DISEASE
MAY 28, 1998
MEETING SITE: VANDERBILT UNIVERSITY, NASHVILLE
TENNESSEE
MINUTES AND CONSENSUS RECOMMENDATIONS:
This document highlights discussions and
presentations given by a group assembled by Dr. Lan Hsiang Wang of the National
Institute of Health on May 28, 1998 on the campus of Vanderbilt University,
Nashville Tennessee. A major purpose of the meeting was to consider the
potential role of cholesterol and lipids, their metabolism, serum carriers and
cell surface receptors in normal and abnormal cardiac development.
Through a series of research presentations, ways were identified in which
cholesterol and related lipids could affect development in general and the
heart in particular. A second purpose was to extend the discussion of
cholesterol to ways in which new and important technological or conceptual
opportunities might be used to further understand mechanisms of congenital
heart disease.
STATEMENT OF RATIONALE FOR THE WORKING GROUP:
Cholesterol, and the synthetic pathway leading to
cholesterol, are central to the formation and function of cell membranes and
the synthesis of a number of steroid hormones. Additionally, cholesterol
is used in the synthesis of isoprene moieties that are used to
posttranslationally modify proteins (e.g. farnesylation) such that the modified
protein can bind to cell membranes, and as a result of membrane association,
the function of the protein is changed.
The reputation of cholesterol has traditionally been
compromised by its exacerbation of arterial intimal thickening into fatty
atherosclerotic plaques. However, while too much cholesterol may be
deleterious, too little can engender birth defects. An evolution in the
understanding of an relationship between birth defects and cholesterol began
with the recognition that the Smith-Lemli-Opitz syndrome (SLOS) was a
cholesterol deficiency problem resulting from an autosomal mutation in a rate
limiting enzyme required for the final step of cholesterol synthesis. A
spectrum of birth defects occur in SLOS including limb, heart, craniofacial and
urogenital. Another common developmental defect with some phenotypic
similarities to SLOS is holoprosencephaly (HPE). The HPE gene was
recently found to be the morphogen, sonic hedgehog (Shh).
Knockouts of Shh in mice produced a head phenotype similar to both
SLOS and HPE. Similarly inhibitors to cholesterol synthesis produced
malformations in mice characteristic of both HPE and SLOS. The link
between Shh and cholesterol appears that the latter is used to
covalently transport and anchor the morphogen to target cell receptors
indicating a potentially important connection may exist developmentally between
the signaling capacity of morphogens and cholesterol.
I. Discussion and Consensus Recommendations Related
to Cholesterol and Its Potential Roles in Development:
- Hedgehog Genes: An overview of the role of the
hedgehog genes in mammalian development was presented. Hedgehog genes can
act as intermediate and long range signaling molecules that function primarily
in pattern formation. Each is a highly conserved protein which two
domains: a signaling domain and an autocatalytic domain. Cholesterol insertion
into a binding site of the catalytic domain appears to be required for its
enzymatic activity. Cholesterol also binds covalently to the signaling
end of hedgehog protein and tethers the morphogen to the cell membrane.
How hedgehog proteins are able to reach their receptors while tethered to
cholesterol is unknown. The key point is that cholesterol modification of
the hedgehog class of signaling morphogens is critical to their function.
It is proposed that cholesterol binding of hedgehog is involved in
transport into target cells or out of a cell that secretes it. One
intriguing possibility is that hedgehog is simply shed from the surface where
it can subsequently serve as intermediate or long range signaling molecule.
Of the three major members of the hedgehog class
of morphogens, Sonic hedgehog (Shh) has been most investigated. It
is expressed primarily in the notochord, CNS, gut, limbs, lung and tooth
epithelium. Shh binds to a receptor, patch, which leads to a
conformational change in a binding partner, smoothened, that possess kinase
signaling activity. Another downstream target is Gli which encodes a
basic helix-loop-helix transcription factor that acts as positive regulator
Shh signaling.
Shh has been shown to directly play a role
in patterning of the limb and CNS. It is expressed by the notochord and
ventral floor plate where the morphogen appears to function as a ventralizing
factor in the formation of neurons in the developing neural tube. In the
limb, Shh is expressed in posterior mesoderm of the limb bud where it
conveys the anterior-posterior polarizing activity traditionally associated
with the ZPA (zone of polarizing activity). The expression of Shh
helped to open the door to an understanding of molecular asymmetry where its
left-sided expression at Hensen's node was linked to the downstream expression
of nodal (one of several members of the TGF supergene family associated with
breaking symmetry). The role of Shh in regulating laterality may involve
a downstream target, ptx-2, frequently used by other genes also
expressed asymmetrically. For example, ptx-2 is primarily
expressed in the left heart forming field. What is unclear is why knockouts of
Shh do not cause asymmetry.
In the developing lung primordium, Shh is
expressed in the epithelium derived from endoderm. This epithelium interacts
with adjacent mesenchyme to promote branching which in Shh knockout mice
was severely restricted. These findings suggest that Shh secreted
by the epithelium may induce adjacent mesenchyme to secrete factors that
promote branching. This observation suggests an inductive paradigm that
may prove useful for future investigations of other epithelial mesenchymal
interactions.
Desert hedgehog (Dhh) appears to be the
only known hedgehog gene to be expressed in the heart (AV canal). Its
function in heart development is unknown. The major known function for
Dhh appears to be in spermatogenesis; within the testis, Sertoli cells
appear to be the principal site of expression. Knockouts of Dhh
result in small testes producing infertile sperm (females appear normal).
Additionally, Leydig cells differentiation appears abnormal leading to
decreased androgen production.
Indian hedgehog (Ihh) appears to be
required for growth, not patterning. It is expressed in the hypertrophic
region of cartilage, particularly in the growth (epiphyseal) plates.
Knockouts lead to dwarfism.
- SLOS and Other Inborn Errors of Cholesterol
Biosynthesis: The recent discovery of abnormal cholesterol biosynthesis in
children with SLOS was discussed by many attendees because it clearly
demonstrated the need for cholesterol in development. In the three
documented inborn errors of cholesterol synthesis, mevalonic aciduria,
desmosterolosis and SLOS, each results in dysmorphic phenotypes that range in
severity, mental retardation and growth disorders. These defects emphasize the
intrinsic role of cholesterol in development and, even though frequency of
these syndromes is on the order of 1 in 20,000 (highest for SLOS), it raises
the question of whether hypocholesterolemia occurs in other birth defects
involving similar dysmorphogies and neurological or growth disorders but whose
etiology is virtually unknown. It is possible that there remain inherited
deficiencies of other enzymes of cholesterol biosynthesis that have not been
identified.
In analyzing the phenotype of the three
disorders, it is of interest that mevalonic aciduria which is a proximal defect
of sterol metabolism and does not result in cholesterol deficiency, also does
not present with significant malformations, while SLOS and desmosterolosis
which are distal defects and result in deficiency of cholesterol are associated
with significant malformations of all organ systems. Earlier animal
studies using pharmacological inhibitors of cholesterol synthesizing enzymes
revealed similar anomalies to those seen in the human syndromes. Some of these
teratogenically induced anomalies could be prevented by cholesterol
supplementation to the mother. These data raised questions as to whether
an adequate amount of cholesterol is needed early in development and may be
more important with regard to the causation of anomalies than the presence of
the proper cholesterol precursors. A consensus of opinion was that future
studies of investigation should include offering sterol analysis to patients
with developmental delays or mental retardation, particularly of the heart,
limbs, palate and genitalia. Similarly, tissue of fetuses with intrauterine
demise and stillborns should be analyzed to further define the spectrum of
hypocholesterolemia in human development.
- Cholesterol, Protein Lipidation and Intracellular
Signaling: The cholesterol synthetic pathway gives rise to intermediates termed
isoprenes that are used to lipidate proteins which frequently leads to
modifications in the function of these proteins. One question discussed
by the group was how changes in the cholesterol pathway could affect
isoprenylated proteins associated with intracellular signaling. One
answer is the outcome observed for the farnesylation (a form of isoprenylation)
of ras in cardiac muscle. Farnesylation of ras increased its association
with cell surface membranes. This association between membrane and
lipidated-ras was found to be reversible and essential for ras activity.
Farnesylation has been suggested to be a regulatable process in embryonic
chick myocytes. Normally, chick myocytes contain a predominance of
unfarnesylated ras in the cytoplasm, however, when myocytes were cultured in
lipoprotein-depleted serum, cholesterol synthesis increased and ras was
farnesylated and shifted the membrane. Movement of ras to the membrane
lead to the activation of genes associated with response to autonomic nerve
signaling. Thus, the observation with ras indicates that increased
cholesterol metabolism may alter the posttranslational modification of a
protein that mediates intracellular signaling.
Other examples of isoprenylated proteins include
nuclear lamins, rho protein, rab proteins, and cdc42.
All these are among a class of proteins involved in G protein signaling,
cytoskeletal rearrangements and vesicle transport. The understanding of
the regulation of the enzymes involved in modification of these proteins, the
involvement of these proteins in developing heart cells, and the effect of flux
through the cholesterol synthetic pathway all need to be considered. A
consensus of opinion was that studies targeted at determining the role of
protein lipidation and isoprenylation as a mechanism to regulate cell
differentiation should be a high priority. These processes are likely to be
critical regulators of embryogenesis by establishing or altering growth factor
responsiveness in developing cells.
- Lipoprotein Secretion and Development: As carriers
of cholesterol, apolipoproteins potentially should play an important role in
development. Indeed, knockouts that variably reduced levels of ApoB have
convincingly shown that lipoproteins are essential for development,
particularly of the CNS and the GI tract. In many ways, the phenotype of
variable ApoB knockouts resembled SLOS. A reduction of 25-30% in
lipoprotein was sufficient to engender maldevelopment. In ApoB -/- mice,
all ApoB synthesis was blocked; most embryos died by day 12. The primary
cause of death appeared to be arrested yolk sac formation. As a
consequence, lipid transport to the developing embryo was deficient and
cholesterol stores were much reduced. One hypothesis proposed was that
any gene that might inhibit lipoprotein production by the yolk sac would lead
to developmental abnormalities. As a test of the hypothesis, a gene which
is expressed in the yolk sac and is required for apoB-lipoprotein assembly was
knocked out. Mice homozygous for this gene [microsomal triglyceride
transfer protein (Mttp)] died around day 10.5; their yolk sacs were pale
compared to wild.ypes owing to an apparent reduction in blood island formation.
These and other results indicate a link between lipoprotein synthesis and
hematopoiesis.
Information was presented that differences in
apolipoprotein secretion and esterification of cholesterol occur in adult
Down's patients which, in some unknown way, protect them from atherosclerosis.
It is also well known that Down's patients are high risk for Alzheimer's
disease possibly due to a gene, amyloid precursor protein, expressed on
chromosome 21. When this is combined with the recently described
association of apolipoprotein E 4 allele with increased risk of Alzheimer's
disease, the implication for future study is that novel insights into mental
retardation and cardiovascular disease could come from studying cholesterol and
lipoproteins in children born with trisomy 21 or in animal models of Down
syndrome (e.g. murine trisomy 16).
A consensus opinion was that future studies were
needed to determine if yolk sac lipoprotein secretion is needed in humans as
well as mice, and, if so, whether it is important simply as means of delivering
triglycerides, fat soluble vitamins, or cholesterol, or if apolipoproteins are
required for morphogenesis in some other way like hematopoiesis or normal
vasculogenesis. For example, do serum lipoproteins have a mitogenic
effect that disrupts normal growth patterns of cells critical to vascular and
heart development, like the neural crest. Another future direction would
be to explore the developmental role of receptors for lipoproteins. Virtually
nothing is known regarding these receptors during development and if the
primary role of these receptors is merely to take up cholesterol or lipids or
if binding of ligands to these receptors mediates intracellular signaling.
II. Programmatic Recommendations:
A series of recommendations with some supportive
justification was also made regarding future directions for research in
congenital cardiovascular diseases with or without implication for cholesterol
or lipids. These are briefly listed below.
- Human Molecular Genetics: As exemplified by SLOS,
human genetic models (syndromes) provide a powerful reminder of the critical
importance of molecular genetics in understanding complex birth defects.
For example, mutations in proteins and enzymes encoded by mitochondrial
or nuclear DNA that are essential for cardiac energy production can cause
sudden death in infants, dilated cardiomyopathy and hypertrophic
cardiomyopathy. Similarly, single gene defects are now being identified
for specific forms of congenital cardiovascular disease. Atrial septal
defects have been linked to mutations in transcription factors such as
Tbx5 (Holt-Oram syndrome) and tinman (Nkx-2.5).
Vasculopathies secondary to mutations of fibrillin-1 (Marfan's syndrome),
elastin (Williams' syndrome and supravalvular aortic stenosis), tenascin
(Ehlers-Danlos syndrome) and jagged (Alagille's syndrome) have been defined.
Also identified are loci for a dominant form of atrial septal defect, AV
septal defect and total anomalous pulmonary return. In animal models,
mutations in right-left dynein have been linked to heterotaxy syndromes,
asplenia and polysplenia. Genes have been traced to specific loci on
chromosome 21 which when duplicated as in Down's syndrome engender AV septal
defects and conotruncal malformations. Specific candidates at these loci
have recently been identified such as DS-CAM (Down syndrome cell adhesion
molecule). Thus, because it is now clear that a single gene modification
can result in life-threatening heart or vessel defects, it is recommended that
as the human genome project matures, both mapping and gene localization studies
should be encouraged. In turn, such studies should be used to develop
animal models in which the mechanisms of human congenital heart disease can be
studied morphologically and physiologically.
- Cellular Mechanisms: As the genes with relevance to
heart disease become identified, the question then becomes how the encoded
proteins affect heart development. If knockouts or knockins produce
phenotypes indicative of functional significance, the challenge then becomes to
determine the morphogenetic mechanism by which the missing, truncated or
overexpressed protein actually modified development. The problem becomes
even more daunting if lethality occurs early in development. Several centrally
defining, morphogenetic mechanisms were proposed that should be studied.
These were epithelial mesenchymal transformations and neural crest
migration and signaling. The former is the process by which cushion
tissues form and fuse to form valvuloseptal primordia. In particular, the
inducers and mediators of transformation processes and the transcription
factors that regulate them were highlighted for study. While neural crest
are widely recognized for their role in septating the aortic sac into the roots
of the aorta and pulmonary trunk, their role within the outlet region of the
heart tube is unknown. What is clear is that two prongs of neural crest migrate
into the truncus and conus regions of the outflow tract of the heart from the
septum which divides the aortic sac. If prevented from reaching the
outflow tract following experimental ablation, Pax3 mutations (Splotch
mouse) or monosomy of 22q11 (DiGeorge syndrome), misalignment of conus septum
with the ventricular septum occurs (e.g. double outlet right ventricle). Yet
nothing is known about the morphogenetic mechanisms by which neural crest might
influence septal alignments. It is recommended that genes known to
produce misalignments (e.g. TGF beta2) or cushion defects (DSCAM) be evaluated
for potential roles in epithelial transformations or cell migrations etc.
Rescue of cellular or tissue phenotypes should be considered in future
studies as the ultimate assay of gene function.
A strong recommendation was also made for the use
of non-mammalian animal models. It was noted how extensively the field of
cardiovascular developmental biology has been advanced by studies in xenopus,
zebrafish, C. elegans and fruit flies. That effort could be enhanced by
continued development of experiments using these systems. The development
of organisms, particularly like Xenopus, provide high throughput screening
systems for genes and gene activities and may also provide a sensitive way to
examine potential mechanisms of oligonucleotide mediated gene conversion and
gene control.
- Technological Considerations: Several opinions were
expressed that technologies for the analysis of genetically altered animal
models had not kept pace with the very rapid increase in the production of
animals with mutant phenotypes. Careful classification and
characterization of phenotypes is essential for ultimately mapping the genes
responsible for normal and abnormal development. Researchers who are well
trained at generating animal models are not often well trained to assess the
results of their labors. To this end, several ideas were proposed.
Many of these concerned electronic mechanisms (bioinformatics) to store
and assemble data. It was suggested that such data could enhance
development of mathematical models that could lead to a better understanding of
the biophysics of developmental processes. Also the development of
imaging modalities that include both live as well as fixed tissue observations
were urged. The use confocal microscopy in a variety of modes including
the tunneling microscope was seen as a way to image large amounts of tissues,
even older whole embryos.
A major consensus was expressed for a center for
Magnetic Resonance Imaging. MRI generates non-distorted 3-dimensional
data of either fixed or living embryos that can be manipulated interactively in
real time and in a fraction of the time required for traditional optical
imaging and allows for creation of visual models for fast and easy
interpretation of complex data. Moreover, using multiple pulse sequences,
tissues with inherent differences in contrast can be readily visualized without
staining and even if differences are slight quantification of image densities
is possible. With use of targeted contrast agents, the potential for
increased resolution is enormously expanded. It was recommended and
endorsed by consensus that to enhance the effectiveness of studying growth and
development of living and fixed embryos that dedicated centers of magnetic
resonance microscopy (MRM) be established that can provide 3-dimensional
quantitative techniques, in utero imaging techniques and finally targeted
mechanisms for use of contrast agents.
CONCLUSION AND SUMMARY RECOMMENDATIONS:
There was consensus that the discussions on
cholesterol and its relationship to embryogenesis be forwarded to NIH staff as
a recommendation for a potential future RFA whereas other recommendations were
proposed as potential areas NIH staff may wish to consider as relevant to
programmatic issues on the study of congenital heart disease.
Last Updated April 2011
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