Birth Defects, Malformations, and Syndromes
John C. Carey and Michael J. Bamshad
Birth defects are relatively common—about 2% of newborns will have a medically significant malformation recognized during the first day of life. However, approximately one half of all defects that are present at birth are not diagnosed until later in infancy. Defects that may not be apparent at birth include abnormalities of the central nervous system, cardiovascular system, and sensory systems (eg, hearing, vision) among others. Collectively, it appears that 4% of infants have a medically significant structural anomaly diagnosed by age 12 months.
Birth defects can be isolated abnormalities or be features of one of the thousands of known syndromes of genetic or environmental etiologies. For example, approximately 75% of children with congenital heart malformations have isolated defects, whereas additional birth defects, often comprising a syndrome, are found in the remaining 25%. The etiology of most birth defects is unknown, although it is estimated that a substantial proportion are caused by mutations in genes that control normal development. Birth defects that arise from an intrinsically abnormal developmental process are called malformations. Birth defects can also result from an alteration of the form, shape, or position of a normally formed body part by mechanical forces and are termed deformations. For example, oligohydramnios can result in abnormal mechanical constraints on the joint mobility of a fetus leading to the formation of contractures (eg, clubfoot). Birth defects may also be caused by external interference with an originally normal developmental process, known as a disruption. For example, strands of amniotic tissue that become tightly wound around a digit can result in truncation of the digit. An abnormal organization of cells into tissues, a dysplasia (eg, a congenital vascular malformation), is also considered a type of birth defect. Of note, malformations and dysplasias are primary disturbances of embryogenesis and histogenesis, respectively. Deformations and disruptions are secondary to a primary extrinsic force.
The presence of a birth defect often evokes an aura of mystery or implies a difference in person-hood. Furthermore, terms such as elfin-like face and harelip implicitly reinforce these differences. Yet families who experience the birth of a newborn with a birth defect wrestle with the same questions about cause, responsibility, and outcome as any other family of a child with a serious pediatric disease. Approaching the diagnosis and management of an infant or a child with a birth defect can also be overwhelming in that thousands of different conditions are associated with birth defects, and strategies to diagnose and treat these conditions change rapidly. A logical and systematic approach to the evaluation of children with birth defects and the collection of phenotypic data are important for both diagnostic and therapeutic reasons. The recognition of a well-characterized disorder, even if the etiology is unknown, provides (1) information on the pattern of inheritance and recurrence risk, (2) the framework and options for the management of future pregnancies, and (3) information that can be used to make general predictions about potential future manifestations and outcomes and about decisions regarding guidelines for routine care and suggestions for educational interventions, especially when a specific behavioral profile has been associated with a condition (eg, Williams syndrome). A specific diagnosis also eliminates the motivation to perform unnecessary testing and enables the use of appropriate screening tools for anticipated problems. For many families, explaining the diagnosis, natural history, and strategy for health care maintenance and anticipatory guidance helps with coping with the uncertainty that typically surrounds genetic disorders.
CLASSIFICATION OF BIRTH DEFECTS
Because our knowledge of the pathogenetic basis of birth defects is limited, all classification schemes of birth defects and malformations are somewhat arbitrary and tentative. Most medical textbooks classify birth defects according to the organ system or body part that is affected (eg, cardiovascular system, limbs). Such classifications can help develop intervention strategies (eg, for surgical palliation) and identification of the general causes of these defects. However, the utility of anatomical classifications becomes limited once specific information on the etiology, natural history, and recurrence is required.
Birth defects can also be classified depending on whether they occur as isolated findings or as a component of multiple congenital anomalies. This particular distinction is probably the most valuable in the evaluation of any infant and child with a birth defect. Compared with children with isolated birth defects, children with multiple birth defects have greater morbidity and mortality and are more likely to have a chromosomal abnormality and/or syndrome diagnosis. Birth defects can also be classified by etiologic categories such as chromosome, single gene, multifactorial, and teratogenic.
Categorization of defects by the developmental process that is perturbed is useful for generating hypotheses about causative pathogenetic mechanisms, although many birth defects can result from the perturbation of more than one pathway, making it difficult to identify the primary disturbance. Although no specific classification is appropriate for all cases, birth defects will be presented according to the developmental process that is disturbed to facilitate understanding of pathogenesis and provide a background for understanding future observations. Accordingly, a brief review of the genetic controls of development, and the cardinal processes that, when disturbed, cause birth defects is provided.
BASIC CONCEPTS OF DEVELOPMENT
Development is the process by which a fertilized ovum becomes a mature organism capable of reproduction. Thus, a single fertilized egg divides and grows to form different cell types, tissues, and organs, all of which are arranged in a species-specific body plan (ie, the arrangement and patterning of body segments). Understanding the pathogenesis of human malformation and genetic syndromes is rooted in developmental biological principles.
Evolution of species requires that development of individual organisms be replicated with high fidelity. Otherwise, it might be difficult to recognize that a group of organisms share similar properties that define a species. In sexually reproducing species, the necessary tools and instructions for building an organism that closely resembles its parents are located in the fertilized ovum (zygote). Much of this information is transmitted from parent to offspring via genes that encode signaling molecules and their receptors, DNA transcription factors, components of the extracellular matrix, enzymes, transport systems, and many other proteins. Mutations in the genes mediating development are a common cause of human birth defects.
Interactions between neighboring cells are often controlled by proteins that can diffuse across small distances to induce a response and are termed paracrine factors because they are secreted into the space surrounding a cell, unlike hormones that are secreted into the bloodstream. Major paracrine-signaling molecules include (1) the fibroblast growth factor (FGF) family, (2) the hedgehog family, (3) the wingless (Wnt) family, and (4) the transforming growth factor β(TGF-β) family. Mutations in genes encoding these molecules may lead to abnormal communication between cells.
Many different mechanisms regulate the expression of a gene. Genes encoding proteins that function to activate or repress other genes are called transcription factors. Transcription factors commonly do not activate/repress only a single target, but regulate the transcription of many genes that, in turn, regulate other genes in a cascading effect.
Extracellular matrix proteins (EMPs) are secreted macromolecules that serve as scaffolding for all tissues and organs. These molecules include collagens, fibrillins, proteoglycans, and large glycoproteins such as fibronectin, laminin, and tenascin. EMPs are not simply passive structural elements. To facilitate cell migration, EMPs must transiently adhere to a cell’s surface, which is accomplished by two families of receptors, integrins and glycosyltransferases. Integrins integrate the extracellular matrix and the cytoskeleton, allowing them to function in tandem.
The process by which ordered spatial arrangements of differentiated cells create tissues and organs is called pattern formation. The general pattern of the animal body plan is laid down during embryogenesis, which leads to the formation of semiautonomous regions of the embryo in which the process of pattern formation is repeated to form organs and appendages. Such regional specification takes place in several steps: (1) definition of the cells of a region, (2) establishment of signaling centers that provide positional information, and (3) differentiation of cells within a region in response to additional cues.
Gastrulation is the process whereby the cells of the blastula are given new positions and neighbors. In the human embryo, gastrulation occurs between days 14 and 28 of gestation. In this process, the embryonic bilaminar disk is transformed into a trilaminar embryo composed of three germ layers: outer ectoderm, inner endoderm, and the interstitial mesoderm. The formation of these layers is a prerequisite for organogenesis. The major structural feature of mammalian gastrulation is the primitive streak, which appears as a thickening of epiblast extending along the anterior to posterior axis.
NEURULATION AND ECTODERM
Once a trilaminar embryo is formed, the dorsal mesoderm and the overlying ectoderm interact to form the hollow neural tube. This event is called neurulation and is mediated by a process called induction, which occurs when the cells of one embryonic region influence the organization and differentiation of cells in a second embryonic region. Induction of the neural tube and transformation of the flanking mesoderm into an amphibian embryo with clear anterior/posterior and dorsal/ventral axes is controlled by a group of cells known as the Spemann-Mangold organizer.
Neurulation is a critical event in development that initiates organogenesis and divides the ectoderm into three different cell populations: (1) the neural tube, which will eventually form the brain and spinal cord, (2) the epidermis of the skin, and (3) the neural crest cells. In humans, neural tube closure begins at five separate sites that correspond to the locations of common neural tube defects such as anencephaly (absence of the brain), occipital encephalocele, and lumbar myelomeningocele. Neural crest cells migrate from the neuroepithelium along defined routes to tissues where differentiation into a variety of cell types such as sensory neurons, melanocytes, neurons of the small bowel, and smooth muscle occurs.
MESODERM AND ENDODERM
The formation of a layer of mesoderm between the endoderm and ectoderm is one of the major events in gastrulation. Mesoderm can be divided into five components: the notochord; dorsal, intermediate, and lateral mesoderms; and head mesenchyme. The notochord is a transient structure that induces the formation of the neural tube and body axis. Dorsal (paraxial) mesoderm is observed on either side of the notochord and differentiates into sclerotomes, myotomes, and dermatomes that form the axial skeleton, appendicular skeleton and skeletal muscles, and connective tissue of the skin, respectively. Intermediate mesoderm forms the kidneys and genitourinary system. Lateral plate mesoderm differentiates into heart, connective tissue of viscera, and the connective tissue elements of the amnion and chorion. Finally, the muscles of the eyes and head arise from head mesenchyme.
The primary function of embryonic endoderm is to form the linings of the digestive tract and the respiratory tree. Outgrowths of the intestinal tract form the pancreas, gallbladder, and liver. A bifurcation of the respiratory tree produces the left and right lungs. The endoderm also produces the pharyngeal pouches that, in conjunction with cells derived from the neural crest, give rise to endodermal-lined structures such as the middle ear, thymus, parathyroids, and thyroids.
Animal body plans have evolved into a wide variety of symmetries. Specification and formation of the axes are critical events in development that determine the orientation of the body plan. The proteins mediating these processes are rapidly being discovered. Many of these mediators have additional roles in patterning of the body plan and tissues.
FORMATION OF THE ANTERIOR/POSTERIOR AXIS
The anterior/posterior axis of a developing mammalian embryo is defined by the primitive streak. At the anterior end of the primitive streak is a structure called the node, which is homologous to the Hensen node in birds and contains many of the same proteins found in the amphibian organizer. Patterning of the anterior/posterior axis is controlled by the HOX genes that encode transcription factors containing a DNA-binding domain of about 60 amino acids called the homeodomain. In Drosophila these genes compose the homeotic gene complex (HOM-C), which has two classes of genes, Antennapedia and Bithorax.
FORMATION OF THE DORSAL/VENTRAL AXIS
Dorsal/ventral patterning of the vertebrate depends on the interaction between dorsalizing and ventralizing signals, which are mediated, in part, by molecules that act in a concentration-dependent fashion. Molecules that can promote multiple positive responses from a field of undifferentiated cells as a function of concentration are called a morphogens. The function of morphogens can be attenuated or inhibited by antagonists, which bind and inactivate them.
FORMATION OF ORGANS AND APPENDAGES
Subsequent to vertebrate axis determination and gastrulation is the formation of organs and limbs, called organogenesis. Many of the proteins used for patterning and growth of organs and limbs are the same molecules used earlier in blastogenesis. However, additional genes that were transcriptionally silent now become active and encode proteins that may act as switches for organ formation or receptors for recognizing patterning information or participate in the expected function of terminally differentiated cells. To date, most of the developmental genes known to cause human birth defects have prominent roles in this period of development. Mutations in genes that disrupt earlier developmental events may be lethal.
Development of the craniofacial region is directly related to the formation of the underlying central nervous system. In mammalian embryos, neural crest cells from the forebrain and midbrain become the nasal processes, palate, and mesenchyme of the first pharyngeal pouch. This mesenchyme forms the maxilla, mandible, incus, and malleus. The neural crest cells of the anterior hindbrain migrate and differentiate to become the mesenchyme of the second pharyngeal pouch and the stapes and facial cartilage. Cervical neural crest cells produce the mesenchyme of the third, fourth, and sixth pharyngeal arches (in humans the sixth pharyngeal arch degenerates), which become the muscles and bones of the neck. The bones of the skull develop directly from mesenchyme produced by neural crest cells via a process called intramembranous ossification. Complete fusion of these bones usually does not occur until adulthood. Premature fusion (synostosis) of the skull bones (craniosynostosis) causes the head to be misshapen and can impair brain growth (see Chapter 177).
DEVELOPMENT OF THE LIMB
The developing tetrapod limb is one of the best understood classical models of morphogenesis. Many of the signaling pathways and transcriptional control elements that coordinate limb development in model organisms such as Drosophila and chick appear to be conserved in mammals, including humans.
The vertebrate limb is composed of elements derived from lateral plate mesoderm (bone, cartilage, and tendons) and somitic mesoderm (muscle, nerve, and vasculature). The signal that initiates induction of forelimbs and hindlimbs appears to arise in the intermediate mesoderm. Once initiated, proximal/distal growth of the limb bud is dependent on a region of ectoderm called the apical ectodermal ridge (AER), which extends from anterior to posterior along the dorsal/ventral boundary of the limb bud.
Mediation of proximal/distal growth by the AER is controlled, in part, by fibroblast growth factors (FGF2, FGF4, and FGF8) that stimulate proliferation of an underlying population of mesodermal cells in the so-called progress zone (PZ).
Defects of the anterior and posterior elements of the upper limb occur in the Holt-Oram syndrome (HOS) and ulnar-mammary syndrome (UMS), respectively. HOS is caused by mutations in the gene TBX5, whereas UMS is caused by mutations in the tightly linked gene TBX3. TBX3 and TBX5 are members of a highly conserved family of DNA transcription factors containing a DNA-binding domain called a T-box.
Many processes must be coordinated simultaneously to construct a specific arrangement of cells and tissues that manifests the properties of an organ. Similar to limb development, formation of parenchymal organs is notable for the reciprocal induction of the epithelium on the mesenchyme and vice versa. This interaction is mediated by secreted signaling molecules that bind to receptors, transduce the signal through various interconnected pathways, and stimulate or repress DNA transcription. Use of the same elaborate networks to form different organs allows for genomic economy while maintaining developmental flexibility.
Once a specialized cell within an organ is terminally differentiated, various proteins turn on its molecular machinery so that it may perform its fated function. Often development of the organ and function of the differentiated cell are interrelated. Epithelial-mesenchymal interactions are prominent in the development of cutaneous structures (eg, hair, sweat glands, breasts), parenchymal organs (eg, liver, pancreas), lungs, thyroid, kidneys, and teeth. These interactions are dynamic such that expression patterns in the epithelia and mesenchyme change over time.
One of the largest organs in the body is the skeleton. In contrast to the development of cranial bones by intramembranous ossification, most skeletal bone formation takes place using a cartilaginous template called endochondral ossification. However, both intramembranous and endochondral ossification are regulated by bone-forming cells called osteo-blasts. The differentiation of osteoblasts is regulated by an osteoblast-specific transcription factor called Cfba1. Targeted disruption of Cfba1 results in mice with a complete lack of ossification of the skeleton. Heterozygous mice have widened cranial sutures, shortened digits, and abnormalities of the shoulder girdle. Similar defects are found in individuals with cleidocranial dysplasia, which is caused by mutations in CFBA1, the human homologue of Cfba1 (1-4).
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