Thalidomide-induced teratogenesis: history and mechanisms

Abstract

Nearly 60 years ago thalidomide was prescribed to treat morning sickness in pregnant women. What followed was the biggest man-made medical disaster ever, where over 10,000 children were born with a range of severe and debilitating malformations. Despite this, the drug is now used successfully to treat a range of adult conditions, including multiple myeloma and complications of leprosy. Tragically, a new generation of thalidomide damaged children has been identified in Brazil. Yet, how thalidomide caused its devastating effects in the forming embryo remains unclear. However, studies in the past few years have greatly enhanced our understanding of the molecular mechanisms the drug. This review will look at the history of the drug, and the range and type of damage the drug caused, and outline the mechanisms of action the drug uses including recent molecular advances and new findings. Some of the remaining challenges facing thalidomide biologists are also discussed.

Keywords: Fgf8; Shh; actin cytoskeleton; angiogenesis; cell death; cereblon; limb development; phocomelia; reactive oxygen species; vascular transition.

Figure 1

Structure of thalidomide enantiomers and packaging. A: Thalidomide is a stereo‐isomer and can exist in two enantiomeric states, depending on the state of the chiral carbon (see asterisk) allowing each form to have slightly different structural moieties. Both enantiomers, R and S, can rapidly interconvert (racemize) in body fluids and tissues and form equal concentrations of each form. B: Thalidomide was sold/distributed as a racemic mix of both enantiomers and called “Distaval” in the UK. These images are from an actual packet of “Distaval,” which was a Physician's Sample and given to women in early pregnancy. Note the safety advice on the packet. Reproduced from Vargesson, BioEssays, 2009,31,1327–1336.

Figure 2

Time sensitive window of thalidomide embryopathy or the “critical period.” Chart indicating when the major outward appearing damage occurred in the embryo following thalidomide exposure.

Figure 3

Embryonic limb development. Limbs form from a bud from the flanks of the embryo. In humans the upper limbs form around a day earlier (day 26) than the lower limbs (day 27). The limb bud consists of two key signaling centers. The apical ectodermal ridge (AER), a thickened epithelium lining the distal tip of the bud and separating the dorsal from ventral surface; and the zone of polarizing activity (ZPA) in the posterior‐distal mesenhcyme. The AER expresses Fgf8 which signals to the mesenchyme to induce Fgf10 and to the ZPA to induce and maintain Shh, which itself feeds back to maintain Fgf8. This feedback loop maintains cell proliferation and limb outgrowth and induces other genes, for example the Hox genes, which establish the pattern of the limb elements, humerus, radius, ulna, and handplate, as well as the soft tissues. The limbs grow out from specific regions of the flank of the embryo and as the limb grows out the limb is patterned proximally to distal, that is, humerus/femur are laid down before the radius, ulna/fibular, tibia, and then the handplate/footplate.

Figure 4

How does thalidomide induce Phocomelia? Phocomelia is the striking characteristic of thalidomide embryopathy which remains the public image of the condition and disaster. Phocomelia, the loss of or severe shortening of the limbs long bones, is rarely seen in other conditions. How can thalidomide cause Phocomelia? Thalidomide and some of its analogs have been shown to reduce or inhibit FGF (and Shh) expression in developing limbs of rabbits and chickens (Hansen et al., 2002; Therapontos et al., 2009; Knobloch et al., 2011). The expression of these genes returns after a few days (Hansen et al., 2002; Knobloch et al., 2011). Moreover, reduction of FGF signaling in developing mouse limbs can cause phocomelic‐like limb injuries through causing increased cell death in proximal tissues (Mariani et al., 2008). Thalidomide exposure induces cell death in chicken embryo limbs (Knobloch et al., 2007), as does CPS49 an antiangiogenic analog of thalidomide (Therapontos et al., 2009). It has been proposed that thalidomide destroys blood vessels, resulting in cell death and loss of the limb signaling pathways (Therapontos et al., 2009). The proximal tissue will be lost permanently and unable to be replaced, due to a combination of the limb being unable to replace the huge number of cells lost, but also the molecular environment in the limb that patterned the proximal elements will no longer be present or be able to be recapitulated (Mahony and Vargesson, 2013). Recovery or reactivation of FGF signaling could allow the limb to recover, but only distal structures would form from the remaining tissue, as the tissue is so close to the apical ridge and influenced by distal signals only. Perhaps a misregulation of these signals could also explain the polydactyly seen in some thalidomide survivors? If the exposure to thalidomide is prolonged or occurred just as the limbs were forming, this could lead to total vessel loss, widespread cell death, and a complete loss of FGF (and Shh) signaling, which is unable to recover and result in Amelia (no limb).

Figure 5

Framework of thalidomide induced embryonic damage. This framework incorporates the majority of the previously proposed models/hypotheses to attempt to provide an explanation for thalidomide embryopathy. Thalidomide and/or a breakdown product after binding a molecular target acts negatively on smooth muscle negative blood vessels, likely affecting the actin cytoskeleton of the endothelial cells, and preventing their proliferation and migration into avascular regions, causing oxidative stress, cell death, and gene expression loss, resulting in tissue damage. In rapidly developing tissues and organs, such as the limbs and internal organs, this would be devastating, causing tissue loss or tissue function loss, preventing growth. The damaged or missing tissues would then also fail to properly recruit and pattern proper chrondrogenesis, nerve innervation, muscle patterning, etc., exacerbating the condition and damage.