Limb defects

Fig. 5.1.2 Cholesterol biosynthesis branch of the isoprenoid biosynthetic pathway. Enzymes are numbered as follows 1 squalene synthase 2 squalene epoxidase 3 2,3-oxidosqua-lene sterol cyclase 4 sterol A24-reductase (desmosterolosis) 5 sterol C-14 demethylase 6 sterol A14-reductase (hydrops-ectopic calcification-moth-eaten, HEM, dysplasia) 7 sterol C-4 demethylase complex (including a 3/ -hydroxysteroid dehydrogenase defective in congenital hemidyspla-sia with ichthyosiform nevus and limb defects, CHILD, syndrome) 8 sterol A8-A7 isomerase (Conradi-Hunermann syndrome CDPX2) 9 sterol A5-desaturase (lathosterolosis) 10 sterol A7-reductase (Smith-Lemli-Opitz syndrome). Enzyme deficiencies are indicated by solid bars across the arrows...

Table 5.1.1 Inherited disorders of isoprenoid/cholesterol biosynthesis. CDPX2 Conradi-Hunermann syndrome, CHILD congenital hemidysplasia with ichthyosiform nevus and limb defects, HEM hydrops-ectopic calcification-moth-eaten, MA/HIDS mevalonic aciduria/ hyperimmunoglobulinemia D and periodic fever syndrome, MIM Mendelian Inheritance in Man... 

As an alternative, primary skin fibroblasts or lymphoblasts of patients suspected to be affected with a cholesterol biosynthesis defect can be cultured for 3-7 days in medium supplemented with fetal calf serum depleted of lipoproteins to induce cholesterol biosynthesis, whereupon the specific defect can be determined by sterol analysis using GC-MS as described above. This procedure will readily identify patients affected with Smith-Lemli-Opitz syndrome, desmosterolosis, lathosterolosis, hydrops-ectopic calcification-motheaten (HEM) skeletal dysplasia and most patients with Conradi-Hunermann syndrome (CDPX2). Patients with congenital hemidys-plasia with ichthyosiform nevus and limb defects (CHILD) syndrome may not be identified with this assay, but they can be readily diagnosed on the basis of their typical clinical presentation. 

Genest DR, Di Salvo D, Rosenblatt MJ, Holmes LB. Terminal transverse limb defects with tethering and omphalocele in a 17 week fetus following first trimester misoprostol exposure. Clin Dysmorphol 1999 8(l) 53-8. 

McCredie J, Kricker A, Elliott J, Forrest J. Congenital limb defects and the pill. Lancet 1983 2(8350) 623. 

Pharmacokinetic information that relates blood concentration to toxic response is critical in defining such dose-response relationships, but information on peak blood concentrations or blood concentrations over time (area under the curve (AUC)) is seldom available. One agent for which such information has been published is 2-methoxyacetic acid (2-MAA), the active metabolite of 2-methoxyethanol. Terry et al. (1994) showed that peak concentration was related to neural tube defects observed after exposure in mice on gestation day 8, whereas area under the curve was shown to be related to limb defects after exposure to 2-MAA on gestation day 11 (Clarke et al. 1992), suggesting that the time of exposure and pattern of development of the susceptible organ... 

Drugs known to be teratogenic include cytotoxics, warfarin, alcohol, lithium, methotrexate, phenytoin, valproate, ACE inhibitors and isotretinoin. Selective interference can produce characteristic anatomical abnormalities, and the phocomelia (flipper-Uke) limb defect was one factor that caused thalidomide to be so readily recognised. (For an account of thalidomide see p. 81.)... 

Reported congenital abnormalities are many and include facial and palate defects, skin and skeletal anomalies, and visceral malformations. Based on one case and a review of six previous reports of malformations after in utero exposure to cyclophosphamide in the first trimester, a distinct embryopathy due to cyclophosphamide has been suggested (68). The proposed phenotype included growth deficiency, developmental delay, craniosynostosis, blepharophimosis, flat nasal bridge, abnormal ears, and distal limb defects chromosomes were normal. 

Administration of fluorouracil to pregnant rats on day 14 of gestation resulted in dose-dependent growth retardation and numerous malformations in near-term fetuses, including hind limb defects and cleft palate (126). After treatment, a number of rapid biochemical and cellular alterations were detectable in embryonic hind limbs and in craniofacial and other tissues, including inhibition of thymidylate synthetase and altered cell cycle progression. In order to assess the importance of these early events in fluorouracil-induced dysmorphogenesis, embryonic mid-facial tissues and hind limbs were dissected 3 or 6 hours after administration of fluorouracil to the dam and placed in explant culture. After 5 days in culture, craniofacial explants were evaluated... 

Pyrimethamine is an inhibitor of dihydrofolate reductase and causes tetrahydrofolate deficiency. It is teratogenic in animals in rats it produces limb defects, cleft palate, and brachygnathia, and in chick embryos micromelia. Fetal death has been seen in rats and hamsters. However, in toxoplasmosis, pyrimethamine, with or without a sulfonamide, has been given to pregnant women without evidence of subsequent abnormalities. Supplementation with folic acid has been advocated to prevent or reduce adverse effects (SEDA-13,812), but it is not known if this could impair efficacy. 

During a 1-year period, 10 babies with major limb defects were born in Stirlingshire maternity units. At least eight of the mothers had had thalidomide prescribed in the early weeks of pregnancy (59,98-101). 

A study in Washington state found that maternal exposure to agricultural chemicals (fertilizers and pesticides) produced an elevated risk of limb defects)23 ... 

Engel LS, O Meara ES, Schwartz SM. Maternal occupation in agriculture and risk of limb defects in Washington state, 1980-1993. Scand J Environ Health 2000 26(3) 193-8. 

Intriguingly, in addition to vertebral malformations and axial truncation, Cdxl-Cdx2 compound mutants also exhibit a low incidence of hindlimb defects, notably polydactyly. This is somewhat perplexing as Cdxl null mutant offspring do not exhibit limb defects, and only transcripts for Cdxl and not Cdx2, are observed in the... 

Fig. 3. Limb defect in Prxl Prx2 embryos. Shown are wildtype and double mutant forelimbs of E14.5 embryos, stained with Alcian blue (cartilage) and Alizarin red (bone virtually absent in this case). At E 12.5 the defect is entirely undetectable, while here, 48 h later it is already very clear. Note that only ulna and radius are affected.

Only treatment at the highest dose reduced the number of live embryos a significant amount (p<0.05) (73.6% survival) when compared to the control incubated with inert ingredients (82.9%). Maneb caused a significant increase in malformations at every dose (p<0.05 or better) with the majority of defects reported as unilateral limb defects consisting of contracted and often shortened lower limbs. Bent tibias, tibiotarsus, and phalanges were also reported (Mad and Arias 1987). 

Temperature changes. Another common gestational problem is high maternal body temperature that translates into high fetal temperature. Fetal hyperthermia can result in severe consequences anencephaly, spina bifida, mental retardation, facial defects, cardiac abnormalities, and limb defects—all classical clinical abnormalities.23... 

Congenital hemidysplasia with ichthyosis and limb defects (CHILD syndrome) Sterol A8,A7-isomerase, sterol C-4 demethylase... 

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