Can contamination of the environment by dioxins cause craniofacial defects?

Abstract

Cleft lip and cleft palate also known as orofacial cleft is a congenital malformation involving the partial or total lack of anatomical continuity of craniofacial tissue. The most common environmental factors that may cause orofacial clefts include pharmaceuticals, alcohol, addictive drugs, and tobacco smoke. Living in the area of industrial factories, garbage, ironworks, crematoria, wastewater treatment plants, and plastic waste landfills also has a significant impact on the development of the craniofacial defects. Some of the main factors causing the formation of congenital craniofacial defects are dioxins, of which emission to the environment is an important environmental and health problem. Dioxins are a diverse group of organic chemical compounds, derivatives of oxanthrene and fumarates, which are organoleptically imperceptible. Acting mainly through induction of inflammation, they influence a number of metabolic processes, including the process of bone mineralization and embryonic development. In this work, we highlight the problem of orofacial cleft including the impact of dioxin on development of this defect and the recommended prevention.

Introduction

Clefts are congenital malformations and consist of partial or complete lack of continuity of anatomical tissues. These defects occurring during the embryological development of the face arise in varying degrees and scope in the area of alveolar bone, lip, and/or palate. ^1,2 They are complex and multisystem disorders and impair vital biological functions, including sucking, chewing, swallowing, and breathing. They cause significant pathological changes in the muscular system, bone structure, and auditory system, as well as articulation disorders, which affect the normal development of speech. Additionally, with the cleft of secondary palate, one of the complication can be hearing loss. ³ Incomprehensible speech which is a feature of some people with cleft can cause emotional disorders, social exclusion, and may have impact on inability to implement their own educational plans. In most cases, people with cleft may need a multidisciplinary medical help. ⁴ Also important is the prevention associated with the negative impact of the environment on the body of the mother. ^5,6

There are many classifications of cleft defects. The best known is the classification developed by Veau in 1931, ⁷ based on the division into four groups: cleft soft palate, cleft hard palate, unilateral cleft lip and palate, and bilateral cleft lip and palate. In 2014, Wang et al. ⁸ have evaluated classification for clefts of the lip, based on Kriens’s and Kernahan and Stark’s systems. ^9,10 Capital letters were used to describe the anatomical extent and form of lip and/or palate cleft. Kriens proposed to use the first letters of the English-language nomenclature: L, A, H, S, H, A, L. According to the International Classification of Diseases and Related Health Problems (ICD-10), clefts were marked as Q, accordingly lip cleft was marked as Q36, palate cleft as Q35, and lip and palate cleft as Q37. ¹¹

In 1979, the European Registry of Congenital Malformations (EUROCAT) was founded. Based on their data, clefts of the primary and secondary palate are twice as often in boys as compared to girls. Whereas isolated clefts of secondary palate are more often found in girls than in boys. ² Frequency coefficient of cleft in children born in 1995 in Poland, established on the basis of research conducted at the Department of Surgery, Institute of Mother and Child in Warsaw, was 1.07 per 1000 live births. ^3,11 According to Polish Registry of Congenital Malformations established in 1997, the incidence of clefts of the primary and/or secondary palate in this year was in the range of 0.8–2.6 per 1000 live births. ^12,13 In 2005–2006, the incidence of lip and/or palate cleft was 14.9 per 1000 live births and was higher than the average of defects in other EUROCAT countries (14.6 per 10,000 live births). ¹⁴

In 2009, in the Hospital and Clinic of Plastic Surgery in Polanica Zdroj in Poland, where two children cases with isolated palate cleft further described in this article were treated, a comprehensive epidemiological analysis of palate clefts in 2500 patients treated in the years of 1973–2003 shows that the most common type of defects were one-sided lip and palate clefts, which coincides with the trends observed in other centers. ¹⁵

Dioxins are one of the main factors giving rise to congenital craniofacial defects. Chemically, these are the polychlorinated aromatic hydrocarbons, derivatives of oxanthrene and fumarates. ^{16 –18} The best known and having the greatest toxicity according to the International Agency for Research on Cancer (IARC) is 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), ^{19 –22} presented in Figure 1 along with other dioxins.

Figure 1.

Structural formulas of representatives of different groups of POPs: (a) PCDD, (b) PCB, (c) PCDF, and (d) TCDD. POPs: persistent organic pollutants; PCDD: polichlorodibenzodioxin; PCB: polychlorinated biphenylene; PCDF: polychlorodibenzofuran; TCDD: 2,3,7,8-tetrachlorodibenzo-p-dioxin.

Dioxins are formed mainly in the course of fuels and waste combustion, during the reaction of thermal waste treatment, household heating, and the incineration of municipal and industrial applications. The combustion of heating fuels and motor also generates the release of dioxins into the environment. ²³ Contamination can occur accidentally as a result of environmental disaster or human error, as in open-pit mine of iron ore in the state of Minas Gerais in the east of Brazil in November 2015, where the dam to the waste receptacle was interrupted and toxic products of combustion were released to the environment. ²⁴ Another important source of dioxin emissions are fires, such as that during a terrorist attack on the World Trade Center where measurements showed a large increase in the concentration of polycyclic aromatic hydrocarbons, not only in Ground Zero but also at a considerable distance from it. ^25,26 The harmfulness of dioxin impacts in the human body is certified by a number of national laws and international Stockholm Convention (Journal of Laws of 2009 No. 14, item. 76) concerning the limitation of emissions of Persistent Organic Pollutants (POPs). ²⁷

Dioxin molecules are structurally similar to steroid hormones and thus their main action is directed toward the male and female gonads, thyroid, uterine endometrium, and other tissues where steroid hormones are produced. Dioxins may also influence the production of estrogenic hormones, which play an important role in osteogenesis. ²⁸ The aryl hydrocarbon receptor (AhR) is a main receptor that plays a role in xenobiotic metabolism and therefore plays an important role in pathomechanism of dioxin intoxication. ²⁹ AhR is also involved in diverse processes, for example, cell proliferation and differentiation. Activation of the AhR by TCDD causes the atrophy of sebaceous glands and development of small skin lesions (hamartoma). In addition, AhR contributes to the development of inflammation through biotransformation of TCDD. ^30,31 Decomposition of TCDD molecule also causes free radicals production and therefore induce inflammation process, which is shown in Figure 2. Moreover, recent study shows a co-expression between AhR and estrogen receptor (ER) in control of detoxification of TCDD. It was shown that TCDD may delay embryonic development by blocking AhR synthesis and modulating expression of estrogen-dependent genes. Estrogens control numerous processes during development, such as cell proliferation and differentiation processes, and they are also involved in osteogenesis and bone formation. Therefore, the loss of estrogens and/or ER (e.g. by being blocked by TCDD) may impact bone formation and structure. ^31,32 Moreover, dioxins disrupt endocrine system by mimicking the action of natural hormones and influencing the processes of feedback loop in the hypothalamic–pituitary–peripheral gland axis. Exposure to dioxins results in immunosuppression and increased susceptibility to infection. ³³ TCDD degradation end products may also inhibit a transfer of ER to the nucleus, delayed activation of estrogen-dependent genes. ³¹ They have a significant impact on the development of the offspring of mothers exposed to dioxins, including low birth weight, developmental disorders of the skeletal system, behavioral disorders, and changes in dentition. ^23,31 Malformations in newborns may have a genomic mechanism and/or the direct impact of dioxin on the embryo during embryogenesis. ^34,35

Figure 2.

Various mechanisms of action of dioxin on the human body. The first mechanism, the extranuclear is based on the oxidative stress resulting from decomposition of the TCDD molecule. The second mechanism is associated with pro-inflammatory activity of dioxins on COX. As a result, the oxidative stress is induced. The third mechanism is implemented by a nuclear receptor AhR. TCDD: 2,3,7,8-tetrachlorodibenzo-p-dioxin; COX: cyclooxygenase; AhR: aryl hydrocarbon receptor.

The article deals with the problem of orofacial cleft including the impact of dioxins on the development of this defect and the recommended prevention.

Etiology of orofacial clefts

World statistics show that 1–2.2 cleft palate cases per 1000 births are observed in the world every year. ³⁶ Geographic area and ethnic features also influence the prevalence and ranges from 1:500 to 1:2500 live births. ³⁶ The prenatal diagnosis is based on prenatal ultrasonography. During this examination, it is relatively easy to diagnose cleft lip or cleft lip/palate. Syngelaki et al. showed that even using two-dimensional or three-dimensional ultrasonography may be insufficient to determine the occurrence of isolated cleft lip palate. ^37,38 Factors responsible for the formation of cleft lip and palate can be divided into genetic and environmental factors. Genetic factors account for approximately 20% of cases and are often the result of synergistic action of genes. Participation of environmental factors in the development of craniofacial defects is estimated to be around 10%. ⁴ Recent studies demonstrate a role for genetic susceptibility to the environmental factors. Thus the interaction of both genes and the environment affects the predisposition to the formation of the clefts.The most common environmental factors are drugs, mainly antibiotics of the tetracycline group, antiemetics, ³⁹ antiepileptic drugs and steroid anti-inflammatory drugs, ⁴⁰ alcohol, ⁴¹ addictive drugs, and tobacco smoke. ^42,43 As many as 4% of craniofacial defects are caused by cigarette smoking by pregnant women. ^39,44 Toxicity of smoke is multifactorial and contains both heavy metals and ring compounds, including dioxins.

A diet deficiency in vitamins A, E, and B and folic acid, together with harmful environmental conditions, affects the potential for development of the defects. ⁴⁵

Pathogenesis of orofacial clefts

Pathomechanism of clefts formation is not fully understood. Malformation begins in the first trimester of pregnancy. Embryology shows that the formation of cleft of primary palate is earlier than secondary one. The primary palate cleft occurs independently on the secondary one or at the same time. Cleft of primary palate is formed between 4 and 7 weeks of fetal life and secondary within 7–12 weeks of gestation. ^{46 –48} There are many theories and views on the pathogenesis of orofacial cleft. The most popular are: (1) the His’s theory says that the cause of the formation of facial clefts is due to the lack of connection among craniofacial nodules; (2) the Fleischmann’s theory says that the mechanism of orofacial clefts is associated with deficient and insufficient penetration of the mesenchyme between the ectoderm of the skin and mucous membranes; (3) the Stark’s theory says that the development and the fusion of three mesenchyme haughtiness determines the potential formation of cleft; and (4) the Veau’s theory says that 6 out of 10 clefts of primary cleft develop together with cleft of secondary palate. ¹¹

The type and extent of the cleft depends on the moment and the exposure time to the toxic substance. As we know, activation of AhR by TCDD alters gene expression and therefore may lead to orofacial deformations, as well as to other developmental defects. It has been also shown that TCDD-induced cleft palate may be caused by disruption of the epithelium by TCDD end products. ⁴⁹

Dioxin accumulation and its impact on orofacial clefts development

Living in the area of industrial plants, garbage, ironworks, crematoria, sewage treatment plants, and plastics landfills entails exposure to POPs. Due to the sources of dioxin emissions, professional groups such as chimney sweeps, firefighters, soldiers, and chemical rescuers are particularly exposed to polychlorinated dibenzodioxins and polychlorinated dibenzofurans. ^20,50 Employees of industrial establishments in which POPs are the by-products are in a risk group. ⁵¹ The survey demonstrated that the awareness of health workers and teachers on the sources of the formation and toxic action of dioxin on the human body is considerably low, between 14% and 16%. ^52,53

Comparing the maps showing the degree of contamination of the environment with maps of incidence of births of children with congenital craniofacial disorders clearly shows the link between cleft incidence and the environmental pollution by dioxins. ^54,55

Dioxins have no color, odor, taste, and are poorly soluble in water. Because of their lipophilic characteristics their absorption from foods containing fat is very high. Consequently, they accumulate in adipose tissue. Toxic effects of dioxins are reported even at quite low concentrations, in the range of pictograms. ⁴⁵ According to World Health Organization norms, ⁵⁶ the maximum daily tolerable dose of dioxin is 1 pg-TEQ/kg/day (0.001 ng-TEQ/kg/day), which attests for their high toxicity. ⁵⁷ They enter the body mainly by the oral route and more than 90% of dioxins are derived from food. To a lesser extent, they enter via inhalation (8%) and through the skin lipids (2%). ⁵⁸ Often the presence of acne on the face is associated with the syndrome of chloracne associated with the poisoning by dioxin. ⁵⁰ The pathways of dioxin entry into the body and their further fate are shown in Figure 3.

Figure 3.

Diagram showing the penetration and transformation of dioxin in the body.

Dioxin concentration is significantly higher in animal food products than in plants. Meat products, milk, dairy products, fish, eggs, fats, and oils are the main sources of dioxin in food. It has been shown that increased consumption of fish from the Baltic Sea, particularly herring and salmon, which contain significant amounts of dioxin, increases exposure and accumulation of these toxic substances in the bodies of consumers. Recently, a correlation between the consumption of milk and the concentration of dioxins was observed in residents of the Netherlands and Belgium. ^{42,59 –61} It has been shown that in the milk and adipose tissue of Eskimos, Italian, and Japanese women, there are substantial amounts of dioxins. This is due to the nature of the diet and the place of living. ⁶²

The elimination of dioxins stored in adipose tissue is a slow process. A consequence of boosting the metabolism of adipose tissue associated with the process of weight loss and lactation is associated with the release of dioxins into the bloodstream and milk. ^63,64 Data show the significant concentrations of dioxins in milk of Eskimos, Japanese, and Italian women who breastfed are several times higher than the permissible for an adult. ⁶⁵ The dioxin content in the bodies of polar bears and their milk is comparable to the concentrations of these compounds in seals, which are their food source as well as the Eskimo milk. ^66,67 All these prove the bioaccumulation and the circulation of dioxin in the food chain. It also shows that dioxin emission to the environment is an important environmental and health problem, which is why in recent years interest in their biological activity has been increased. Besides it also stresses out importance of the food testing for the dioxin contamination to avoid scandals such as Chicken scandal in Belgium or pig meat in Ireland. ^68,69

Dioxins, which are factors giving rise to craniofacial defects, can accumulate in the body for a long time. They have strong biological effects that may manifest itself even in the second generation, which is not directly exposed to these compounds. Recent studies using mice show an increased frequency of clefts in an environment contaminated with toxic dioxin with an increase in cases of infertility and miscarriages. ⁷⁰ Based on the statistical data from industrialized areas in Poland, where the percentage of infertility and miscarriages reaches over 30%, it is concluded that the factor responsible for that may be dioxins. Their teratogenic effect has been proven. TCDD in small doses is toxic to embryos and developing individuals. ^34,71,72 Similar conclusions were reached from studies in China, TCDD (64 mg/kg) was given to mice at 10 weeks of pregnancy intragastrically thereby causing the 100% cleft palate in offspring. ⁷³ Moreover, abnormalities in bone mineralization and lower concentrations of calcium and magnesium as well as disorders of the teeth development were observed in newborns from mothers exposed to dioxins. ^31,74 Dioxins pass to the fetus through the placenta resulting in disorders of organogenesis including skeletal system and stomatognathic apparatus. ^75,76

As an example of the impact of dioxin accumulation on the development of cleft palate, two cases can be described briefly. The first case involves a patient who was diagnosed with cleft of secondary palate. Interview with the mother during pregnancy did not indicate such a serious condition of a child. The mother was 25 years old. During the pregnancy, she did not suffer of any diseases, including infectious diseases, and did not take any drugs. Until becoming pregnant she worked as a cook; however, during pregnancy she did not work. Before and during pregnancy she lived in the immediate vicinity—1.5 km from factory producing plastic. Her diet in the first trimester contained a large amount of fish products. There was no evidence of genetic defects in the child. Family history shows no genetic defects occurring in the family of the patient. ⁷⁷

The second case presents cleft in male patient who was diagnosed with a cleft of primary and secondary palate. Patient’s mother reported frequent colds in the first trimester of pregnancy. Moreover, the survey shows that the pregnant mother consumed large amounts of marine fish. Her first child was not burdened with any birth defect, there were no genetic defects in the family. In the immediate family of above cases, no craniofacial defects were observed before, which largely excludes hereditary origin of cleft defects. ⁷⁸

In both examples, genetic factors of craniofacial defects were excluded. Therefore it has been suggested that the dioxins are responsible for these malformations.

Prevention of orofacial clefts

Knowledge of the sources of dioxin emissions and ways of their penetration into the body contributes to isolation of professional groups with particular risk to dioxin exposure, which determines the method of prevention. Prevention of craniofacial malformations should be based on the elimination of harmful agents and avoiding foods high in dioxin content. These include some medicines, alcohol, drugs, and tobacco smoke. Percentage of dioxin contamination of various foods is as follows: 35.1% in milk and its products; 58.8% in meat, poultry, and eggs; and 6.6% in fish and fish products. ⁷⁹ A potential threat may also be smoked salmon and smoked Baltic sprats, as well as grilled products in which the content of dioxins repeatedly exceeds the maximum allowed levels. ^{80 –82} For effective prevention of malformations, a diet should be rich in vitamins A, E, and B and folic acid. In addition, it is important to avoid exposure to contamination of the external environment, including heavy metals and POPs. ^39,44,83 Proper diet can reduce the incidence of miscarriage and pregnancy complications, and can prevent a reduced birth weight in infants. ^84,85

Tocopherol, which is one of the most important antioxidants, does not accumulate in the body and was shown to reduce the amount of free radicals generated during inflammation and dioxin poisoning. In addition, it causes limited availability of AhR and ER to dioxins. ^23,72,86,87 It was shown that the 3 weeks of administration of the high dose of tocopherol decreased the levels of inflammatory cytokines induced by administration of TCDD and inflammation. In particular, a significant reduction in the concentration of tumor necrosis factor was shown. ⁸⁸ A deficiency of tocopherol and vitamin D3 in some climate zones requires supplementation of these vitamins. ⁸⁹ Another important factor in reducing the incidence of cleft palate is folic acid supplementation before and during pregnancy. Experimental studies on mice carried out in Wellington Laboratory in Canada suggested that folic acid can reduce the magnitude of the toxic impact of TCDD confirming its anti-teratogenic effect with a decrease in the incidence of defects from 55.56% to 17.50%. ⁸⁸

Previous studies in mice show that supplementation with folic acid by pregnant women reduces negative effects of TCDD intoxication and thus reduces the risk of craniofacial palate occurrence. ⁴⁹

The knowledge about the factors causing cleft defects should be disseminated especially among physicians who have direct contact with pregnant women or those planning future pregnancies. Doctors can inform about the importance of the various factors that cause the formation of cleft, as well as provide recommendations for proper diet and how to protect against dioxins. It seems important to provide children and adolescents with educational programs on latest knowledge about environmental contamination and how to prevent from various diseases including orofacial cleft.

Summary

Based on the surveys, it has been found (1) no hereditary factors are the cause of reported cases of hard palate cleft, and (2) there is a link between the lifestyle of mothers, their nutrition, and the possibility of dioxin poisoning resulting in the birth of children with palate cleft.