Variation in Geo-ethnic Distribution Need for Prospective Monitoring

Neural tube defect (NTD) is the most common congenital malformation of central nervous system. However, its prevalence varies greatly from place to place and population to population. The highest incidence of NTD has been reported from Ireland and Wales (6.38-10.92 per 1000 births), whereas its incidence in other European countries has been only 0.1-0,6 per 1000 births (l). The prevalence of NTD in the US and worldwide is about 1 per 1000(1). Other parts of the world  with high prevalence of NTD are northern India(2), northern China(3), Egypt and Lebanon(l). Its prevalence in Palestinians too has been reported to be high(4). Interestingly, compared to northern provinces in China, the prevalence of NTD in southern China is only 1 per 1 000. Its prevalence in Japan also is low. 

The prevalence of NTD from different parts of India has been reported to vary from 0.5 to 1 1 per 1000 births(5-8). In general the prevalence in northern states, namely, Punjab, Haryana, Delhi, Rajasthan, UP and Bihar has been much higher (3.9-9.011000) compared to eastern, western and southern parts of the country (0.5-2.6411 000). One exception to this statement is the reported high incidence of NTD from Davangere in Kamataka(8,9). The reason for high prevalence of NTD in Punjab has been attributed to high prevalence of NTD in Sikhs, which had remained high even in rnigrant Sikhs(l 0, 1 1). However, it is not clear whether the high prevalence of NTD in Sikhs is due to their sociocultural practices or genetic makeup. On the other hand the high prevalence of NTD in Davangere has been attributed to consanguinity(9).  The prevalence of NTD in consanguineous couples was found to be 16.3-20.611 000 compared to 5.9-8.41 1000 in couples without consanguinity. However, with similarly high consanguinity rates in other southern states, the prevalence of NTD from Mysore-(1 2) and Madras(l 3) has not been high. If these data were true, careful analysis of geo-ethnic factors could lead to interesting leads. For this purpose it would be useful to collect prospective data from representative areas of the country on a regular basis particularly since temporal variation in the prevalence of NTD has been reported from other parts of the world(I4,15).The informative populations cold be valuable in hunt for genes giving rise to NTD.

Genetic Counseling and Secondary Prevention-Need for Training of Primary Doctors in Principles of Medical Genetics

Prevention of recurrence of the disease after birth of one affected child is known as secondary prevention. This is the usual recourse available for prevention of most genetic disorders. The risk of recurrence of NTD after birth of one affected child is 3-5%, which is 1 0 times higher than that of general population(16). It increases to - 10% after birth of two affected children and to -25% after three such births. Periconceptional folic acid supplementation and antenatal diagnosis can prevent the birth of NTD child in next pregnancy. It is, therefore, important that all women who have given birth to a child with NTD be referred for genetic counseling prior to conception of the next child. It is also important that the affected child be examined by a geneticist to make correct diagnosis of the disorder since there are several monogenic syndrornes(16) and chromosomal disorders(17) which are associated with NTD. These syndromes carry a very different risk of recurrence and may not be amenable to prevention by folic acid supplementation. In case of still births and neonatal deaths, where it may not possible to obtain immediate genetic advice, it is important to take a photograph, a whole body radiograph and to carry out chromosomal study and autopsy, if feasible, of the affected child. This should also be done where pregnancy is terminated for antenatally detected NTD. Besides genetic factors, the environmental factors that may contribute to the causation of NTD, for example, insulin dependent maternal diabetes or intake of valproic acid or carbamazepine during pregnancy, also need to be excluded(16).

Genetic counseling deals with communication of risk of recurrence/occurrence of the disease as well as its burden to the given family. On basis of this information various reproductive options are discussed with the couple and non-directive assistance is provided to reach an appropriate decision. This includes discussion on availability, utility, limitations and safety of various tests and procedures for carrier screening and antenatal diagnosis that can be availed by the couple. The ultimate objective is to reduce genetic burden as far as possible and to achieve the desired reproductive goal of the couple without undue harm to mother and foetus. While definitive counselling and antenatal diagnosis is provided by the medical geneticist, for initial referral and follow up it is necessary that primary care providers, namely, the pediatricians and obstetricians are also equipped with relevant information and skills in genetic outselling for problems like NTD etc.

Primary Prevention

One of the newly emerging areas in the field of clinical genetics is the concept of primary prevention. It implies prevention of the birth of an affected child prior to its occurrence in any family. Primary prevention requires targeting of preventive measure(s) to entire population or to high-risk individuals, if the latter can be identified by suitable screening strategies. Obviously it is both cost and effort intensive activity which can be considered only when suitable intervention is available for this purpose. NTD has emerged as an ideal condition for primary prevention. Two feasible approaches for achieving primary prevention for NTD are described below.

Maternal Serum Alpha-fetoproteiii (msAFP) Screening to Identify Presence of Open NTD in Fetus in-utero

One of the tests for antenatal diagnosis of NTD is to measure msAFP at ]6th- 1 8th week of gestation(] 8, 1 9). Fetal liver synthesizes this protein. It is present in amniotic fluid in high amounts. From amniotic fluid a small amount of AFP leaks into maternal serum. In presence of open NTD much higher amounts of AFP ooze into amniotic fluid and from there to maternal serum. Thus, the level of msAFP is significantly higher if the fetus in-utero carries an open NTD. At cut-off value of greater than 2.5XMOM (Multiple of Median), more than 90% cases of anencephaly and 80% of spina bifida eystica may he detected by msAFP measurement(] 9). The specificity of the test, however, is not very high since it may be elevated in several other fetal disorders as well(20). The utility of combining serum uE3 measurement along with msAFP has been recently proposed(21). The diagnosis of NTD can be confirmed by high-resolution fetal ultrasonography andlor amniotic fluid acetyl-choline-esterase estimation, both of which are capable of making correct diagnosis of NTD in great majority of cases. The results of msAFP screening have been so encouraging, that several countries have adopted it as routine screening test for NTD for all pregnant women, irrespective of the past and family history of NTD.

Periconceptional Folic Acid Supplementation

One of the most remarkable developments in the field of teratogenesis during last two decades has been the demonstration of efficacy of periconceptional folic acid supplementation in prevention of NTD(22,23). The double blind randomized trial of Medical Research Council, Great Britain has shown that supplementation of 4 mg folic acid per day for at least one month prior to conception to 3 months post conception reduces the risk of recurrence of NTD by 70%(24). Its efficacy in the Indian population has also been demonstrated (un-published ICMR Multicentric trial results). The timing of folic acid supplementation for prevention of NTD is very critical since neural tube in humans closes between days 17-30 post-ovulation, which corresponds to day 2 to 1 5 post-LMP. In a parallel study it was shown that 0.8 mg of folic acid may prevent occurrence of even the first NTD(25). This has opened the avenue for primary prevention of NTD except for the fact that most human pregnancies are neither preplanned nor such compliance on mass scale appears to be feasible. As an alternative it has been proposed to fortify breakfast cereals/bread to provide the recommended daily allowance of folic acid namely, 0.4 mg per day to all women in the reproductive age group(26). However, the efficacy and safety of this approach still needs to be validated. Perhaps identification of susceptible individuals may simplify the task.

The mechanism of action of folic acid in presenting the occurrence of NTD has been a subject of - speculation. Women who have given birth to a NTD child do show marginally lower serum and red cell folate levels but the difference statistically is not significant(27). Many of these women have been found to have higher levels of scrum homoeysteine (and methionin) indicating a metabolic block in the folic acid pathway(28). It has been proposed that considerably larger amount of folic acid (ten times of daily requirement) is needed to overcome this metabolic block although no dose response data is available on the efficacy of folic acid. Another aspect of folic acid supplementation that needs to be examined iswhy folic acid is effective in only 70% of the women? What else could be useful for the remainder 30% is another challenge to he resolved.

Identification of Genes Predisposing to NTD

The common sporadic type of isolated NTD is considered to he a polygenic multi-factorial disorder(I6). It implies that multiple genes are involved in predisposition to NTD. But besides genes, environmental factors also play an important role in causation of the disorder. The role of genes in causation of NTD is supported by higher risk of recurrence in first degree relatives compared to general population. However, the genes predisposing to NTD in humans have not yet been identified. The observation that perioconceptional folic acid supplementation can prevent recurrence and even the first occurrence of NTD has led to investigation of the role of genes involved in folic acid metabolism(29). It has been reported that C to T mutation at nuelcotide 677 in the MTHFR gene is 1.7-1.9 (95% C.I. 1.1-3.1) times more prevalent in mothers, fathers and fetuses affected with NTD compared to general population(30,31) but this association has not been found in all the studies(32,33). Since the process of neural tube development is conserved in nature there is hope that animal models may provide lead for identification of homolgous elements in humans by syntany. Several spontaneous as well as transgenic models in mice already exist which look quite promising(34). One of the candidate genes, Pax 3 which has been found to be responsible for NTD in the Splotch mouse model, has been investigated in humans. While in isolated families Pax 3 as well as Paxl have been implicated in humans but the mutations of Pax genes or its linkage has not been found in the common variety of NTD(35-37). For identification of human genes, familial cases of NTD are an important resource. Once the predisposing genes are identified they can serve as tool to screen for high-risk individuals who can be targeted for primary prevention.

Need for Starting Preconception Clinics

For prevention of genetic diseases by antenatal diagnosis routine antenatal care is not adequate. Even if women reach the antenatal clinic immediately after recognition of pregnancy it would already be 6-8 weeks of gestation. The time left to plan for antenatal diagnosis by chorion villus sampling, which should be done at 1 1 - 1 2 weeks of pregnancy, is then not adequate, particularly if the mutation causing the disease is required to be characterized. Further, for conditions like NTD the critical period of intervention is already over by that time. For genetic counseling and antenatal diagnosis to be effective the couple must seek genetic advice prior to conception. In other words the conception in such families should be preplanned. For prevention of genetic disorders we need to start preconception clinics where individuals with high risk of having a child with genetic disease could be screened and those who are found to be at risk are counselled and managed.

S.S. Agarwal,
Professor of Eminence, Medical Genetics,
Sanjay Gandhi Postgraduate Institute of
Medical Sciences,
Lucknow 226 014, India.