Correlation of trisomy 13 with atelencephalic aprosencephaly

Introduction

Folate is essential in the human body for the biosynthesis of purines and pyrimidines, as well as for the processes of cell division and epigenetic changes. From the food, folates are absorbed by the intestinal villi and transported by the portal vein to the liver. After methylation in the liver, the resulting 5-methyltetrahydrofolate (5-methyl THF) enters the blood stream and reaches the cells. 5-Methyl THF is involved in the synthesis of the precursors for DNA and RNA and in the conversion of homocysteine to methionine. Methionine is involved in the formation of the DNA methylating agent S-adenosylmethionine [1]. Deficiency of folate in cells may lead to aberrant DNA methylation, point mutations and chromosomal abnormalities, such as defective recombination of chromosomes and aneuploidy [2].

Vitamin B₁₂ and folic acid deficiency, as well as cholesterol deficiency are risk factors for the expression of neural tube defects (NTDs), while teratogenic substances, such as ethanol, retinoic acid, ochratoxin A and cyclopamine, are risk factors for the expression of facial and brain phenotypes [3].

Trisomy 13 or Patau syndrome is an aneuploidy. The syndrome is a numerical chromosomal aberration in which the 13th chromosome has three copies instead of the usual two (47, XX/XY+13). The change in the normal quantity of chromosomes in trisomy 13 is usually the reason for the development of abnormalities of the prosencephalon. Changes in the number of gene copies in trisomy and triploidy can interrupt or even repress the signaling pathways such as the sonic hedgehog (SHh) and the planar cell polarity signaling pathways, which are crucial for the development of the forebrain [4].

The specific effects of gene regulatory mechanisms on the closure of the neural tube and its defects are debatable. The SIX homeobox 3 (SIX3) gene has a vital role in the development of the forebrain and eyes, and mutations therein are found in 5% of anomalies of the prosencephalon and, in particular, holoprosencephaly (HPE). The zinc finger protein (ZIC2) gene is located in the 13th chromosome. Mutations of ZIC2 are found in 9% of the cases of HPE and most frequently occur de novo (72%) [5]. The exact functions of this gene are not yet fully clarified, but it is known that this gene partakes in the early neurogenesis by interacting with other key genes. In such a way, for example, HPE is explained as a result of an impaired interaction between the ZIC2 and the SHh genes [6].

The transforming growth-β-induced factor (TGIF) is the only gene that is described in a case of transmission of the mutant allele of the gene from clinically healthy parents to a strongly affected child. The TGIF gene is a transcriptional corepressor of the TGIF signaling pathway and inhibits the action of retinoids. It is possible that mutations in this gene can lead to the accumulation of retinoic acids in the front of the prosencephalon.

Aprosencephaly is a NTD in which the prosencephalon does not develop at all. Causes of the malformation are not known, but according to Menkes [7], the emergence of aprosencephaly is due to a folic acid deficiency. So far, 200 genes have been described to be associated with NTDs. The defects of the neural tube that relate to atelencephalic aprosencephaly result from the cumulative effect of the action of a plurality of genes and environmental factors.

The aim of this study was to present a rare phenotype-genotype correlation of atelencephalic aprosencephaly in a fetus with a free trisomy 13 karyotype.

Materials and methods

The material for this study was a female fetus obtained by a termination of pregnancy during the 26th gestational week at the Center of Maternity and Neonatology (Tunis, Tunisia). The reason for the pregnancy interruption was a detection of HPE by prenatal ultrasound. After obtaining authorization from the ethics committee of the Center of Maternity and Neonatology, a classical autopsy and karyotyping of the fetus were performed.

The mother was 22 years old and had one previous vaginal delivery. There was no evidence of a consanguineous marriage. She was not supplemented with folic acid, either prior or during her pregnancy.

The biometric parameters of the fetus correspond to the 26–27th gestational week, with a weight of 900 g, vortex-talon length of 320 mm, vortex-coccygis length of 210 mm, cranial perimeter of 195 mm and length of foot of 52 mm. Karyotyping discovered a free trisomy 47, XX+13.

Examination of the head and face revealed microcephaly and cyclopia. Cyclopia was represented by an eye slit in the upper middle part of the face, with one upper and one lower eyelid with no eyelashes and eye brows and a missing eyeball. The lack of external nostrils and nose is pathognomonic of an agenesis of the nose and nasal cavity. Ear auricles were hypoplastic, poorly formed and highly attached to the head (Figure 1). Examination of the mouth and palate found a cleft palate of 15 mm, a fissure on the left cheek also 15 mm in length and macroglossia.

Figure 1:

Fetus with cyclopia, agenesia nasalis and cleft cheek.

Examination of the skull found craniostenosis, an agenesis of sella turcica and right temporal epidural hematoma. The skull base was covered with a shiny dura mater, and the right half was hemorrhagic. The brain weighed just 8 g (instead of 95 g) and had an agenesis of both cerebral hemispheres, corpus callosum, septum pellucidum, fornix, ventriculus lateralis and tertius, eyeball and optical nerve, pituitary gland, rhinencephalon and all prosencephalic structures. The diencephalon was rudimentary, occupying the medial portion of the skull base, located high and protruding like a tree stump. A configuration of a rudimentary hypothalamus was identified (Figure 2). The brain stem was present, and the mesencephalon and the pons were formed. The cerebral parts forming the rhombencephalon were present. The medulla oblongata had well-formed olives, located on the anterior surface of the brain at the place of the pyramids, but no pyramids. Cerebellar structures were hypoplastic; the foliation of the hemispheres and vermis was not developed enough or well shaped; the fourth ventricle was wide open (Figure 3). The morphology of the brain structures proved the malformation atelencephalic aprosencephaly.

Figure 2:

Atelencephalic aprosencephaly.

Figure 3:

Brain stem.(A) Rudimentary diencephalon; (B) hypoplastic cerebellar structures; (C) pons; and (D) well-formed olives, no pyramids.

Examination of the internal organs revealed correctly lobulated but hypoplastic lungs. Examination of the heart, kidneys, umbilical arteries and bladder found no abnormalities. The uterus was hypoplastic and bicornuate.

Microscopic histological examination found pancreatic heterotopias in the spleen.

Results and discussion

The present study describes a rare brain malformation where the telencephalon was missing and the diencephalon was rudimentary (atelencephalic aprosencephaly). There are only a few cases described in the literature, but this is the first case with a karyotype of free trisomy 13 originating from a non-consanguineous marriage. Atelencephalic aprosencephaly belongs to the group of NTDs and represents a defect in the closure of the anterior neuroporus, occurring between day 24 and day 26 of the embryonic development. It is a variation of atelencephaly in which the telencephalon is missing, of aprosencephaly where the telencephalon and the diencephalon are missing and of pseudo-aprosencephaly, which represents a primitive form of HPE [8, 9].

On the one hand, atelencephalic aprosencephaly is a very rare brain malformation, but, on the other hand, this study found it to be the most common abnormality of a karyotype of free trisomy 13. In our case, there were many of the typical symptoms of trisomy such as underweight and reproductive system abnormalities (hypoplasia of the uterus), but one of the most typical signs, hexadactyly, was missing. The most common facial phenotypes correlated with trisomy 13 were cyclopia, cebocephaly and ethmocephaly, with the most common being brain phenotype HPE [10].

The facial phenotype cyclopia with nasal agenesia and cleft palate correlated with atelencephalic aprosencephaly. This confirms the assertion that a severe facial phenotype is correlated with an even more severe cerebral one and that the genetic modification is a key modulator of the phenotype [11].

Trisomy 13 or Patau syndrome has an incidence of about 1 in 16,000 births. The pathogenetic mechanism of the aberration is not exactly known, but it is the most common cause of miscarriage, especially during the first trimester of pregnancy [12]. According to Solomon et al. [13], about 32–41% of patients with HPE have an abnormal number of chromosomes, which, in most cases, results in stillbirth.

The age of the mother appears to be a frequent risk factor for nondisjunction of the maternal meiotic division [14]. The thermally unstable form of methyltetrahydrofolate reductase (MTHFR) is a risk factor for NTDs, but maternal polymorphisms at either of the two folate metabolism enzymes, methylenetetrahydrofolate reductase (MTHFR) and methionine synthase reductase, are considered to be genetic contributors to human meiotic nondisjunction in trisomy [15]. The mother’s age in this particular case is 22; she has had one prior vaginal delivery and is of non-consanguineous marriage.

The genesis of atelencephalic aprosencephaly is most likely multifactorial, and only further research can explain such karyotype-phenotype correlations. Therefore, any suspicion of central nervous system abnormalities during prenatal ultrasound examination at the 11th and 22nd week of gestation must be supplemented with a biochemical triple test of α-fetoprotein, β-estradiol and human chorionic gonadotropin in the blood of the mother. A multidisciplinary approach is needed for the diagnosis – SCAN, interference reflection microscopy, genetic analysis and karyotyping by fluorescent in situ hybridization).

A deficiency in the folate metabolism could lead to hyperhomocysteinemia due to an incomplete processing of methionine and to the accumulation of homocysteine in the body. The increase in plasma levels of homocysteine is associated with genetic defects in enzymes involved in the biochemical remethylation, such as MTHFR; therefore monitoring of homocysteine in the plasma is recommended.

Prevention remains as the most important approach to NTDs. Numerous studies have proven the protective effect of increased folate levels in the mother, in the form of supplementation with folic acid during the periconceptional period and the first months of pregnancy. Some studies confirm the existence of a cause-effect link between folic acid supplementation and malformations of the central nervous system, both in high risk pregnancies and in pregnancies with a low risk. The need for folate is increased by 0.4 mg/day during pregnancy. Therefore a dose of 0.4 mg (400 μg) of folic acid per day is recommended for women with no previous births of children with NTDs who have decided to have a child. This administration of folic acid should begin at an early period, at least 4 weeks before conception, to be beneficial to the closure and differentiation of the neural tube.

For patients who have had previous pregnancies and births of children affected by NTDs, the recommended dose of supplementation with folic acid in the form of oral daily intake is increased to 1000 μg in the periconceptional period until the 12th week of gestation.

Conclusion

To determine the exact role of folates in atelencephalic aprosencephaly and defects of the neural tube in general, a larger quantity of cases needs to be examined through biochemistry and molecular biology studies. Atelencephalic aprosencephaly associated with a karyotype of free trisomy 13 proves the risk of recurrent free trisomy 13. Prenatal diagnosis is of great importance in the combat against congenital malformations due to the non-mandatory nature of prenatal karyotype tests. Despite the small risk, it is desirable to inform parents about its accidental and non-hereditary nature in a probable future pregnancy.

Prophylactic supplementation of folates during the first 3 months of pregnancy should be encouraged.