Biology Notes on Edward Syndrome | Genetics

The below mentioned article provides notes on Edward syndrome.

Aneuploidy for the autosome in man often produces both physical and mental defects. One well-known condition of 18-trisomy or E- trisomy is a clinical syndrome and the child generally dies before birth.

In 1980, Edward and Patau simultaneously discovered a new syndrome associated with the presence of an extra group E chromosome, here considered to be a No. 18 chromosome.

Nondisjunction during anaphase of mitot­ic or meiotic nondisjunction or nondisjunction in zygote lead to the mosaicism.

In 90% of cases death occurs within the first six months. But Hook (1965) recorded a mentally retarded female who was living at 15 years of age.

The ratio of affected females to males is 3 to 1. This difference in sex ratio has been attributed to the survival advantages of the genetically more balanced XX females.

77% die during 3 months of post mental condition.

It is a syndrome characterised by multiple congenital malformation in which virtually every organ system is affected.

i) Birth weight is under six pounds (2.7 kg).

ii) Multiple abnormalities, especially of bones characterised by short sternum, enlarged occiput, micrognathe, laterally compressed cranium with increased fronto-occipital length.

iii) Hypertonicity.

iv) Usually wide apart eyes with microcrania.

v) External auditory meatus slightly to extensively folded with certain auricular folding and low set ears.

vi) Clenched fists with 2nd digits overlap­ping the 3rd or the 5th digit overlaping the 4th.

vii) Umbilical cord occasionally with single umbilical artery.

viii) Male cryptorchidism.

ix) Ventricular septal defect and latent duc­tus arteriosus.

x) Fingers are held permanently flexed but with the distal joint extended.

xi) The fingertips almost always have their dermal patterns in the forms of arches.

xii) Horse shoe/or double kidney.

xiii) Eventration of diaphragm.

xiv) In a mentally retarded child having an isochromosome of the short arm of chromosome number 18 has recently been reported.

xv) Micrognathia.

xvi) Prominent occiput, failure to thrive.

xvii) Heterotropic pancreatic tissue.

xviii) Cytogenetically:

(a) The additional E-chromosome was detected radio-autographically as chromosome number 18 and later by Quinacrine Mustered fluorescent micros­copy various modifi­cations of Giemsa banding (g-banding) tech­niques.

(b) Translocation E-trisomy individuals are known as translocation Mongols.

Remarks:

Affected individuals have a short life span and usually die in early infancy, although occasionally patients survive into early childhood. There is approximately a 3:1 female to male sex ratio

Maternal age is to be a factor in Trisomy E 18. According to Smith (1964), 52.5% of the mothers of such offspring were 35 years or older at the time of birth of the affected child.

1. The abnormal finding is an extra chro­mosome number 18, giving a total chromo­some count of 47.

2. If there is only a small additional part of the 18 chromosome, then the effect on the phenotype might be a modified trisomy syn­drome.

3. The trisomy 18 syndrome is occasion­ally associated with translocation between chromosome 18 and chromosomes in group B, D and G and with mosaicism.

4. The high maternal age in trisomy 18 implies that nondisjunction is usually maternal.

Patients with Edward syndrome have a single barr body and with the XX chromosome or normal females and an extra chromosome of the group which normally includes just the two number 17 and two number 18 chromo­somes, so the trisomy is either 17 or 18 was a matter of controversy.

1. Double trisomies:

Double primary nondisjunction has been observed in which trisomy 18 has been com­bined in the same individual with mongolism or trisomy X. Gagon (1961) studied a patient who was trisomic for chromosome 18 and 21.

The phenotype expression of the patient was the additive effect of Trisomy 18 and mon­golism somewhat different in the clinical mani­festations reported in two patients with com­bined trisomy 18 and trisomy X.

2. Translocation and Trisomy 18 syn­drome:

Brodic and Dallaire (1962) reported trans­location of an extra 18th chromosome piece to another chromosome in a few patients. The patient’s young mother proved to have a balanced translocation between part of a num­ber 18 and a D-chromosome plus a small acen­tric chromosome fragment. The affected child had two 18 chromosomes plus translocation chromosome and the small acentric fragment.

Another patient with trisomy for the long arms of the 18th chromosome was reported by Rohde (1963). This patient had an 18-18 chromosome translocation plus a nor­mal 18th chromosome and a typical phenotypic expression peculiar to the trisomy 18 syndrome.

3. Mosaicism:

Koulischer (1963) described a case of trisomy 18 mosaicism. The patient was a female child who died at 11 months of age and showed mental ratardation, hypertonicity; failure to thrive, prominent occiput, low set ears, small mandibles, short sternum, flexion of fingers, talus feet and dorsiflexion of big toes.

4. The 18th chromosome:

Some workers have identified the extra chromosome in the syndrome as chromosome number 17 while others interpret it as a number 18 chromosome.

Actually, it is very difficult to recognise chromosome numbers 17 and 18 clearly. Patau has observed that this difficulty is encountered if the chromosomes are not carefully identified under bright microscopy with Feulgen stained preparations. Preliminary investigations utilising tritiated thymidine sug­gest that the extra chromosome involved has the morphological and DNA replication char­acteristics of the 18th chromosome .

Levine (1971) provides a comprehensive list of defects that have been observed in tri­somy 18. Compared with male births, more than twice as many trisomy 18 females are born.

This is thought to reflect a greater male mortality in the fetal period. 50% of the patients die by two months of age, and only a few have known to survive beyond several years of age. Again, it should be noted that many of the affected fetuses may be aborted and that live born frequencies represent only a portion of trisomic 18 zygotes. Older mothers have an increased risk of trisomy 18 offspring.