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ORIGINAL CONTRIBUTION
Year : 2002  |  Volume : 56  |  Issue : 5  |  Page : 225-229
 

Translocation down syndrome


1 Institute of Genetics, Hyderabad, India
2 Institute of Immunohematology, Mumbai, India

Correspondence Address:
A Jyothy
Institute of Genetics, Hyderabad
India
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PMID: 12649945

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How to cite this article:
Jyothy A, Mallikarjuna Rao G N, Kumar K, Babu Rao V, Uma Devi B, Reddy P P. Translocation down syndrome. Indian J Med Sci 2002;56:225-9

How to cite this URL:
Jyothy A, Mallikarjuna Rao G N, Kumar K, Babu Rao V, Uma Devi B, Reddy P P. Translocation down syndrome. Indian J Med Sci [serial online] 2002 [cited 2014 Oct 30];56:225-9. Available from: http://www.indianjmedsci.org/text.asp?2002/56/5/225/11979


Down syndrome (DS) is the most common intellectual disability in man with characteristic facies and mental retardation. The frequency of Down syndrome in Indian population is estimated to be 1 in 920 births. [1] In general 90% of Down syndrome are due to pure trisomy 21 resulting from non­disjunctional error of chromosome 21 during gametogenesis in one on the parents. 6 to 7% are of mosaic type (presence of normal and abnormal cell types) and 3 to 4% are due to trans­location of chromosome 21 to D or G group chromosomes. The present study was undertaken in referred Down syn­drome cases to find the frequency of the translocation Down syndrome, origin of translocated chromosomes and the role of parental age in the predisposition to translocation, as the family history, parental age, parental karyotype, pregnancy outcome play an important role in the diagnosis and genetic counseling.


 ¤ Materials and Methods Top


We investigated 1021 cases with clinical features of Down syndrome attending Cytogenetic division at the Institute of Genetics and Hospital for Genetic diseases from different parts of the state of Andhra Pradesh. A complete clinical assessment and information pertaining to age, birth order, pedigree, parental age were recorded in special case proformas. The age of the subjects ranged from newborns to 25 years. Cytogenetic investigations were carried out on peripheral blood lymphocytes of DS cases and their parents according to the modified method of Moorhead et al. [2] GTG banding was performed according to Seabright. [3] CGB banding according to Arrighi and Hsu [4] and Silver staining of nuclear organising regions according to Howell and Black. [5] 20 to 25 well spread metaphases were scored for the detection of chromosomal anomalies and the origin of translocated chromosomes.


 ¤ Results and Discussion Top


Cytogenetic analysis was carried out for 1021 cases of Down syndrome. Out of which 892 cases (87.37%) showed pure trisomy, cases (4.5%) were of translocation type, and 83 cases (8.13%) had mosaicism. Sex ratio of male to female was 1.4:1 and that of translocation was 1.1:1.Translocation in Down syndrome is usually of Robertsonian type with the fusion of chromosome 21 to D or G group chromosomes. Most frequent forms are t( 21;21) and t(14;21). [6] The other less frequent translocations are t(13;21), t(15;21) and t(21;22). The occurrence of these can be sporadic or secondary if one of the parent happens to be a translocation carrier in balanced state. In our study the frequency of translocation was estimated to be 4.5%. This is in accordance with earlier reports important factor in genetic counselling. Parental ages of Down syndrome at the time of conception were available for only 885 cases out of 1021 cases studied. Out of which 775 cases were of trisomy 21, 69 cases were of mosaicism and 41 cases were of translocation Down syndrome. In [Table 2] we compared parental ages of pure trisomy 21 with translocation Down syndrome. It is evident from the present data that 80.7% pure trisomy Down syndrome children were born to mother's <30 years age and ranging from 3.6% to 5.1 % of Stoll et al 1998 [7] and Kim et al 1999 [8] . [Table 1] shows the types of translocations encountered in the present study. The most frequent was of the t(14;21) type (43.47%), followed by t(21;21) type 36.95%). The frequency of other translocation Down syndrome like t(13;21, t(15;21), t(21;22) and translocated mosaics were below 5%. Parental age at conception is an 49.41 % to father's <30 years. In case of translocation 90.24% were born to mother <30 years age and 70.74% to father <30 years.

Among 41 translocation cases 39.02% (n=16) were born to mother's age <20 years, 36.58% (n=15) to mother's age between 20-25 years, 14.63% (n=6) born to mother's age between 25-30 years and only 4 (9.75%) to mother's age >30 years. The mean maternal age of translocation Down syndrome cases was estimated to be 23.17 years. Most of the translocation DS children (n=31) were born to mothers <25 years of age and were first born in the birth order. This can be explained by the fact that most of the women were primigravidas (n=18) and the incidence of miscarriage is less in women conceiving at first pregnancy for unknown reasons. Hence it is clear from the data that most of the translocated DS children were born to younger mothers when compared to pure trisomy 21. Parental karotypes are therefore essential for all patients with a translocation to be sure that the rearrangement was not inherited. When either parent is found to be a balanced carrier of any chromosome rearran­gement, the siblings of both the patient and the affected partner are at risk. Prenatal diagmosis must be offered to these younger mothers if the karyotype analysis indicates an altered chromosomal pattern.

Parental origin of translocation DS (n=46) cases is presented in [Table 3]. Of the 46 cases, 5 were originated from the mother and 1 from the father. 26 were denovo in origin. In 14 cases the parental study was not possible, hence the origin of the translocated chromosome could not be traced out. Most of the translocation DS children were first borns in the birth order (n=25), followed by second borns (n=12) and there after the number decreased with the increasing birth order. In one interesting case of t(14;21), parental study revealed carrier status in both the parents. Mother had translocated chromosome in 100% of her cells and father showed translocated chromosome in only 45% of his cells. History of consanguinity revealed first cousin marriage. Karyotype of siblings showed mosaic translocation in 2 male sibs and normal chromosomal constitution in 2 female sibs. The risk for translocation Down syndrome can be estimated by investigating carrier status of the translocation chromosomes in both the parents. In case of t(21;21) if one of the parent is a carrier, the chance of recurrence is 100%. I n case of t(21:22) the risk is below 5% if one of the parent is a carrier and in case of D/G translocation if mother is a carrier the recurrence risk is below 10% and if father is a carrier the risk is below 5%. Chromosomal investigations, family history, pedigree analysis, parental ages and parental karyotypes are essential factors in offering genetic counselling, estimating the risk for the next conception. Advances in prenatal diagnosis are some of the preventive measures that can be suggested to the parents of a child with DS and help them in decision making to alleviate the burden of further birth of a DS child.


 ¤ Summary Top


Cytogenetic investigations carried out on 1021 cases of Down syndrome revealed translocation in 46 cases. The most frequent was of t(14;21) and t(21;21) types. Most of the translocation DS cases (n=31) were born to younger mothers (<25 years), when compared to pure trisomy 21 DS cases. Parental karyotypes, family history and parental ages has helped us greatly in offering genetic counseling, prenatal diagnosis and estimating the risk for the next conception.

 
 ¤ References Top

1.Verma I C Genetic cause of mental retardation in India. The genetics of mental retardation, biochemical, psychosocial and ethical issues, E.B Hook and J.M.Berg, eds (Kluwer Acad Publ, Dordrecht, The Netherlands) 1988. pp.99-106.  Back to cited text no. 1    
2.Moorhead P S, Nowell P C , Mellman W J, Battips D M Hungerford D A. Chromosome preparation of leucocytes cultured from human peripheral blood. Exp Cell Res 1960,20:613-616.  Back to cited text no. 2    
3.Seabright M A. A rapid banding technique for human chromosomes. Lancet 1971, 11;9:71-2  Back to cited text no. 3    
4.Arrighi FE Hsu TC. Localization of hetero­chromatin in human chromosomes. Cytogen­etics 1971;10:81-86.  Back to cited text no. 4    
5.Howell W M Black D A. A rapid technique for producing silver stained nucleolus organizer regionsand trypsin-Giemsa bands on human chromosomes. Hum Genet 1978; 43:53-56.  Back to cited text no. 5    
6.MuttomD, Alberman E,Hook E B. Cytogenetic and epidemiological findings in Down syndrome, England and Wales 1989 to 1993. National Down syndrome cytogenetic register and the association of clinical cytogenetics. J Med Genet 1996;33; 387-394.  Back to cited text no. 6    
7.Stoll C, AlembikyY, Dott B, Roth M R Study of Down syndrome in 238,942 consecutive births. Ann Genet 1998; 41; 44-51.  Back to cited text no. 7    
8.Kim S S, Jung S C, Kim H J, Moon H R Lee J S. Chromosome abnormalities in a referred population for suspected chromosomal aberrations: a report of 4117 cases. J korean Med Sci 1999;14:373-6.  Back to cited text no. 8    



 
 
    Tables

  [Table 1], [Table 2], [Table 3]

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