Genetic abnormality' in fragile X-syndrome: (PGI June 2007)

Ans.: A (Trinucleotide repeat disorder) "Fragile X syndrome is the prototype of the disease in which the mutation is characterized by a long repeating sequences of three nucleotidesRobbins 7th/ 181Fragile X syndromeSecond most common genetic causes of mental retardation , after Down syndromeThe only distinctive feature that can be detected in at least 90% of postpubertal males with fragile -X syndrome is macro-orchidismc-Long face with a large mandible & large everted earsHyperextensible joints, a high arched palate & mitral valve prolapseQ is noted in some patientsAs with all X-linked disease. Fragile X syndrome affects malesTable: Selected Trinucleotide Repeat DisordersDiseaseLocusRepeatTriplet Length (Normal/Disease)InheritanceGene ProductX-chromosotnal spinobulbar muscular atrophy [SBMA)Xq11-q12CAG11-34/40-62XRAndrogen receptorFragile X-syndrome (FRAXA)Xq27.3CGG6-50/200-300XRFMR-1 proteinFragile X-syndrome (FRAXE)Xq28GCC6-25/>200XRFMR-2 proteinDystrophia myotonica (DM)19ql3.2-q13.3CTG5-30/200-1000AD, variable penetranceMyotonin protein kinaseHuntington disease (H D)4pl6.3CAG6-34/37-180ADHuntingtonSpinocerebellar ataxia type 1 (SCA1)6p213-21.2CAG6-39/40-88ADAtaxin 1Spinocerebellar ataxia type 2 (SCA2)12q24.1 ACAG15-31/34-400ADAtaxia 2Spinocerebellar ataxia type 3 (SCA3); Machado Joseph disease (MD)14q21CAG13-36/55-86ADAtaxin 3Spinocerebellar ataxia type 6 (SCA6, CACNAIA)I9pl 3.1-13.2CAG4-16/20-33ADAlpha 1A voltage- dependent L-type calcium channelSpinocerebellar ataxia type 7 (SCA7)3p21.1-pl2CAG4-19/37 to >300ADAtaxin 7Spinocerebellar ataxia type 12 (SCA12)5q31CAG26/66-78ADProtein phosphatase 2 ADentorubral pallidoluysiane atrophy (DRPLA)12pCAG7-23/49-75ADAtnopbin 1Friedreich ataxia (FRDA1)9ql3-21GAA7-22/200-900ARFrataxinNote: AD, autosomal dominant; AR, autosomal recessive; XR. X-linked recessive.