American Journal of Medical Genetics 23581-587 (1986)

PREVALENCE OF THE FRAGILE-X SYNDROME I N MENTALLY RETARDED BOYS I N A SWEDISH COUNTY.

K a r l - H e n r i k Gustavson, Hans B lomqu is t K:son and Gosta Hobngren Department o f C l i n i c a l Genet ics , U n i v e r s i t y H o s p i t a l o f Uppsala, Departments o f P e d i a t r i c s and C l i n i c a l Genet ics , U n i v e r s i t y H o s p i t a l o f UmeG, Sweden.

ABSTRACT

I n an e t i o l o g i c a l s t u d y o f an u n s e l e c t e d s e r i e s o f m e n t a l l y r e t a r d e d c h i l d r e n (14 < 70) b o r n 1959-1970 i n a n o r t h e r n Swedish county , 12 o f 205 boys (5.9%) were found t o have a f r a g i l e s i t e on t h e d i s t a l end o f t h e C-chromosome ( f r a ( X ) ( q 2 7 ) ) . The i n c i d e n c e o f t h e f r a ( X ) syndrome was c a l - c u l a t e d t o be 1:1500 boys i n t h i s county . I f t h i s is t r u e for t h e whole o f Sweden, 30-40 new cases o f t h e f r a ( X ) syn- drome s h o u l d be b o r n y e a r l y i n Sweden. This must be con- s i d e r e d a m i n i m u m f i g u r e , s i n c e a c e r t a i n p r o p o r t i o n o f i n d i - v i d u a l s w i t h f r a ( X ) a r e n o t observed i n groups o f m e n t a l l y r e t a r d e d p a t i e n t s .

Nex t t o t r i s o m y 21, t h e f r a g i l e X syndrome i s t h e most common s p e c i f i c cause o f men ta l r e t a r d a t i o n among m e n t a l l y r e t a r d e d boys i n Sweden.

Keywords: f r a g i l e X , men ta l r e t a r d a t i o n , p r e v a l e n c e

Address r e p r i n t r e q u e s t s t o K a r l - H e n r i k Gustavson, M.D., Department o f C l i n i c a l Gene t i cs , U n i v e r s i t y H o s p i t a l , S-751 85 Uppsala, Sweden.

0 1986 Alan R. Liss, Inc.

582

INTRODUCTION

Gustavson, Blomquist K:son, and Holmgren

Few epidemiological studies of the frequency of the fra(X) syndrome in unselected material of mentally retarded patients have been published. Sutherland (1982) found no case of the fra(X) syndrome in 1019 newborn infants and concluded that the syndrome is much less common in such a population than in a population of retarded individuals.

Herbst & Miller (1980) calculated the incidence of fra (X) to be 0.92 per 1000 newborn males in British Columbia. However, Sutherland (1982) considered the figures published by Herbst & Miller a considerable overestimate. He found the fra(X) syndrome in 1.8% and fragile sites in autosomal chro- mosomes in l.’l!% of 444 institutionalized retarded males in Adelaide, Australia. Turner et a1 (1980) found that 5 of 72 (7%) mildly mentally retarded girls in Sidney, Australia carried the fra(X).

The aim of the present study was to estimate the preva- lence of the fra(X) syndrome in a series of mildly and severely mentally retarded children from a well defined population.

MATERIAL AND METHODS

An unselected series of children born in the county of Vasterbotten in Northern Sweden during the years 1959 to 1970 and alive at age one year was analysed retrospectively con- cerning mental retardation, defined as a stable condition of IQ less than 70 and present before age 18 years.

In thorough search procedure we checked the register of the Board for Provision and Services to the Mentally Retarded (BPSMR) in all counties o f Sweden, the case records of men- tally retarded children who had received care at the depart- ment of Pediatrics and Child Psychiatry in the county of Vasterbotten, and the death registers. The official system of ten-digit identification numbers assigned to all Swedish residents made it possible to trace all children born in the county.

The total number born in the county in 1959-1970 and alive at age one year was 40,871. Of these, 332 (8.1 per thousand were defined as mentally retarded (Gustavson et al,

Fragile-X in Mentally Retarded Boys 583

1977; B lomqu is t e t a l , 1981).

The 332 c h i l d r e n were c l a s s i f i e d i n t o t h e f o l l o w i n g main groups: 1 ) p r e n a t a l 2 ) p e r i n a t a l 3 ) p o s t n a t a l / p s y c h o t i c and 4 ) u n t r a c e a b l e e t i o l o g y ( T a b l e I ) . C h i l d r e n c l a s s i f i e d i n t o t h e groups chromosomal ( a n e u p l o i d ) , neurometabolic/-cutaneous and p e r i n a t a l were c o n s i d e r e d t o have a known d i a g n o s i s and cause o f t h e i r men ta l r e t a r d a t i o n . They were exc luded f rom t h i s s tudy . C h i l d r e n c l a s s i f i e d i n t o t h e groups : o t h e r here- d i t a r y , o t h e r p r e n a t a l , p o s t n a t a l , p s y c h o t i c and m u l t i f a c t o - r i a l ( a l t o g e t h e r 140 b o y s ) , were s e l e c t e d f o r t h i s s tudy . For v a r i o u s reasons i t was i m p o s s i b l e t o r e c e i v e blood-samples f rom 51 o f t hese boys. E leven o f them were dead, and 40 d i d n o t want t o p a r t i c i p a t e . B l o o d samples were ana lysed f rom t h e r e m a i n i n g 89 boys ( 6 4 % ) , concern ing t h e occur rence o f f r a ( X ) .

B lood samples were c o l l e c t e d f rom t h e p a t i e n t s i n 10 m l h e p a r i n i z e d v a c u t a i n e r tubes . The b l o o d was c u l t u r e d i n MEM (F low Labs) w i t h o u t f o l i c a c i d and w i t h 5% f e t a l c a l f serum. A l l c u l t u r e s were supplemented w i t h phy tohemagg lu t i n in , a n t i - b i o t i c s and L-g lu tamine. The c u l t u r e t i m e was 72 h, t h e l a s t 90 m inu tes i n t h e presence o f co lcemide. A i r - d r i e d chromosome p r e p a r a t i o n s were made w i t h r o u t i n e methods. The s l i d e s were s t a i n e d i n 10% Giemsa. As a r u l e a t l e a s t 100 c e l l s were ana lysed. When a f r a g i l e s i t e on p resumpt i ve X-chromosome was observed, t h e s l i d e was d e s t a i n e d and G-banded f o r i d e n t i f i - c a t i o n o f t h e chromosome ( S u t h e r l a n d 1979; Leverska e t a l , 1981).

RESULTS

F r a ( X ) chromosomes were seen i n 12 o f 89 m e n t a l l y r e - t a r d e d boys s e l e c t e d f o r t h e s t u d y ( i . e . 13.576, Tab le I ) ; i n t h e t o t a l g roup o f m e n t a l l y r e t a r d e d males t h e f r a ( X ) syn- drome was found i n 5.9%.

I n t h e o r i g i n a l g roup o f boys, t h e percentages were 7.3% i n t h e s e v e r e l y and 4.5% i n t h e m i l d l y r e t a r d e d boys (Blorn- q u i s t e t a l , 1982, 1983). There was no c o r r e l a t i o n between t h e i n t e l l e c t u a l l e v e l and t h e percentage o f f r a ( X ) ( T a b l e 11).

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Tab le I. C l a s s i f i c a t i o n o f t h e O r i g i n a l M a t e r i a l o f M e n t a l l y Retarded Boys Accord ing t o t h e Main Cause o f t h e R e t a r d a t i o n and t h e R e s u l t s o f t h e I n v e s t i g a t i o n Concerning X Chromosomes w i t h a F r a g i l e S i t e .

Gustavson, Blomquist K:son, and Holmgren

T o t a l Number Number w i th number ana lysed f r a ( X)

P r e n a t a l Chromosomal (aneup lo idy ) 41 Neuro-metabolic/cutaneous 17 Other h e r e d i t a r y / m u l t i - f a c t o r i a l 35 27 8 Other p r e n a t a l 28 7 0

P e r i n a t a l 14

Unt raceab le 66 50 4 R o s t n a t a l / p s y c h o s i s 10 5 0

T o t a l m a t e r i a l 205 89 12

Table 11. Cy togene t i c Data on t h e 12 Retarded Boys w i t h F ra (X) Syndrome.

Case I n t e l l e c t u a l No. o f c e l l s w i t h f r a ( X ) / l e v e l No. o f c e l l s ana lysed

ROY

LL** KL** KG* JL LGN*** PGN*** JB LM MOS GP

MMR MMR MMR MMR MMR SMR SMR SMR SMR SMR SMR

4/100 15/50 6/50 4/46 3/131,2/83

16/100 18/100

5/100 30/100

7/100 20/100

R S SMR 2/100 MMR: M i l d men ta l r e t a r d a t i o n ( IQ : 50-70) SMR: Severe menta l r e t a r d a t i o n ( I Q < 50) * F i r s t cous ins ** B r o t h e r s *** B r o t h e r s

Fragile-X in Mentally Retarded Boys 585

DISCUSSION

To ou r knowledge, no p r e v i o u s e p i d e m i o l o g i c a l s t u d y has been r e p o r t e d concern ing t h e p r e v a l e n c e o f t h e f r a ( X ) syndrome i n an u n s e l e c t e d p o p u l a t i o n o f m e n t a l l y r e t a r d e d c h i l d r e n .

The 89 boys examined c o n s t i t u t e d 64% o f t h e 140 boys s e l e c t e d f o r t h i s s tudy , drawn o u t o f a t o t a l s e r i e s o f 332 c h i l d r e n w i t h MR. The number o f i n d i v i d u a l s n o t examined m i g h t be c o n s i d e r e d h i g h . However, a t t h e t i m e for t h e f o l l o w - up, 11 boys were dead and, f o r v a r i o u s reasons, t h e o t h e r 40 boys c o u l d n o t p a r t i c i p a t e i n t h i s s tudy . Most o f t h e boys t h a t d i d n o t want t o p a r t i c i p a t e i n t h e s t u d y were m i l d l y r e t a r d e d and i t was found t o be ex t reme ly d i f f i c u l t t o m o t i - v a t e many teenagers t o p a r t i c i p a t e , s i n c e t h e y d i d n o t i den - t i f y themselves as m e n t a l l y subnormal.

Twelve o f t h e boys had t h e f r a ( X ) . S ince t h e c h i l d r e n i n t h e o t h e r e t i o l o g i c a l groups a l l had ano the r con f i rmed main cause o f men ta l r e t a r d a t i o n , i t i s u n l i k e l y t h a t t h e y s h o u l d a l s o possess a f r a ( X ) c o n s t i t u t i o n , b u t i t may be p o s s i b l e i n a few cases. However, we m igh t m iss cases w i th f r a ( X ) i n t h e boys who c o u l d n o t be examined. Thus, t h e p r e - va lence f i g u r e s g i v e n i n t h e p r e s e n t s tudy must be cons ide red a minimum.

I n a s tudy o f modera te ly m e n t a l l y r e t a r d e d i n s t i t u t i o n a - l i z e d males i n Ade la ide A u s t r a l i a , S u t h e r l a n d (1981) found t h e f r a ( X ) syndrome i n 276, and i n h i s s t u d y o f 1019 newborn i n f a n t s (1982) he found none w i t h t h e f r a ( X ) . P o s s i b l y s m a l l s e r i e s , e t h n i c and s e l e c t i o n f a c t o r s may e x p l a i n t h e d i f - f e rences i n t h e r e p o r t e d p reva lences o f t h e f r a ( X ) syndrome i n m e n t a l l y r e t a r d e d i n d i v i d u a l s .

S ince 12 boys w i th t h e f r a ( X ) syndrome were seen i n t h e combined s e r i e s o f s e v e r e l y and m i l d l y m e n t a l l y r e t a r d e d boys i n t h i s a rea c o l l e c t e d d u r i n g a 12-year p e r i o d , t h e mean y e a r l y i n c i d e n c e o f t h e f r a ( X ) fo rm o f men ta l r e t a r d a t i o n m i g h t be c a l c u l a t e d t o be about 1:1500 newborn boys. This g i v e s 30 t o 40 new male cases o f t h e f r a g i l e X syndrome i n Sweden eve ry yea r , and t h e t o t a l number o f male p a t i e n t s w i th f r a ( X ) i n Sweden t o be a t l e a s t 2400. This must be cons ide red a m i n i m u m f i g u r e , s i n c e , as d e s c r i b e d i n t h e p r e s e n t s tudy , a c e r t a i n p r o p o r t i o n o f i n d i v i d u a l s w i t h f r a ( X ) a r e n o t ob- se rved i n groups o f m e n t a l l y r e t a r d e d p a t i e n t s .

586

ACKNOWLEDGEMENTS

Gustavson, Blomquist K:son, and Holmgren

T h i s s t u d y was suppor ted by t h e Savstaholm Foundat ion , The Swedish M e d i c a l Research C o u n c i l ( p r o j e c t No. 19X-05445) and t h e M e d i c a l Research C o u n c i l o f t h e Swedish L i f e Insu - rance Companies (Svenska l i v f o r s a k r i n g s b o l a g s namnd for med ic insk f o r s k n i n g ) .

REFERENCES

B lomqu is t H K:son, Gustavson K-H, Holmgren G (1981): M i l d men ta l r e t a r d a t i o n i n c h i l d r e n i n a n o r t h e r n Swedish county . J Ment D e f i c Res 25:169-186.

B lomqu is t ti ;<:son, Gustavson K-H, Holmgren G, Nordenson I, j w e i n s A (1982) : F r a g i l e s i t e X chromosomes and X - l i n k e d men ta l r e t a r d a t i o n i n s e v e r e l y r e t a r d e d boys i n a n o r t h e r n Swedish coun ty . A p reva lence s tudy . C l i n Genet 21:209-214.

B lomqu is t H K:son, Gustavson K-H, Holmgren G, Nordenson I, P i l s s o n - S t r i e K (1983) : F r a g i l e X syndrome i n m i l d l y m e n t a l l y r e t a r d e d c h i l d r e n i n a n o r t h e r n Swedish county . A p reva lence s tudy . C l i n Genet 24:393-398.

Gustavson K-H, Holmgren G , J o n s e l l R , B lomqu is t H K:son (1977) : Severe men ta l r e t a r d a t i o n i n c h i l d r e n i n a n o r t h e r n Swedish county . J Ment D e f i c Res 21:161-180.

Herbs t DS, M i l l e r JR (1980) : N o n s p e c i f i c X - l i n k e d men ta l r e t a r d a t i o n 11: The f requency i n B r i t i s h Columbia. Am J Med Genet 7:461.

Leverska MA, Webb GC, Pavey SM (1981) : Chromosome band ing r e q u i r e d for s t u d i e s on X - l i n k e d men ta l r e t a r d a t i o n . Lancet i: 49.

S u t h e r l a n d GR (1979): H e r i t a b l e f r a g i l e s i t e s on human chromosomes. I. E f f e c t s o f c o m p s i t i o n o f c u l t u r e medium on exp ress ion . Am J Hum Genet 31:125-135.

S u t h e r l a n d GR (1982) : The p o p u l a t i o n c y t o g e n e t i c s o f f r a g i l e s i t e s ( A b s t r a c t s ) . The 6 t h I n t e r n a t i o n a l Congress o f Human Genet ics , 12-1 9 Sept 1981 . Jerusalem I s r a e l . p. 44.

Fragile-X in Mentally Retarded Boys 587

Sutherland GR (1982): Heritable fragile sites on human chro- mosomes VIII. Preliminary population cytogenetic data on the folic-acid-sensitive fragile sites. Am J Hum Genet 34:452-458.

Turner G, Brookwell R , Daniel A, Selikowitz M, Zilibowits M (1980a): Heterozygous expression o f X-linked mental retardation and the marker X: fra(X) ( q 2 7 ) . New Eng J Med 303:662-664.

Edited by John M. Opitz and James F. Reynolds

Received for publication July 17, 1985; revision received September 12, 1985.