Hum Genet (1994) 94: 240- 246 Springer-Verlag 1994 Dominique F. C. M. Smeets Ute Moog Corry M. R. Weemaes G. Vaes-Peeters Gerard E M. Merkx Jeanette R Niehof - Guus Hamers ICF syndrome: a new case and review of the literature Received: 28 January 1994 / Revised: 3 March 1994 Abstract Patients with ICF syndrome can be recognized by the presence of a variable immunodeficiency, instabil- ity of the pericentromeric heterochromatin of, in particu- lar, chromosomes 1,9, and 16 in cultured peripheral lym- phocytes, and a number of facial anomalies. Recently, aberrations at the molecular level have been described, consisting of alterations in the methylation pattern of clas- sical satellite DNA, in a number of patients. ICF syn- drome is considered to be inherited in an autosomal reces- sive manner and may be rare, as only 14 patients have been described thus far. We present a new case, a boy with agammaglobulinemia, who was extensively studied by means of classical cytogenetics and fluorescent in situ hy- bridization. All patients previously reported in the litera- ture are reviewed. Introduction At the end of the nineteen seventies, two patients were in- dependently reported who both had specific facial charac- teristics, immunodeficiency, and instability of certain cen- tromeric regions upon cytogenetic testing (Hulten 1978; Tiepolo et al. 1979). The acronym ICF (immunodefi- ciency, centromeric instability, facial anomalies) was coined by Maraschio et al. (1988), after publication of other similar cases. Up to now, 14 patients with ICF syn- drome have been reported. Patients displaying this syn- drome are mostly admitted to hospital during the first year D. F. C. M. Smeets (5:~1). G. F. M. Merkx . J. P. Niehof Department of Human Genetics, University Hospital Nijmegen, P.O. Box 9101, NL-6500 HB Nijmegen, The Netherlands U. Moog - G. Vaes-Peeters. G. Hamers Department of Molecular Biology and Genetics, University of Limburg, Limburg, The Netherlands C. M. R. Weemaes Department of Paediatrics, University Hospital Nijmegen, Nijmegen, The Netherlands of their life because of severe recurrent respiratory infec- tions attributable to combined immunodeficiency. On cy- togenetic investigation of cultured peripheral lympho- cytes, many structural aberrations are found, originating from the heterochromatic regions of, in particular, chro- mosomes 1 and 16 and, much less frequently, of chromo- some 9. The third recognizable feature of ICF syndrome comprises a number of specific facial dysmorphisms that are present in virtually all patients. ICF syndrome has been postulated to be inherited in an autosomal recessive mode. Recently, an embryonic-like methylation pattern of classical satellite DNA was ob- served in several ICF patients (Jeanpierre et al. 1993); this pattern is probably directly related to the heterochromatic instability. Here, we report a new case of ICF syndrome that was extensively studied by a combination of routine cytogenetics and fluorescence in situ hybridization (FISH); we also integrate our patient into a review of all previously reported patients. Materials and methods Clinical investigations The proband, a boy, was born in 1992 as the first child of healthy non-consanguineous parents. After the 35th week of gesta- tion, growth retardation was reported. Delivery was at 38 weeks. The patient was small for gestational age (2055 gram, 45 cm). Within the perinatal period, he had to be treated for hypo- glycemia. During the first half year of his life, he failed to thrive and a slight psychomotor retardation became apparent. At the age of 10 months, the proband suffered from respiratory syncytial virus-induced bronchiolitis. He was referred to our hospital at the age of l 1 months because of agammaglobulinemia. On admission, his weight was 6150 gram (< P3), height 67.5 cm (< P3), and head circumference 44 cm (= PI0). His face was typical with a broad flat nasal bridge, telecanthus, epicanthic folds, low set ears, and protrusion of the tongue. Furthermore, he had a large abdomen, and thin arms and legs (Fig. 1). He was able to sit, but could not yet walk. lmmunoglobulins were administered, and his height and weight increased. Two weeks after the substitution treatment had started, the patient developed pericarditis from which he recovered sponta- neously. At the age of 15 months, he ceased growing again, be-
Domin ique F. C. M. Smeets �9 Ute M o o g Corry M. R. Weemaes �9 G. Vaes-Peeters Gerard E M. Merkx �9 Jeanette R Niehof - Guus Hamers
ICF syndrome: a new case and review of the literature
Received: 28 January 1994 / Revised: 3 March 1994
A b s t r a c t Patients with ICF syndrome can be recognized by the presence of a var iable immunodef ic iency , instabil- ity of the per icent romeric heterochromat in of, in part icu- lar, ch romosomes 1 ,9 , and 16 in cul tured per ipheral lym- phocytes , and a number of facial anomal ies . Recently, aberrat ions at the molecu la r level have been descr ibed, consis t ing of al terat ions in the methyla t ion pattern of clas- sical satell i te DNA, in a number of patients. ICF syn- d rome is cons idered to be inheri ted in an autosomal reces- sive manner and may be rare, as only 14 patients have been descr ibed thus far. We present a new case, a boy with agammaglobu l inemia , who was extens ively studied by means of classical cytogenet ics and f luorescent in situ hy- br idizat ion. Al l patients prev ious ly repor ted in the litera- ture are reviewed.
Introduction
At the end of the nineteen seventies, two patients were in- dependent ly repor ted who both had specif ic facial charac- teristics, immunodef ic iency , and instabi l i ty o f certain cen- t romeric regions upon cytogenet ic test ing (Hulten 1978; Tiepolo et al. 1979). The ac ronym ICF ( immunodef i - ciency, cent romeric instabili ty, facial anomal ies) was coined by Marasch io et al. (1988), after publ ica t ion of other s imilar cases. Up to now, 14 patients with ICF syn- d rome have been reported. Patients d isp laying this syn- d rome are most ly admit ted to hospital during the first year
D. F. C. M. Smeets (5:~1). G. F. M. Merkx . J. P. Niehof Department of Human Genetics, University Hospital Nijmegen, P.O. Box 9101, NL-6500 HB Nijmegen, The Netherlands
U. Moog - G. Vaes-Peeters. G. Hamers Department of Molecular Biology and Genetics, University of Limburg, Limburg, The Netherlands
C. M. R. Weemaes Department of Paediatrics, University Hospital Nijmegen, Nijmegen, The Netherlands
of their l ife because o f severe recurrent respira tory infec- tions at tr ibutable to combined immunodef ic iency . On cy- togenet ic invest igat ion of cul tured per ipheral lympho- cytes, many structural aberrat ions are found, or iginat ing f rom the heterochromat ic regions of, in particular, chro- mosomes 1 and 16 and, much less frequently, of chromo- some 9. The third recognizable feature of ICF syndrome compr ises a number o f specific facial dysmorph i sms that are present in vir tual ly all patients.
ICF syndrome has been pos tu la ted to be inheri ted in an autosomal recess ive mode. Recently, an embryon ic - l ike methyla t ion pattern of classical satell i te D N A was ob- served in several ICF patients (Jeanpierre et al. 1993); this pat tern is p robably direct ly related to the heterochromat ic instabili ty. Here, we report a new case of ICF syndrome that was extens ively s tudied by a combina t ion of routine cytogenet ics and f luorescence in situ hybr id iza t ion (FISH); we also integrate our patient into a review of all previous ly reported patients.
Materials and methods
Clinical investigations
The proband, a boy, was born in 1992 as the first child of healthy non-consanguineous parents. After the 35th week of gesta- tion, growth retardation was reported. Delivery was at 38 weeks. The patient was small for gestational age (2055 gram, 45 cm). Within the perinatal period, he had to be treated for hypo- glycemia. During the first half year of his life, he failed to thrive and a slight psychomotor retardation became apparent. At the age of 10 months, the proband suffered from respiratory syncytial virus-induced bronchiolitis. He was referred to our hospital at the age of l 1 months because of agammaglobulinemia. On admission, his weight was 6150 gram (< P3), height 67.5 cm (< P3), and head circumference 44 cm (= PI0). His face was typical with a broad flat nasal bridge, telecanthus, epicanthic folds, low set ears, and protrusion of the tongue. Furthermore, he had a large abdomen, and thin arms and legs (Fig. 1 ). He was able to sit, but could not yet walk.
lmmunoglobulins were administered, and his height and weight increased. Two weeks after the substitution treatment had started, the patient developed pericarditis from which he recovered sponta- neously. At the age of 15 months, he ceased growing again, be-
cause of malabsorption of unknown cause. This malabsorption was still present when the proband was 2 years old, and he had hardly grown during the last 9 months. At 18 months, he started to walk. Speech development appeared to be delayed.
Immunological studies
The proband clearly suffered from total agammaglobulinemia with all classes of serumimmunoglobulins being below the level of de- tection (IgG < 0.45 g/l, IgA < 0.06 g/l, IgM < 0.05 g/l, IgD < 1 IU/ml, IgE < 2 IU/ml). Cellular immunity appeared to be normal with a normal in vitro response of peripheral blood lymphocytes to pokeweed and phytohemagglutinin stimulation. The cell marker test also showed normal values (s Ig + cells = 16%, CD3+ cells = 82%, CD4+ cells = 55%, CD8+ cells = 16%). No c Ig + cells were present in the bone marrow.
Cytogenetic studies
Cytogenetic studies were performed on cultured peripheral lym- phocytes (48-h and 96-h cultures), bone marrow cells (direct preparations), and Epstein-Ban" virus (EBV)-transformed B-lym- phocytes. Chromosome slides were made according to routine pro- cedures. Metaphases from all cultures were analyzed after GTG- banding, whereas fluorescence in situ hybridization (FISH) was performed on 48-h and 96-h lymphocyte cultures.
FISH
FISH studies were performed with probes pUC 1 and pHUR 195, which detect DNA sequences at the pericentromeric heterochro- matin of chromosomes 1 and 16, respectively, and with whole chromosome paints for chromosomes 1 (pBS1) and 16 (pBS16). All probes were labeled by standard nick translation with biotin- 14-dATP (Gibco, BRL) or dig-11-dUTP (Boehringer). In situ hy- bridization was performed as described elsewhere (Suijkerbuijk et
al. 1991). Briefly, 200 ng labeled probe DNA of chromosome 1 and chromosome 16 (pBS1 and pBS16, respectively) was mixed with a 15-fold concentration of Cot 1 DNA (serving as competitor DNA). After a 10-min denaturation at 80~ the probe DNA was reannealed at 37~ for 10 min. Labeled centromeric probes (pUC 1 and pHUR 195) were applied directly and not prehybridized with competitor DNA. Pretreated standard chromosome preparations were heat-denatured and hybridized overnight at 37~ with two denatured probes at the same time. The biotin-labeled probes were detected by three layers of avidin/fluorescein isothiocyanate (FITC), diluted 1 : 500, once in 5% non-fat dry milk in 4 x SSC (1 • SSC = 150 mM NaC1/15 mM sodium citrate, pH 7.0)/Tween-20, and twice in 5% non-fat milk in PN-buffer (100 mM Na2PO4.2 H20/100 mM NaH2PO4.H20/0.05% Triton X-100). The digoxi- genin-labeled probes were detected by rhodamine-conjugated sheep anti-digoxin antibody, diluted 1 : 20 in PN-buffer, and Texas-Red- conjugated rabbit anti-sheep antibody diluted 1 : 100 in PN-buffer. Finally, the slides were mounted in antifade medium. Preparations were studied with a Zeiss Axiophot epifluorescence microscope equipped for DAPI (4,6-diamidino-2-phenylindole), FITC, and Texas Red epifluorescence. The microscope was fitted with a charge-coupled device (CCD) camera that was connected to an Apple Macintosh computer for processing of the images. Micro- graphs were taken on 400-Asa Kodak Ektachrome film.
Results
At the age o f 8 months, the first cytogenet ic invest igat ion o f the patient was per formed on peripheral lymphocytes
that were routinely cultured for 96 h. The patient showed a normal male karyotype; 46,XY. However , hal f o f his GTG-banded cells displayed aberrations of ch romosomes 1 and/or 16; these aberrations were recognized as breaks, deletions, i sochromosomes , triradial figures, interchanges be tween per icentromeric regions of ch romosomes 1 and 16, and multiradial configurat ions (Fig. 2). In all aberrant
242
Fig,2a-i Multiradial figures detected in cultured peripheral lym- phocytes (96 h culture), a-b Only one chromosome 1 involved; c-f both chromosomes 1 involved; g-i both chromosomes 1 with parts of chromosome 16 (note that in each configuration the num- ber of chromosome 1 short arms never exceeds two, whereas the number of long arms varies considerably)
ceils, two short arms of both chromosomes 1 and chromo- somes 16 were present, with a varying number of long arms of these two chromosomes, indicating that breaks occurred just below the centromere within the heterochro- matin region on the proximal long arm.
The cytogenetic study was repeated and extended by applying FISH (Fig. 3) at the ages of 11 and 15 months, with exactly the same results in the routine cultures. Moreover, it was noticed that many micronuclei were pre- sent in every culture. However, after shortening the cul- ture period from 96 h to 48 h, the percentage of aberrant cells dropped from 50% to 12%, and the aberrations were of a much reduced complexity, mostly appearing as sim- ple breaks or deletions. The same was found with regard to the number of micronuclei. In the 96-h cultures, nu- merous micronuclei were present, whereas these were vir- tually absent in the 48-h cultures. Moreover, FISH studies established that the micronuclei mostly incorporated parts of chromosomes 1 or 16 (Fig. 3 d-e). The type of tissue culture medium used had no influence on the number of aberrations of chromosomes 1 and 16, nor on the fre- quency of micronuclei (results not shown).
In 32 uncultured and unstimulated bone marrow cells, no aberrant metaphases were found, whereas in cultured EBV-transformed B-cells, abnormal chromosomes l and/or 16 were present in 16% of the 50 cells analyzed. These aberrations appeared to be much less complex than those found in the 96-h lymphocyte cultures and consisted mainly of breaks, deletions, and sometimes a triradial fig- ure.
Both parents of the proband were cytogenetically in- vestigated. They showed no aberrations in either chromo- some 1 or 16 in 50 cultured peripheral lymphocytes.
Discussion
Our patient clearly presented all signs of ICF syndrome, including complete agammaglobulinemia, specific facial anomalies, and the characteristic cytogenetic manifesta- tion of the syndrome. Although the proband had a normal karyogram, like all patients with ICF syndrome, aber- rations in chromosomes 1 and/or 16 were found in about half of his routinely cultured peripheral lymphocytes. Whereas, in our case, aberrations were found only in chromosomes 1 and 16, aberrations in the pericentromeric region of chromosome 9 and rarely in chromosomes 2 and l0 have been reported (Table 1). Chromosomal aber- rations typical of ICF syndrome comprise despiralization of the heterochromatin, chromatid and chromosome breaks, somatic pairing, and interchanges between homol- ogous and non-homologous heterochromatic regions (Table 1). They lead to the loss or gain of whole chromo- some arms (only long arms) and multiradial figures of the chromosomes involved (Haas 1990). The overall fre- quency of cells with chromosomal aberrations tends to in- crease with increasing culture time (Tiepolo et al. 1979; Maraschio et al. 1988; present case), although the oppo- site has also been reported (Turleau et al. 1989). More- over, aberrations become more complex with increasing culture time (Hulten 1978: Tiepolo et al. 1979; Howard et al. 1985; present case). Studies on interphase nuclei consistently show an increased number of micronuclei and/or nuclear protrusions (Fasth et al. 1990; Maraschio et al. 1992). FISH has clearly established that parts of the most instable chromosomes (i.e., chromosomes 1 and 16) are present in these micronuclei (Maraschio et al. 1992; Fig. 3 d-e).
Cultured peripheral lymphocytes always display nu- merous aberrations, whereas other tissues, such as cul- tured fibroblasts, EBV-transformed B cells, and bone mar- row, show only a few aberrant cells, or none at all. If aber- rations are present in these tissues, they are mostly of a much reduced complexity than those in lymphocytes (Howard et al. 1985; Maraschio et al. 1989; Turleau et al. 1989; Fasth et al. 1990; Gimelli et al. 1993). Nevertheless, in one case, ICF syndrome was reported to be diagnosed
Fig. 3 a--e Selection of FISH results with paints for chromosome 1 (pBSI) in blue and for chromosome 16 (pBS 16) in pink (a---e) or in yellow (d, e). a---c Large arrow Multiradial figures of chromosome 1 (a), or chromosomes 1 and 16 (b, e); small arrow both chromo- somes 16, one showing a break (a), or the remaining normal chro- mosome 16 (b, e). d, e A micronucleus incorporating (parts of) chromosomes 1 and 16 (yellow), respectively. Small arrow Incor- porated chromosome; large arrow remaining chromosome in the nucleus
244
Table 1 Cytogenetic findings in patients with ICF syndrome (+ feature present, - feature absent, blank report not informative, "[" in- creased, N normal)
a Cases: 1 Hulten 1978; 2 Tiepolo et al. 1979; 3 Fryns et al. 1981; 4 Howard et al. 1985; 5 Valkova et al. 1987; 6 Maraschio et al. 1988; 7 Carpenter et al. 1988; 8 Turleau et al. 1989; 9, 10 Fasth
et al. 1990; 11 Bauld et al. 1991; 12 Kieback et al. 1992; 13 pre- sent case; 14, 15 Gimelli et al. 1993
on cu l tu red amn io t i c ce l ls at 34 w e e k s o f ges ta t ion (Fasth et al. 1990). In a second prenatal case o f I C F s y n d r o m e , the d iagnos i s was m a d e on a cu l tu red b lood sample o f a 2 0 - w e e k - o l d fetus ( Jeanp ie r re et al. 1993).
Parenta l c h r o m o s o m e s o f pat ients wi th I C F s y n d r o m e are a lways normal , wi th one e x c e p t i o n desc r ibed by Car- pen te r et al. (1988), w h o o b s e r v e d 2 .7% and 5% o f ce l ls wi th h e t e r o c h r o m a t i n abnorma l i t i e s in the p r o b a n d ' s fa- ther and mother , respec t ive ly .
I C F s y n d r o m e is not a c h r o m o s o m e b reakage s y n d r o m e
A l t h o u g h I C F s y n d r o m e shares a n u m b e r o f fea tures wi th the k n o w n c h r o m o s o m e ins tabi l i ty synd romes , such as spon taneous c h r o m o s o m e instabi l i ty, i m m u n o d e f i c i e n c y , and p robab ly a u t o s o m a l r eces s ive inher i t ance (see be low) , no hype r sens i t i v i t y to phys ica l and /o r c h e m i c a l agents , or an inc reased i n c i d e n c e o f neop la s i a and skin abnorma l i - ties, all o f wh ich are o t h e r w i s e typ ica l for c h r o m o s o m e b reakage s y n d r o m e s ( C o h e n and L e v y 1989), have been r epor t ed in pat ients wi th I C F s y n d r o m e thus far. M o r e - over , the typ ica l ly inc reased c h r o m o s o m a l b r eakage seen in c h r o m o s o m e b reakage s y n d r o m e s is not c o n f i n e d to a f ew spec i f ic c h r o m o s o m a l r eg ions as in I C F s y n d r o m e . The re fo r e , I C F s y n d r o m e has to be r ega rded as a d is t inct entity, not re la ted to the c lass ica l c h r o m o s o m e b r e a k a g e synd romes .
| C F s y n d r o m e : a r e c o g n i z a b l e ent i ty
As yet , 15 pat ients wi th I C F s y n d r o m e h a v e b e e n re- por ted, i nc lud ing the p resen t case. In Tab le 2, w e h a v e s u m m a r i z e d a n u m b e r o f c l in ica l and i m m u n o l o g i c a l man i fe s t a t ions o f these pat ients . F r o m this, it appears that I C F s y n d r o m e is a spec i f ic r e c o g n i z a b l e ent i ty wi th a h igh d e g r e e o f s imi la r i ty b e t w e e n the cases.
T h e ch i ld ren are born at t e rm m o s t l y af ter an uneven t - ful p regnancy . S o m e t i m e s , they are dys t roph ic at bir th and of ten fail to thr ive . In all pat ients , i m m u n o d e f i c i e n c y is present , wi th at least two but of ten three c lasses o f im- m u n o g l o b u l i n s s t rongly r educed or even c o m p l e t e l y ab- sent, l ead ing to a g a m m a g l o b u l i n e m i a . As a c o n s e q u e n c e , the pa t ien ts suffer f r o m m a n y recur ren t in fec t ions , in m o s t cases e v e n dur ing the first yea r o f thei r life. P u l m o n a r y in- fec t ions , d iges t i ve d isorders , and f e e d i n g p r o b l e m s deter- m i n e fur ther d e v e l o p m e n t , and the deg ree o f g r o w t h retar- da t ion and hypo t roph ia , wh ich m a y p r o g r e s s i v e l y b e c o m e m o r e s eve re (Fryns et al. 1981; Va lkova et al. 1987; Tur- leau et al. 1989; Fas th et al. 1990). F o u r o f the 15 k n o w n pat ien ts have d i ed b e c a u s e o f seve re infec t ions . D e v e l o p - men ta l de lay is genera l but o f a m a r k e d variabi l i ty . B o y s and gir ls m a y be a f fec ted and, f r o m the l imi ted n u m b e r o f pat ients k n o w n , no sex p r e d o m i n a n c e is apparent .
Fac ia l d y s m o r p h i s m s h a v e been repor t ed in all but one pa t ient wi th I C F s y n d r o m e ( G i m e l l i et al. 1993); they are mos t ly mi ld and are cha r ac t e r i z ed by ep ican th ic folds , hype r t e lo r i sm , f lat nasal br idge , m a c r o g l o s s i a wi th pro- t rud ing tongue , and m i l d m i c r o g n a t h i a (Table 2). Various
Table 2 Clinical findings in patients with ICF syndrome ($ reduced, r missing, w weeks, t term, other symbols as in Table 1)
C a s e n o . a
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Sex 6 4 9 c~ 2 9 9 4 4 9 9 c~ 4 9 4 Pregnancy complications - (+) - - - + - Birth at t t t t t t t t 34 w t t t IU growth retardation - + . . . . + + + + + Failure to thrive + + + + - + +
Immunodeficiency + + + + + + + + + + + + + + + IgG $ ,1, .], .1, .1, ,1. ,I, $ r N N IgA $ $ r $ r J, .J, J, r $ r N N IgM $ r ,1, r ,[, $ ,[. r $ r ,[, .1. IgE $ $ $ r $ $
T-lymphocytes $ N $ N N ,[, $ N ,[. $ B-lymphocytes $ N N 1" N N N N
neurological defects have been described in four patients (Tiepolo et al. 1979; Valkova et al. 1987; Carpenter et al. 1988; Gimel l i et al. 1993). Epilepsy, as in the case re- ported by Kieback et al. (1992), may not be regarded as typical, since the child 's mother is an epileptic. Addit ional features are rarely reported: short stature in two cases (Hulten 1978; Bauld et al. 1991), cleft palate (Howard et al. 1985), and a short t runk and thin arms and legs in the present case.
However, notwiths tanding the presence of a number of typical clinical symptoms, convinc ing evidence that a patient suffers from ICF syndrome is only obtained from cytogenetic studies of cultured peripheral lymphocytes showing the presence of the characteristic chromosomal aberrations (Figs. 2, 3, Table 1).
Etiology
ICF syndrome has been postulated to be inherited in an autosomal recessive mode (Valkova et al. 1987; Maras- chio et al. 1988; Fasth et al. 1990; Kieback et al. 1992; Gimell i et al. 1993; Jeanpierre et al. 1993). Consanguin i ty was found to be present in two families (Fasth et al. 1990; Bauld et al. 1991) and, recently, two families have been described with two affected sibs (Fasth et al. 1990; Gimel l i et al. 1993). In three other families, early death,
recurrent infections, and/or immunodef ic iency are re- ported to occur in the sibships (Tiepolo et al. 1979; Val- kova et al. 1987; Fasth et al. 1990). Thus, a striking accu- mulat ion of affected children and deaths during preg- nancy or infancy exists in at least some families. Parents of the patients show no signs of ICF syndrome, although there is one report of a small percentage of cells with aberrant chromosomes typical for ICF syndrome in both the father and the mother (Carpenter et al. 1988). Further- more, ICF syndrome has been found to occur in males and females. Taken together, these observations indicate an autosomal recessive inheri tance of ICF syndrome, al- though participation of viral infections during further etiopathology has also been implicated (Haas 1990). In- deed, certain viruses may specifically integrate into the pericentromeric heterochromatin of chromosomes 1 and 16 (Shaul et al. 1986), and subsequently damage these chromosomes (Haas 1990). However, Jeanpierre et al. (1993) have recently reported undermethylat ion of classi- cal satellite DNA, but not of c~-satellite DNA, in four pa- tients with ICF syndrome. This was probably the result of an inherited defect and not of any viral integration. Clas- sical satellite D N A is located in the pericentromeric re- gions of chromosomes 1, 9, and 16, and on the distal long arm of the Y chromosome. Methylat ion of these se- quences, normal ly almost complete in leukocyte DNA, is reduced or absent in ICF patients, thus mimicking the ger-
246
minal and embryonic pattern of undermethylat ion (Jean- pierre et al. 1993). This f inding again points to an autoso- mal recessive defect with a high recurrence risk. Since classical satellite DNA is also located at the distal long arm of the Y-chromosome, one would expect this chro- mosome to be involved in the ICF-specific chromosomal aberrations, although this is obviously not the case in any of the eight male patients so far described. The reason for this discrepancy is presently not known, but might em- anate from small sequence differences in these large hete- rochromatin regions.
Assuming that ICF syndrome is inherited in an autoso- mal recessive mode and considering the few cases de- scribed thus far, one would expect the populat ion fre- quency of the gene defect to be low. However, in view of the mild phenotypic expression of the syndrome, with no severe congenital malformations or severe mental handi- cap in patients with ICF syndrome, it is feasible that a number of cases go undiagnosed because no cytogenetic studies are performed. Therefore, we strongly advise chromosomal investigations to be performed in all pa- tients who display general immunodef ic iency accompa- nied by mild facial dysmorphisms.
Acknowledgements We thank Doreen Elsevier, Jos6 van Gaal, Bert Janssen, and Ineke Wienen for excellent technical assistance, and Dr. Ineke van de Burgt for critically reading the manuscript.
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