HIDRATACIÓN DE LAS ARCILLAS

A pert syndrom e is characterised by turribrachycephaly, m idfacial retrusion w ith a variable degree o f hypertelorism , and symm etrical com plex com plete syndactylies. T h e first description o f A p ert syndrom e is eponym ously attributed to 1906 (Apert, 1906), though previous p o s t - m o rtem cases had been described (W heaton, 1894). A p ert rep o rted a fifteen - m o n th old girl presenting in 1896 at the L ’H opital des Enfants-M alades in Paris w ith a high brachycephaUc head, severe syndactyly o f all four Hmbs and a cleft palate. T en years later, he had collected eight m ore cases from the literature and coined the term

"acrocephalosyndactyly", to describe the appearance o f a flattened occiput, tall forehead and the syndactyly deform ity. A case series was rep o rted in 1920 (Park and Pow ers, 1920), and in 1960 a collection o f 54 U nited K ingdom cases were rep o rted and sub — classified (Blank, 1960). Blank distinguished two clinical categories: (1) 'typical' acrocephalosyndactyly, the group to w hich A pert's nam e n ow com m only applied; and (2) o th er form s, com prising phenotypes n o w otherw ise attributed, grouped as 'atypical' acrocephalosyndactyly. T h e ‘true A p e rt’ was distinguished by the m id - digital hand m ass, or ‘spatulate h a n d ’, w ith a single nail com m on to digits 2-4, found in A p ert syndrom e and lacking in the n o n — A pert group {see belotû). T hirty-nine o f the 54 in Blank’s series were o f the ‘true A p e rt’ type.

A pert syndrom e is the second com m onest craniofacial syndrom e, after C rouzon syndrom e, accounting for 4.5% o f aU craniosynostosis cases (Cohen et al, 1992). E stim ates o f the frequency o f A p e rt syndrom e vary. P ooled data from 7 geographic areas (across N o rth A m erica, D enm ark, Spain and Italy) gives a birth prevalence o f 15.5 per 1,000,000 births (C ohen et al, 1992). Racial variation in incidence has been rep o rted in a Californian series (Tolarova et al, 1997). B irth prevalence was calculated at 12.4 cases p er million live births (confidence interval [Cl] 8.6,17.9), and the calculated m utation rate was 6.2x10^ p er gene per generation (com pared to 7.8x10'^ p er gene p er generation as rep o rted by C ohen and

K reiborg; 1992). Asians had the highest prevalence (22.3 p er m illion Uve births; C l 7.1,61.3) and H ispanics the low est (7.6 per million. C l 3.3-16.4). M ost cases o f A p e rt syndrom e are sporadic, though reports o f vertical transm ission suggest an autosom al do m in an t inheritance (Blank, 1960d^ewanda et al, 1993) consistent w ith the subsequent d em onstration o f causal activating m utations o f FG FR 2. A p ert syndrom e has a particularly restricted genotype am ongst the F G F R 2 — related craniofacial dysostosis syndrom es, in that ~ 99% cases result fro m two closely related m utations in the extracellular dom ain o f the rec e p to r (Wilkie et al, 1995a;01dridge et al, 1999). T hese cytosine to guanine transversions (C934G and C 937G in exon I I I a /U /7 ) predict the neighbouring am ino acid m is-sense substitutions Ser252Trp and

P ro253A rg respectively in the ‘linker — region’ sequence betw een the I g ll and I g ll l loops w hich form the putative ligand binding region (S-1.4).

In the first series rep o rt o f 40 unrelated patients, Wilkie (1995) found that the Ser252Trp and Pro253A rg substitutions occur in 63% and 37% o f patients, respectively. In a similar

ind ep en d en t re p o rt o f 36 A pert patients in the same year, S252W and P253R w ere given frequencies o f 71% and 26% , respectively (Park et al, 1995c). Similar frequencies have subsequently been rep o rted by a n u m ber o f authors (M oloney et al, 1996;Passos-Bueno et al, 1998afLajeunie et al, 1999;von G ernet et al, 2000). M oloney et al (1996) in studying 118 patients w ith new m utations, in com bination with an in d ep en d en t series o f 48 patients, estim ated germ hne m utation rates for C 934G (Ser252Trp) and C 937G (Pro253Arg) to be 5x10 and 2.7x10'*^ respectively, w hich are the highest transversion rates in the hum an genom e. Previous observations o f a paternal age effect in the genesis o f A p ert F G F R 2 m utations w ere confirm ed in 55 sporadic cases. T he authors conclude that w ithin 95% confidence hm its, ‘m o st o r aU cases o f A pert syndrom e arise by paternal m u ta tio n ’, and that the m utation arises in sperm atogenesis (M oloney et al, 1996).

T h e n arrow genotype, and relatively restricted craniofacial and hm b phenotype am ongst A p e rt patients suggests that the tryptophan and arginine substitutions in the hnker region have particularly specific effects up o n receptor signalhng in A p e rt skeletogenesis. T h e A pert p h enotype has, how ever, been linked to o ther F G F R 2 m utations in rare cases. T hree further types o f m utation o f the F G F R 2 — linker have been described, although n o t all are exlusive to A p e rt syndrom e. A C—T m utation that predicts a Ser252Leu substitution correlates w ith b o th m ild C ro u zo n syndrom e (craniosynostosis w ith norm al Hmbs) and a norm al phenotype in m em bers o f the same family. T h e Ser252Phe substitution, resulting from a double nucleotide substitution (CG to TT) o f residues 934 and 935, generates the A pert phenotype, (O ldridge et al, 1997)(Lajeunie et al, 1999). H ow ever, a com plex C G C —> T C T m utation, predicting a double am ino acid substitution o f b o th Ser252Phe com bined w ith Pro253Ser causes a P feiffer syndrom e variant w ith m ild craniosynostosis, broad thum bs and big toes, fixed extension o f several digits, and only m inim al cutaneous syndactyly (O ldridge et al, 1997). It therefore appears, that whilst the substitution o f wild — type serine for phenylalanine at p o sition 252 will encode A p ert syndrom e, the relationship is exquisitely sensitive. T h e effects o f the Ser252Phe m utation are m odified (with a decreased severity o f the Hmb phenotype) by the substitution o f proHne by serine at neighbouring position 253. F u rth erm o re the

Ser252Leu m utation at the same site generates either a norm al appearance o r m ild craniofacial dysostosis; and in addition Ser252Trp, whilst m o st frequently correlated w ith A pert

1998b). T h e relationship o f these m utations to receptor function and phenotypic outcom e are fu rth er considered below (S-1.4).

G iven this data, it m ight be supposed that the A pert p h en otype w ould be restricted to specific am ino acid substitutions in the F G F R 2 — linker. H ow ever, in rare cases, the A pert phenotype does n o t appear to be exclusive to m utations in exon 7 / I I I a o f F G F R 2. Atypical A p e rt syndrom e has been Linked to a T feiffer m u tation’ (Schell et al, 1995d^ajeunie et al, 1995a); a nucleotide substitution, 1119 — 2A— G , at the 3 ' splice site upstream o f exon 9 /I I I c (Passos-B ueno et al, 1997). T he patient displayed a craniosynostosis p h enotype w ith com plete digital syndactyly o f the feet and 3-4 syndactyly o f the hands. F urtherm ore, in a study o f 260 A p e rt patients, w hilst 258 had causative m utations in the established dipeptide o f the linker region, the rem aining 2 patients had A lu-elem ent insertions in o r near exon 9, w hich was correlated in one patient to an upregulation o f the I g lllb iso form o f F G F R 2 (O ldridge et al, 1999). A pert syndrom e thus dem onstrates a com plex and subtle relationship w ith the range o f causative m utations. T h e subsequent sections o f this thesis review the A pert phenotype and the current inform ation available from w hich to draw genotype — p h enotype correlations.

The Craniofacial Phenotype

Craniofacial features include a wide turri - brachycephaly, w hich tapers dow n fro m the parietal to the supraorbital region (Pig 1.3-3). T here is a steeply inclined, flattened forehead, and shortened, shallow p o sterio r fossa w ith short, asym m etric cranial base. T h e anterior cranial fossa and tuberculum sellae, pituitary fossa, dorsum seUae and clivus are all shortened and w idened. T here is reduced orbital volum e and significant proptosis, w ith hypertelorism , and an expanded, dysplastic ethm oidal labyrinth. T h e lesser wings o f the sphenoid bone, form ing the p o sterio r wall o f the orbit and u p p er b o rd er o f the superior orbital fissure are angled obliquely up and laterally producing a characteristic radiographic sign. T h e sum m ary effect is that o f the skuU base being foreshortened and angled, such th at there is an increasing lateral angulation o f the cranial fossae above.

T h e cranial sutures are affected in a broadly consistent fashion. T h ere is a w ide - open calvarial defect from the ro o t o f the nose to the posterior fontanelle in m idsagittal plane, encom passing the territory o f the m etopic and sagittal sutures and the anterior fontanelle. P rogression to bony fusion in this m id — sagittal plane is by the form ation o f islands o f bone in the presum ptive m esenchym al connective tissue, w hich coalesce to bony fusion by 36 m o n th s w ithout the sutures ever having been truly form ed (K reiborg and C ohen, Jr., 1990;K reiborg and Cohen, Jr., 1991;Cohen and K reiborg, 1994). T rue frontal encephalocele probably does n o t occur b u t reflects pro tru sio n o f the frontal fossa anteriorly via the calvarial

defect. T h e intracranial volum e is consistently greater than controls (C ohen and K reiborg, 1994;G osain et al, 1995) and the brain is large w ithin the skull w ith an expanded head height, although the head circum ference norm alises at birth and slows thereafter (C ohen and

K reiborg, 1994). In contrast, true fusion o f the coronal suture in m o st cases begins in - utero,

com m encing at the cranial base and extending cephalad (K reiborg and C ohen, Jr., 1990). C lover — leaf skull has been rep o rted in association w ith A p e rt syndrom e (G osain et al, 1997), b u t occurs in a m inority (4%) o f cases (Cohen, Jr. and K reiborg, 1994).

R eports o f open coronal sutures in A p ert syndrom e m ay reflect variance in age o f o n set (Cohen, Jr. and ICreiborg, 1994;Lajeunie et al, 1999), b u t are consistent w ith reports o f persistently open synchondroses at the A pert skuU base (K reiborg et al, 1993). T he variability in pathology o f b o th the skuU base chondrocranium and m em branous calvarium suggests that there are heterogeneous effects o f A pert F G F R m utations u p o n hum an m odes o f

ossification, and this is explored further in subsequent sections o f this thesis.

A p e rt syndrom e patients dem onstrate consistent and typical facial features. T h ere is severe m idface retrusion and secondary ocular proptosis w ith lagophaknos. O cular subluxation o f the globe o n to the cheek m ay occur, in conjunction w ith the retruded m idface and m arked p ro tru sio n o f the greater wing o f the sphenoid, w ith elevation o f the lesser wing (K reiborg et al, 1999). Maxillary and nasal height is reduced (K reiborg et al, 1999). T h e nasal ro o t is depressed, such that the nasal bridge is h u m ped and beak — like. D eviated nasal septum is com m on. H ypertelorism , and dow n slanting palpebral fissures (the anti - m ongoloid cant), are c o m m o n features (C ohen and K reiborg, 1996). A deeply w rinkled forehead skin is com m on. T h e retru d ed midfacial phenotype gives rise to a skeletal and dental class I II m alocclusion and relative pseudo - m andibular prognathia, though the m andible itself rem ains w ithin control param eters (K reiborg et al, 1999). T he u p per dental arch is crow ded and V- shaped, w ith an anterior open bite and p o sterio r cross bite. D ental anomalies include severely delayed eruption, ectopic eruption, and shovel-shaped incisors. M alocclusion is severe w ith a

m andibular over jet, anterior and p o sterio r crossbites, and severe crow ding o f teeth (K reiborg and C ohen, 1992).

T h e A p e rt craniofacial m orphology (n=12) has been com pared to th at o f C rouzon syndrom e (n=19) in an age — banded series o f cases studied w ith serial 3D -C T scans. W hilst the

operative history o f som e cases m ake the interpretation o f the u p per calvaria data

problem atic, the study currently provides the best longitudinal analysis o f the com parative cranial and skull base m orphology o f the two syndrom ic groups (K reiborg et al, 1993). T he com parative m orphology is tabulated below:

Cranial and sk u ll b ase features o f A pert and C rouzon sy n d ro m es (K reiborg at al, 1993): Apert C rouzon Calvaria A g e d 0-1 year C oronal synostosis

M edian sagittal diastema, including m edian plane fontanelles, w ith no m idline sutures form ed.

O p en squam osal and lam bdoid sutures.

T hin calvarial bone

Synostosis o f coronal, sagittal, m etopic, and squam osal sutures. Fontanelles close early.

L am bdoid suture may rem ain unfused in early infancy

T hin calvarial bo n e

N o m edian plane diastem a Increased digital m arkings

Calvaria A g e d 1-4 years

Delayed closure o f fontanelles and m edian diastem a by coalescing islands o f bone form ation. N o m edian plane sutures form. Digital m arkings appear late

Pan — synostosis and progressive closure o f fontanelles

Skull B ase O p en synchondroses in early infancyh w hich fuse late (> ly r), w ith delayed closure o f the spheno — occipital synchondrosis. Enlarged

sella turcica. W idened ethm oid from birth w ith a depressed cribriform plate. V — shaped anterior cranial fossa.

Synchondroses close consistently early (< ly r) in progressive m anner. E nlarged sella turcica w ith narrow floor. E th m o id n o t w idened, and cribriform plate undisplaced. T h in clivus and tendency for basilar kyphosis.

^ also rep o rted by H olten and K reiborg (H olten, 1994)(K reiborg et al, 1976).

O f n o te is th e tendency for the sutures and synchondroses in the C rouzon p h enotype to fuse early, com pared to their delayed closure in A pert syndrom e. T h e A p ert p h enotype displays

the pathognem onic unossified m edian diastema, w hich is n o t seen in C rouzon syndrom e; and also develops a w idened ethm oid with V — shaped anterior fossa. T his is clinically m anifest as hypertelorism , w hich is also n o t a m ajor feature o f the C rouzon phenotype. T h e C rouzon cases, in com pany w ith early closure o f sutures and synchondroses, m anifest early radiological signs o f raised intracranial pressure such as a w idened sella turcica and digital m arkings (S-4).

P an - sutural closure, correlated w ith raised intracranial pressure and herniation o f the cerebellar tonsils th ro u g h the foram en m agnum , has been associated w ith the C rouzon p henotype to greater extent than the A pert phenotype in a corroborative in d ep en d en t study (Cinalli et al, 1995). T h e genotype — phenotype correlations w hich m ay account for these differences are discussed in Sections 3 and 4 o f this thesis.

O phthalm ic m anifestations include rep o rted absence o f the superior rectus m uscle (C uttone et al, 1979;M orax and Pascal, 1982) and intrinsic extraocular m uscle dysplasia (Margolis et al, 1977a). Clinical sequaelae o f this include a consistent divergent up gaze and esotropic

dow ngaze (C ohen and K reiborg, 1996). O cular albinoid findings (MargoUs et al, 1977b), congenital glaucom a, keratoconus and ectopic lens are rare observations. Secondary ophthalm ic concerns include corneal exposure keratitis and ocular subluxation. Visual loss and ocular atrophy are serious concerns and reflect untreated, sustained rises in intracranial pressure (T h om pson et al, 1995a;Taylor et al, 2000).

A bnorm alities o f palatal m orphology are com m on. Cleft palate is a consistent finding, and in the rem aining population the palate is highly arched, w ith a deep m edian groove o r

‘pseudocleft’ created by swelling in the u p per arch m ucosa (Peterson and Pruzansky, 1974). G o rh n has postulated that the high arched and laterally swollen palate results from

com pression o f the u p p er dental arch during A pert craniofacial m orphogenesis, and is secondary to the midfacial retrusion and hypertelorism (G orlin et al, 1990). H ow ever, similar palatal m orphology in C rouzon syndrom e w ithout the incidence o f cleft palate (Peterson and Pruzansky, 1974) suggests that the A p ert m utations m odulate a particularly sensitive role for F G F R 2 in hum an palatogenesis (S-3.4). F urtherm ore, w hilst the palates o f b o th A p e rt and