Interacciones químicas

Coronal synostosis — variability of phenotype defined by the P2S0K mutation in F G F R 3

TSIon syndrom ic’ coronal synostosis is associated w ith a variety o f cranial phenotypes th at are distinct from the eponym ous syndrom es associated with m utations in F G F R 2 and F G F R l. T h e com m on cranial phenotypes are plagiocephaly associated w ith a unicoronal synostosis, and brachycephaly, associated w ith bicoronal synostosis. T h e com bined incidence o f these phenotypes is calculated as 1 in 2,100 - 2,500 (Lajeunie et al, 1995b). T h e plagiocephaly phenotype consists o f a ridged, fused suture in the coronal territory on one side causing a flattened forehead on the same side. T here is elevation o f the sphenoid o n th at side at its articulation w ith the coronal ring. T he orbit is therefore also elevated in a vertical dystopia, and there is a resultant facial scoliosis. O n the opposite side o f the skull there is frontal bossing, and the o rb it is low er than its partner. T he coronal, orbital, and skull base asym m etry may extend to involve the maxillary arch and create a skeletal and dental cross - bite. T he bicoronal synostosis phenotype is brachycephahc w ith a w ide flat forehead, w hich may be bossed forw ards above the supraorbital plane. T here is no dystopia and the m idface is n o t recessed as seen in the ‘C rouzon — Pfeiffer’ group o f F G F R 2 related syndrom es. T here are no specifically related dental problem s. ‘N o n — syndrom ic’ synostosis, or ‘sim ple synostosis’ has a low incidence o f intracranial anomaly. Intracranial hypertension occurs in 17% (n=74; including a p ro p o rtio n o f sagittal and m etopic cases) o f single suture synostoses (T hom pson et al, 1995b).

T h e involvem ent o f the coronal suture in m any o f the craniosynostosis syndrom es, and the recognised m utabihty o f the Pro252 site in F G F R l and the P ro253 site in F G F R 2 resulted in the exam ination o f the hom ologous site in FG F R 3 for m utations causing craniosynostosis (Bellus et al, 1996). T he F G F R 3 — P250R m utation was dem o n strated in two famihes w ith phenotypes rem iniscent o f Pfeiffer syndrom e in w hom there was no linkage to chrom osom es 8 o r 10. Subsequent analysis o f 65 unrelated cases w ith craniosynostosis and n o n — classifiable hm b anom aly revealed the F G F R 3 — P250R m utation in 10 individuals. T h e affected patients show ed great intra- and interfam ihal variation and craniosynostosis was n o t consistently present. T h e hm b phenotypes w ere predom inantly norm al, b u t displayed radiographic evidence o f sh o rt m iddle phalanges.

T h e retrospective analysis o f a further 61 cases from 20 unrelated families revealed the po in t m utation 749C— G , encoding the Pro250A rg substitution in a range o f phenotypes w ith coronal synostosis (M uenke et al, 1997). T h e group included a kindred previously rep o rted as linking to chrom osom e 4p (HoUway et al, 1995) and nam ed the Adelaide type, and a kindred previously described as brachydactyly — craniosynostosis (Glass et al, 1994). T h e phenotype defined by the FG F R 3 - Pro250A rg m utation is very broad. It includes uni- and bi- coronal synostosis, although norm ocephalic and m acrocephalic individuals w ith o u t craniosynostosis w ere also noted; and in these aU had abnorm al radiographic findings in the extremities. A m inority o f cases with craniofacial features had n o radiographic lim b anomaly. Facial findings included m ild m idface hypoplasia, high arched (non — cleft) palate, rmldly dow n — slanting palpebral fissures, and ptosis (M uenke et al, 1997). In a subsequent report, n o association o f F G F R 3 — P250R with o ther form s o f single suture synostosis o r anterior plagiocephaly w ithout coronal synostosis was found, in an otherw ise wide cranial phenotype (G ripp et al,

1998b), w hich includes pansynostosis in one rare case (Golla et al, 1997). T h e analysis o f family m em bers o f probands with coronal synostosis and the F G F R 3 — P250R m utation indicates th at the m utation may be associated with very m ild cranial phenotypes and subtle limb phenotypes (M oloney et al, 1997). It is likely that the coronal synostoses p resen t a heterogeneous group o f conditions, in th at only a m inority (~40% ) test positive for the F G F R 3 — P250R m utation (M oloney et al, 1997;Gripp et al, 1998b).

A lthough the hm b phenotype is clinically norm al in m o st cases, there is a range o f subchnical radiographic anomaly. This includes thim ble — like m iddle phalanges, coned epiphyses and carpal and tarsal bone fusions (M oloney et al, 1997). T h e tarso — m etatarsal fusions

characteristic o f Jackson — Weiss syndrom e are n o t seen (M uenke et al, 1997). Brachydactyly, b u t n o t syndactyly, is n o ted in som e patients, together w ith short, b u t n o n — deviated great toes. T h e phenotypic range is wide and subtle in b o th the lim b and craniofacial skeleton, and there is n o apparent correlation betw een hm b and craniofacial phenotype (M oloney et al, 1997;M uenke et al, 1997;Reardon et al, 1997). D evelopm ental delay is n o ted in a m inority o f patients w ith the Pro250A rg m utation (M uenke et al, 1997;Reardon et al, 1997). Sensorineural hearing loss is also identified in som e patients.

I t is o f note th at no n e o f the defining skeletal m anifestations o f the dw arfism dysplasias are p rese n t in the craniosynostosis F G F R 3 — P250R group. T his suggests th at the activating m echanism o f the m utation is discrete, and independent o f the range o f m utations causing achondroplasia, thanatophoric dysplasia and hypochondroplasia. F G F R 3 — P250R , in com pany w ith the dwarfism m utations in FG FR 3, is hkely to be an activating m utation, as patients w ith the 4p - deletion syndrom e lack any o f the features o f skeletal dw arfism or

craniosynostosis. T h e FG FR 3 — P250R m utation defines a genetic syndrom e in w hich the phenotype is so variable as to m ake clinical diagnosis and genetic counselling difficult. T he phenotypic crossover betw een similar phenotypes w ithout the m utation (in the m ajority o f cases) is very great. F urtherm ore, phenotypes previously described as Saethre — C hotzen have linked to chrom osom e 4p and subsequently dem onstrated the Pro250A rg m u tatio n (M uenke et al, 1997;Golla et al, 1997). T he potential m echanism s th at may define the various FG F R 3 phenotypes are considered in Sections 3 and 4 o f this thesis.

Saethre — C hot^n Syndrome: a variable phenotype o f F G F R and T W IS T mutations

T h e Saethre C hotzen syndrom e (SCS) is eponym ously nam ed following initial rep o rts in the early 1930s. In the family described by Saethre (1931), a m o th e r and tw o daughters show ed m ild craniosynsotosis, facial asymmetry, low frontal hairline, brachydactyly and partial soft tissue syndactyly o f index and m iddle fingers and the third and fourth toes (Saethre, 1931). C hotzen (1932) described similar m alform ations in a father and tw o sons w h o also had hypertelorism , sh o rt stature, deafness and intellectual com prom ise (C hotzen, 1932). T he variability in the phenotype has been em phasised in m any reports (Pantke et al, 1975;Marini et al, 1991;Niem ann-Seyde et al, 1991;Reardon and W inter, 1994).

Characterizing features include; rmld and asymmetrical craniosynostosis, low -set frontal hairHne, parrot-beaked nose w ith deviated septum , ptosis o f the eyelids, strabism us, refractive error, tear duct stenosis, dystopia canthorum , brachydactyly and abnorm al derm atoglyphic patterns. M uenke has suggested that m inim al diagnostic criteria should include tear duct abnorm alities, palatal anomalies including cleft palate, parietal foram ina, brachydactyly, 2 /3 sim ple syndactyly o f hands, and bifid hallux w ith a lateral deviation (M uenke et al, 1997). C om plete haUucal reduplication has been noted, and probably brings the syndrom e o f R obinow and Sorauf (Robinow and Sorauf, 1975) in to the Saethre C hotzen group (Reardon and W inter, 1994), an observation w hich has latterly gained m olecular credibility (ICunz et al, 1999).

T h e craniosynostosis phenotype is variable, and may include coronal, m etopic, lam bdoid and sagittal sutures in variable patterns (H unter et al, 1976;Reardon and W inter, 1994). Cranial defects include parietal foram ina and late - closing fontanelles. V ertebral fusion is

u n com m on. Auricular m alform ations are com m on including a long and p ro m in e n t ear crus, rep o rted as a consistent sign (Carter et al, 1982). A small pinna, w ith sh o rt columella, m ild sim ple syndactyly and craniosynostosis has been described (Aase and Sm ith, 1970;Kopysc et al, 1980;Kurczynski and C asperson, 1988d-egius et al, 1989), and o th er nasal anom alies, such

as long thin nose also occur (Aase and Smith, 1970). E xtracranial skeletal m anifestations have been further catalogued by A nderson (A nderson et al, 1996c;A nderson et al, 1998b). E lbow synostosis, n o ted rarely by R eardon and W inter (1994), was n o t seen in this series, and only 3 /1 5 cases had m ild 2 /3 syndactyly. T h e hand phenotype included radiographic evidence o f anom alies o f the thum bs, fingers, m etacarpals, and the radius. CHnodactyly was a com m on finding. Epiphyseal anomahes o f the distal phalanx o f the th u m b w ere n o ted in 7 o f 15 patients. T here was a variable delay o f the bone age.

Saethre C hotzen syndrom e is predom inantly famihal, and o f such great variabihty and subtlety o f phenotype that figures o f incidence are probably unrehable (R eardon and W inter, 1994), although 1 in 25,000 to 1 in 50,000 has been suggested (H ow ard et al, 1997). T h e

heterogeneity o f the phenotype, with variable anom ahes o f b o th osseous and epithehal developm ent, suggests that the causative gene is m ultifunctional in the control o f the developm ent o f head m esenchym e, and that its effect is dose dep en d en t o r m odified by variable control m echanism s.

Linkage studies in famihal SCS (B rueton et al, 1992;van H erw erden et al, 1994dLewanda et al, 1994b) lead to association with m arkers in chrom osom e 7p21. T h e T W IS T gene, broadly expressed in m urine head m esenchym e (Stoetzel et al, 1995), m aps to the sam e region and p resented a candidate (Bourgeois et al, 1997). F urtherm ore, w hilst T W IS T knock o u t mice are em bryonic lethal and faü with cranial neural tube closure, the heterozygotes survive and presen t w ith subtle craniofacial and hm b anomahes (El G h ouzzi et al, 1997;Bourgeois et al, 1998).

A range o f m is-sense, non-sense, deletion, and 21 bp duphcation m utations in the T W IS T gene has been dem onstrated in famihal and sporadic SCS (El G h ouzzi et al, 1997;H ow ard et al, 1997;Rose et al, 1997) and is reviewed by R ose and M alcom (1997). C om plete T W IST deletions are also associated w ith SCS, and correlate w ith the presence o f learning difficulties (Johnson et al, 1998), suggesting the involvem ent o f neighbouring genes in inteUectual developm ent. Balanced translocations in 7p21 in SCS patients th at do n o t in terru p t the coding sequence o f T W IST have been interpreted to exert deleterious effects u p o n T W IST expression by positional effect (Rose et al, 1997).

D escrip tio n o f the SCS phenotype in patients with 7p deletions (Chotai et al, 1994) suggested that the T W IS T hum an phenotypes w ould result from haplo — insufficiency, and this is c o rro b o rated by the wide range and type (deletion, non-sense, insertion) o f causative m utation, affecting various functional dom ains o f the protein. T h e similarity o f the m ouse

T W IS T null heterozygote phenotype to hum an SCS also su p ports this contention. T W IS T m utations affecting the alpha — helix are predicted to disrupt dim érisation, w hich by analogy w ith experim ental evidence in the related E47 b H L H , leads to loss o f function (Rose et al, 1997). T W IST m utations affecting the loop — sequence are predicted to have the same effect, and the term ination m utations, in affecting a severely truncated protein, w ould also com ply w ith this m olecular m odel o f T W IST haploinsufficiency in SCS.

T h e variability in the SCS phenotype suggests that the developm ental pathogenesis o f the T W IS T m u tan t is dose - dependent. T here is som e evidence that the nonsense m utations resulting in a severely truncated transcription factor correlate w ith severity o f phenotype (H ow ard et al, 1997), and that m icrodeletions o f the T W IST gene also result in severe intellectual com prom ise (Johnson et al, 1998). T he range o f m utations is wide, how ever, and no firm genotype — phenotype correlations have been draw n fro m a large n u m b er o f

patients.T he T W IST + / - m ouse displays similar heterogeneity o f phenotype, and was imtially considered phenotypicaUy norm al. T here is hindleg reduplication o f digits in 50% , w hich may involve variable extents o f the digit ray, and is seen in the hallux o f SCS patients. Squam osal and interparietal bo n e dysplasias are noted, together w ith accelerated calvarial ossification (El G h o u zzi et al, 1997;Bourgeois et al, 1998).

T h ere is considerable overlap o f the phenotypes o f SCS and phenotypes o f F G F R 3 — P250R (M uenke et al, 1997), and the Pro250A rg m utation in F G F R 3 has been identified in SCS phenotypes w ithout T W IST m utations (Rose et al, 1997;Paznekas et al, 1998). F urtherm ore, the W 2 6 9 — 270 deletion in F G F R 2 was also n o ted in a single sporadic SCS case, identified by the sam e phenotypic criteria (Paznekas et al, 1998). This suggests th at F G F R and T W IST m utations occupy the sam e signalling pathway. Evidence for this com es fro m Drosophila,

w hich shares 68% twist sequence conservation w ith IKenopus, m ouse and m an, rising to com plete conservation in the loop — region (H ow ard et al, 1997). T h e D F R1 and D F R 2 genes in Drosophila, which are related to the vertebrate F G F R family are d ependent u p o n the twist pro tein to function in the induction o f Drosophila m esoderm and endoderm layers (Shishido et al, 1993). This suggests th at the T W IS T gene is an upstream regulator o f F G F R expression. It rem ains to be seen, how ever, w h ether vertebrate T W IS T differentially regulates F G F R

expression in m em branous and endochondral ossification, as the h um an T W IS T phenotypes do n o t display a dwarfing dysplasia.

Crouî^n syndrome y^canthosis nigricans

A canthosis nigrican is a velvety hyperplasia o f the skin, w hich com m only affects flexure creases. T here is hyperpigm antation and accentuation o f the skin m arkings, distributed in the neck axiUae and o ther flexural areas. A n hereditary autosom al do m in an t form exists, and the condition may also be associated with endocrine problem s, drug com plications, or occult m alignancy (Koizum i et al, 1992).

A canthosis nigricans in association with C rouzon syndrom e m ay affect unusual distributions, such as eyelids and face, as well as chest abdom en and breasts. It takes o nset in childhood and always by puberty. Histologically, it is similar to o ther form s o f the disease, consisting o f m arked papillom atosis and a thin hyperpigm ented epiderm is (Meyers et al, 1995). It m ay be associated w ith m ultiple m elanocytic naevi o f the face and neck (Meyers et al, 1995;Wilkes et al, 1996;Gines et al, 1996). A female preponderance is n o ted (Breitbart et al, 1989;K oizum i et al, 1992;Meyers et al, 1995), w hich may reflect co - genetic o r horm o n al influence. T h ere is a high association w ith choanal atresia, w hich is sporadic in the F G F R 2 — related C rouzon ph en o ty p e (Breitbart et al, 1989;Meyers et al, 1995).

T h e C ro u zo n — A canthosis nigricans (Cr — A N ) variant is associated w ith the A la391G lu m utation in the transm em brane region o f FG F R 3 (Meyers et al, 1995). A n acidic, bulky, glutam ic acid residue is substituted for a neutral hydrophobic alanine, and the m utation is specific to the C r-A N phenotype, b u t lies w ithin a few residues from the tw o m utations encoding achondroplasia (ACH). T hese m utations, also specific to the phenotype, substitute neutral fo r neutral residues (Gly375Cys) or neutral glycine for basic, bulky, arginine

(Gly380Arg) in the m o st com m on case. T he specific m echanism o f functional activation o f these m u ta n t receptors may generate the great differences in their phenotypes. I t is likely that