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A Mutational Hotspot in the 2B Domain of Human Hair Basic Keratin 6 (hHb6) in Monilethrix Patients

1998; Elsevier BV; Volume: 111; Issue: 5 Linguagem: Inglês

10.1046/j.1523-1747.1998.00362.x

1523-1747

Bernhard Korge, Eugene Healy, Colin S. Munro, Mark Birch-Machin, Susan C. Holmes, S. DARLINGTON, Henning Hamm, Andrew G. Messenger, Jonathan Rees, Heiko Traupe, Claudia Pünter,

Wnt/β-catenin signaling in development and cancer

Monilethrix is an inherited hair dystrophy in which affected, fragile, hairs have an unique beaded morphology. Ultrastructural studies suggest a defect in filament structure in the cortex of the hair, and the hard keratins of hair and nail are thus candidate genes. In several families with autosomal dominant monilethrix, the disorder has been linked to the type II keratin gene cluster at chromosome 12q13. Recently, causative mutations in the critical helix termination motif in the 2B domain of the human hair basic keratin 6 (hHb6) have been identified. We now report the results of sequencing this domain in 13 unrelated families or cases with monilethrix. Five of the 13 had the same mutation as previously found, a G to A transversion leading to a lysine for glutamic acid substitution (E413K) in the 2B domain (residue 117 of the 2B helix) of hHb6. The mutation was confirmed by a restriction fragment length polymorphism assay developed for this purpose, and, as this mutation is evidently a common cause of the syndrome, for use in screening other cases. In eight families or cases, however, including three in whom linkage data are consistent with a defect at the type II keratin locus, no mutation was found in this domain of hHb6. Monilethrix is an inherited hair dystrophy in which affected, fragile, hairs have an unique beaded morphology. Ultrastructural studies suggest a defect in filament structure in the cortex of the hair, and the hard keratins of hair and nail are thus candidate genes. In several families with autosomal dominant monilethrix, the disorder has been linked to the type II keratin gene cluster at chromosome 12q13. Recently, causative mutations in the critical helix termination motif in the 2B domain of the human hair basic keratin 6 (hHb6) have been identified. We now report the results of sequencing this domain in 13 unrelated families or cases with monilethrix. Five of the 13 had the same mutation as previously found, a G to A transversion leading to a lysine for glutamic acid substitution (E413K) in the 2B domain (residue 117 of the 2B helix) of hHb6. The mutation was confirmed by a restriction fragment length polymorphism assay developed for this purpose, and, as this mutation is evidently a common cause of the syndrome, for use in screening other cases. In eight families or cases, however, including three in whom linkage data are consistent with a defect at the type II keratin locus, no mutation was found in this domain of hHb6. human hair basic keratin restriction fragment length polymorphism Monilethrix (“necklace hair”) is a hereditary disorder, characterized by a dystrophic alopecia. It is so named because of typical beaded hair shafts showing a regular periodicity of nodes, of normal diameter, and abnormally narrow internodal segments. Non-beaded hairs are also fragile. The effect on scalp hair is variable and ranges from normality or mild occipital hair loss to near total alopecia. In some cases alopecia persists throughout life; in others regrowth of apparently normal hair may occur around puberty or in pregnancy (Alexander and Grant, 1958Alexander Jod Grant P. Monilethrix: report of 3 cases with extensive family history.Scot Med J. 1958; 3: 356-360PubMed Google Scholar). Prominent follicular keratoses are present in up to 90% of patients (Tietze, 1995Tietze K. Monilethrix. Genetik, Klinik, Begleitsymptomatik und Krankheitsverlauf. Inaugural Thesis, Medical Faculty of the University of Münster, Germany1995Google Scholar) and nail defects may be manifest, for example, as a “spoon nail” (koilonychia) (Heydt and Zur Kenntnis des Monilethrix-Syndroms, 1963Heydt G.E. Zur Kenntnis des Monilethrix-Syndroms Arch Klin Exp Dermatol. 1963; 217: 15-29Crossref PubMed Scopus (16) Google Scholar;Tietze, 1995Tietze K. Monilethrix. Genetik, Klinik, Begleitsymptomatik und Krankheitsverlauf. Inaugural Thesis, Medical Faculty of the University of Münster, Germany1995Google Scholar). Monilethrix-like hair dystrophies have been reported in association with mental retardation and other ectodermal dysplasias (Sfaello and Hariga, 1967Sfaello Z. Hariga J. Monilethrix associéà la debilité mentale: étude d’une famille.Arch Belg Dermatol Syphil. 1967; 23: 363-371PubMed Google Scholar;Geormaneanu et al., 1976Geormaneanu M. Walter A. Lazarescu P. Spinocerebellare Ataxie assoziiert mit Monilethrix, eigenartige Gesichtszüge und Zahnanomalien.Monat Kinderheil. 1976; 129: 647-649PubMed Google Scholar), but most families show no extracutaneous features. Most pedigrees show autosomal dominant inheritance with high, but not complete, penetrance (Deraemaeker, 1957Deraemaeker R. Monilethrix: report of a family with special reference to some problems concerning inheritance.Am J Hum Genet. 1957; 9: 195-201PubMed Google Scholar;Birch-Machin et al., 1997Birch-Machin M.A. Healy E. Turner R. et al.Mapping of monilethrix to the type II keratin gene cluster at chromosome 12q13 in three new families, including one with variable expressivity.Br J Dermatol. 1997; 137: 339-343Crossref PubMed Scopus (25) Google Scholar). Electron microscopic studies in monilethrix identify defects in the microfibrillar structure of the cortex of hair shafts, affecting both nodal and internodal regions (De Berker et al., 1993De Berker D.A. Ferguson D.J. Dawber RpR Monilethrix, a clinicopathological illustration of a cortical defect.Br J Dermatol. 1993; 128: 327-331Crossref PubMed Scopus (35) Google Scholar), and vacuolation of cortical trichocytes (Ito et al., 1990Ito M. Hasimoto K. Katsumi K. Sato Y. Pathogenesis of monilethrix: computer stereography and electron microscopy.J Invest Dermatol. 1990; 95: 186-194Abstract Full Text PDF PubMed Google Scholar). Hence genes encoding structural proteins of hair are candidates for causative defects. The major structural proteins of hair are the relatively cysteine-rich “hard” keratins, also found in nail, whose genes are located in the same clusters as the “soft” keratins at 17q12-q21 (type I, acidic keratins) and 12q13 (type II, basic keratins) (Rogers et al., 1995Rogers M.A. Nischt R. Korge B. et al.Sequence data and chromosomal localisation of human type I and type II hair keratin genes.Exp Cell Res. 1995; 220: 357-362Crossref PubMed Scopus (51) Google Scholar). Several groups have reported that the defect in monilethrix maps to the type II keratin gene cluster at 12q13 (Healy et al., 1995Healy E. Holmes S.C. Belgaid C.E. Stephenson A.M. McLean Whi Rees J.L. Munro C. A gene for monilethrix is closely linked to the type II keratin gene cluster at 12q13.Hum Mol Genet. 1995; 4: 2399-2402Crossref PubMed Scopus (51) Google Scholar;Stevens et al., 1996Stevens H.P. Kelsell D.P. Bryant S.P. Bishop D.T. Dawber Rpr Spurr N.K. Leigh I.M. Linkage of monilethrix to the trichocyte and epithelial keratin gene cluster on 12q11–13.J Invest Dermatol. 1996; 106: 795-797Crossref PubMed Scopus (38) Google Scholar). 1Korge BP, Richard G, Pünter C et al. Monilethrix links to the keratin type II cluster at 12q13 and cloning of a possible candidate gene. J Invest Dermatol 106:843 1996 (abstr.)1Korge BP, Richard G, Pünter C et al. Monilethrix links to the keratin type II cluster at 12q13 and cloning of a possible candidate gene. J Invest Dermatol 106:843 1996 (abstr.) As yet, no linkage to the type I cluster has been shown, although in at least one family the defect does not appear to map to either major keratin gene cluster (Richard et al., 1996Richard G. Itin P. Lin S.P. Bon A. Bale S.J. Evidence for genetic heterogeneity in monilethrix.J Invest Dermatol. 1996; 107: 812-814Crossref PubMed Scopus (23) Google Scholar). Knowledge of hair keratins and their patterns of expression in man is less well developed than for “soft” keratins, but the same principles of keratin biology appear to apply. Keratin intermediate filaments (IF) are composed of heterodimers containing paired acidic and basic keratins, with a highly homologous central rod domain, and functional specificity conferred by the variable N- and C-terminal domains. In man, evidence to date suggests at least seven acidic and at least four basic hair keratins (Rogers et al., 1997Rogers M.A. Langbein L. Praetzel S. Moll I. Krieg T. Winter H. Schweizer J. Sequences and differential expression of three novel human type II hair keratins.Differentiation. 1997; 61: 187-194Crossref PubMed Scopus (50) Google Scholar), but more may exist. Four basic keratins have been cloned (Rogers et al., 1995Rogers M.A. Nischt R. Korge B. et al.Sequence data and chromosomal localisation of human type I and type II hair keratin genes.Exp Cell Res. 1995; 220: 357-362Crossref PubMed Scopus (51) Google Scholar,Rogers et al., 1997Rogers M.A. Langbein L. Praetzel S. Moll I. Krieg T. Winter H. Schweizer J. Sequences and differential expression of three novel human type II hair keratins.Differentiation. 1997; 61: 187-194Crossref PubMed Scopus (50) Google Scholar;Bowden et al., 1998Bowden P.E. Hainey S.D. Parker G. Jones D.O. Zimonjic D. Popescu N. Hodgins M.B. Characterization and chromosomal localization of human hair-specific keratin genes and comparative expression during the hair growth cycle.J Invest Dermatol. 1998; 110: 158-164Abstract Full Text Full Text PDF PubMed Scopus (33) Google Scholar), 1Korge BP, Richard G, Pünter C et al. Monilethrix links to the keratin type II cluster at 12q13 and cloning of a possible candidate gene. J Invest Dermatol 106:843 1996 (abstr.) of which human basic hair keratin 5 (hHb5) is expressed in the proliferative pool of the hair matrix, and hHb1, hHb3, and hHb6 in the cortex. A basic partner for hHa2 in the cuticle is also postulated (Rogers et al., 1997Rogers M.A. Langbein L. Praetzel S. Moll I. Krieg T. Winter H. Schweizer J. Sequences and differential expression of three novel human type II hair keratins.Differentiation. 1997; 61: 187-194Crossref PubMed Scopus (50) Google Scholar). In eight families or isolated cases with monilethrix,Winter et al., 1997aWinter H. Rogers M.A. Gebhardt M. et al.A new mutation in the type II hair cortex keratin hHb1 involved in the inherited hair disorder monilethrix.Hum Genet. 1997 a; 101: 165-169Crossref PubMed Scopus (89) Google Scholar,Winter et al., 1997bWinter H. Rogers M.A. Langbein L. et al.Mutations in the hair cortex keratin hHb6 cause the inherited hair disease monilethrix.Nature Genet. 1997 b; 16: 372-374Crossref PubMed Scopus (156) Google Scholar) have now identified mutations in hHb6, and most recently, in hHb1. The hHb6 defect in six pedigrees or cases was a G to A transversion, encoding a lysine residue in place of a glutamic acid at position 413 (E413K) in the helix termination peptide (numbering of amino acid residues relates to the sequence published byBowden et al., 1998Bowden P.E. Hainey S.D. Parker G. Jones D.O. Zimonjic D. Popescu N. Hodgins M.B. Characterization and chromosomal localization of human hair-specific keratin genes and comparative expression during the hair growth cycle.J Invest Dermatol. 1998; 110: 158-164Abstract Full Text Full Text PDF PubMed Scopus (33) Google Scholar; EMBL/Genbank AJ000263) of the keratin 2B domain (residue 117 of the 2B helix). In the seventh family, a point mutation in hHb6 at the third base of the same codon was found, causing an aspartate substitution. In this paper, we report the results of sequencing the 2B domain of hHb6 in 13 more unrelated cases or families with monilethrix. In five of these we have again identified the E413K mutation, for which we have also developed an restriction fragment length polymorphism (RFLP) assay, but in the remainder we have found no mutations in this domain. Most of the 13 pedigrees have previously been studied Table 1, and in five that have been subject to linkage analysis, this confirms or is consistent with linkage to 12q13. Affected members of all families displayed the typical phenotype of beaded and broken hairs Figure 1. Linkage and DNA studies were approved by the local research ethics review committee.Table 1Cases studied and linkage dataIdentifierOriginCasesZmax aMaximum two point lod score in linkage analysis to one or more markers at or near type II keratin gene cluster; nd, not done.Linkage referenceCommentS bPedigrees in which E413K mutation is identified.Scotland179.63 (D12S96)Healy et al., 1995Healy E. Holmes S.C. Belgaid C.E. Stephenson A.M. McLean Whi Rees J.L. Munro C. A gene for monilethrix is closely linked to the type II keratin gene cluster at 12q13.Hum Mol Genet. 1995; 4: 2399-2402Crossref PubMed Scopus (51) Google ScholarFamily 1 inAlexander and Grant, 1958Alexander Jod Grant P. Monilethrix: report of 3 cases with extensive family history.Scot Med J. 1958; 3: 356-360PubMed Google ScholarO bPedigrees in which E413K mutation is identified.Ireland92.65 (D12S96)Healy et al., 1995Healy E. Holmes S.C. Belgaid C.E. Stephenson A.M. McLean Whi Rees J.L. Munro C. A gene for monilethrix is closely linked to the type II keratin gene cluster at 12q13.Hum Mol Genet. 1995; 4: 2399-2402Crossref PubMed Scopus (51) Google ScholarK bPedigrees in which E413K mutation is identified.England113.39 (D12S398)Birch-Machin et al. 1997Follicular keratoses only in some membersISpain72.69 (D12S96)Birch-Machin et al. 1997DScotland51.06 (D12S339)Birch-Machin et al. 1997Linkage to type I keratin gene cluster excludedLScotland2ndBirch-Machin et al. 1997Family 3 inAlexander and Grant, 1958Alexander Jod Grant P. Monilethrix: report of 3 cases with extensive family history.Scot Med J. 1958; 3: 356-360PubMed Google ScholarM1 bPedigrees in which E413K mutation is identified.Germany1ndKorge, unreported: neither parent affectedM2Germany72.43 (D12S96)Footnote 1M3Germany3ndTietze, 1995Tietze K. Monilethrix. Genetik, Klinik, Begleitsymptomatik und Krankheitsverlauf. Inaugural Thesis, Medical Faculty of the University of Münster, Germany1995Google ScholarM4 bPedigrees in which E413K mutation is identified.Germany5ndTietze, 1995Tietze K. Monilethrix. Genetik, Klinik, Begleitsymptomatik und Krankheitsverlauf. Inaugural Thesis, Medical Faculty of the University of Münster, Germany1995Google ScholarM5Germany3ndTietze, 1995Tietze K. Monilethrix. Genetik, Klinik, Begleitsymptomatik und Krankheitsverlauf. Inaugural Thesis, Medical Faculty of the University of Münster, Germany1995Google ScholarM6Germany4ndTietze, 1995Tietze K. Monilethrix. Genetik, Klinik, Begleitsymptomatik und Krankheitsverlauf. Inaugural Thesis, Medical Faculty of the University of Münster, Germany1995Google ScholarM7Germany4ndTietze, 1995Tietze K. Monilethrix. Genetik, Klinik, Begleitsymptomatik und Krankheitsverlauf. Inaugural Thesis, Medical Faculty of the University of Münster, Germany1995Google Scholara Maximum two point lod score in linkage analysis to one or more markers at or near type II keratin gene cluster; nd, not done.b Pedigrees in which E413K mutation is identified. Open table in a new tab DNA was extracted from blood or salivary samples obtained from family members, and polymerase chain reaction (PCR) amplification performed by standard methods, using the Expand Long template PCR system (Boehringer, Mannheim, Germany). The forward primer 5′-CCCTCAGCGATGCCCGCTGCAAG-3′ was picked from the published sequence (GenBank No X99142 and EMBL/Genbank AJ000263) and the reverse from the sequence of intron 7 of hHb6: 5′-CTGGTTGCAGGGTGGGGAGGTTA-3′. PCR was performed at 94°C, then 35 cycles (94°C, 1 min; 65°C, 1 min; 68°C, 2 min) followed by a 7 min extension at 68°C. After shrimp alkaline phosphatase/exonuclease 1 treatment, PCR products were directly sequenced using the PCR Product and Sequencing Kit (USB/Amersham Life Science, Cleveland, OH). In each of four families (S, O, K, and M4) and in one sporadic case with clinically normal parents (M1) we identified an identical mutation in the helix termination motif of the 2B domain of hHb6 Figure 2. The G to A transversion, encoding a lysine residue in place of glutamic acid at position 413 (E413K), was the same as that identified in six families or cases byWinter et al., 1997aWinter H. Rogers M.A. Gebhardt M. et al.A new mutation in the type II hair cortex keratin hHb1 involved in the inherited hair disorder monilethrix.Hum Genet. 1997 a; 101: 165-169Crossref PubMed Scopus (89) Google Scholar,Winter et al., 1997bWinter H. Rogers M.A. Langbein L. et al.Mutations in the hair cortex keratin hHb6 cause the inherited hair disease monilethrix.Nature Genet. 1997 b; 16: 372-374Crossref PubMed Scopus (156) Google Scholar). The presence of the E413K (numbering of amino acid residues relates to the sequence published byBowden et al., 1998Bowden P.E. Hainey S.D. Parker G. Jones D.O. Zimonjic D. Popescu N. Hodgins M.B. Characterization and chromosomal localization of human hair-specific keratin genes and comparative expression during the hair growth cycle.J Invest Dermatol. 1998; 110: 158-164Abstract Full Text Full Text PDF PubMed Scopus (33) Google Scholar; EMBL/Genbank AJ000263) mutation in family K was of particular interest because in this family, follicular keratoses were a prominent or sole feature in several cases (Birch-Machin et al., 1997Birch-Machin M.A. Healy E. Turner R. et al.Mapping of monilethrix to the type II keratin gene cluster at chromosome 12q13 in three new families, including one with variable expressivity.Br J Dermatol. 1997; 137: 339-343Crossref PubMed Scopus (25) Google Scholar). The recurrent mutation is likely to have arisen independently, because standardization of markers for haplotyping the region of the type II keratin cluster in the British families (S, O, and K) did not suggest common ancestry (Healy et al., 1995Healy E. Holmes S.C. Belgaid C.E. Stephenson A.M. McLean Whi Rees J.L. Munro C. A gene for monilethrix is closely linked to the type II keratin gene cluster at 12q13.Hum Mol Genet. 1995; 4: 2399-2402Crossref PubMed Scopus (51) Google Scholar;Birch-Machin et al., 1997Birch-Machin M.A. Healy E. Turner R. et al.Mapping of monilethrix to the type II keratin gene cluster at chromosome 12q13 in three new families, including one with variable expressivity.Br J Dermatol. 1997; 137: 339-343Crossref PubMed Scopus (25) Google Scholar), and in case M1, the mutation appears to have arisen de novo, being excluded in both parents. In our remaining eight pedigrees, sequencing of the whole of the 2B domain of hHb6 identified no mutations, despite the fact that in all three families in whom linkage analysis has been performed, data are suggestive of or consistent with linkage to 12q13 Table 1. The domain, encoding the helix termination peptide, is thought to be critical for higher order assembly of keratin intermediate filaments (Steinert et al., 1995Steinert P.M. IF Pathology: Molecular consequences of rod and end domain mutations.in: Parry Dad Steinert P.M. Intermediate Filament Structure Med Intelligence Unit. Austin, RG Landes1995: 145-175Google Scholar), and the vast majority of pathogenic mutations in diseases due to keratin gene defects lie in either this or the corresponding helix initiation peptide in the 1A domain (McLean and Lane, 1995McLean Whi Lane E.B. Intermediate filaments in disease.Curr Opin Cell Biol. 1995; 7: 118-125Crossref PubMed Scopus (209) Google Scholar;Korge and Krieg, 1996Korge B.P. Krieg T. The molecular basis for inherited bullous diseases.J Mol Med. 1996; 74: 59-70Crossref PubMed Scopus (45) Google Scholar). The helix termination sequence TYR(R/K)LLEGEE is highly conserved throughout both epidermal keratins and hair keratins. The recurrent G to A transversion at position 413 in five of the 13 pedigrees of monilethrix we have studied, and six of eight other families or cases (Winter et al., 1997aWinter H. Rogers M.A. Gebhardt M. et al.A new mutation in the type II hair cortex keratin hHb1 involved in the inherited hair disorder monilethrix.Hum Genet. 1997 a; 101: 165-169Crossref PubMed Scopus (89) Google Scholar,Winter et al., 1997bWinter H. Rogers M.A. Langbein L. et al.Mutations in the hair cortex keratin hHb6 cause the inherited hair disease monilethrix.Nature Genet. 1997 b; 16: 372-374Crossref PubMed Scopus (156) Google Scholar), is likely to have arisen independently. Identical mutations in the corresponding nucleotide represent an even higher proportion – 10 of 14 – of reported mutations in keratin 2e in ichthyosis bullosa of Siemens (Kremer et al., 1994Kremer H. Zeeuwen P. McLean Whi et al.Ichthyosis bullosa of Siemens is caused by mutations in the keratin 2e gene.J Invest Dermatol. 1994; 103: 286-289Crossref PubMed Scopus (86) Google Scholar;McLean et al., 1994McLean Whi Morley S.M. Lane E.B. et al.Ichthyosis bullosa of Siemens – a disease involving keratin 2e.J Invest Dermatol. 1994; 103: 277-281Crossref PubMed Scopus (92) Google Scholar;Rothnagel et al., 1994Rothnagel J.A. Traupe H. Wojcik S. et al.Mutations in the rod domain of keratin 2e in patients with ichthyosis bullosa of Siemens.Nature Genet. 1994; 7: 485-490Crossref PubMed Scopus (124) Google Scholar;Jones et al., 1997Jones D.O. Watts C. Mills C. Sharpe G.R. Marks R. Bowden P.E. A new keratin mutation in ichthyosis bullosa of Siemens.J Invest Dermatol. 1997; 108: 354-356Abstract Full Text PDF PubMed Scopus (26) Google Scholar), and recently, the equivalent mutation in the keratin 3 gene has been found in Meesmann’s corneal dystrophy (Irvine et al., 1997Irvine A.D. Corden L.D. Swensson O. et al.Mutations in cornea-specific keratin K3 or K12 genes cause Meesmann’s corneal dystrophy.Nature Genet. 1997; 16: 184-192Crossref PubMed Scopus (183) Google Scholar). The common mechanism is likely to be spontaneous deamination of a methylated cytosine in a CpG dinucleotide giving rise to thymine (Holliday and Grigg, 1993Holliday R. Grigg G.W. DNA methylation and mutation.Mutation Research–Fundamental and Molecular Mechanisms of Mutagenesis. 1993; 285: 61-67Crossref PubMed Scopus (247) Google Scholar). As a result of ineffective mismatch repair the opposite strand is propagated as adenine instead of guanine. In our remaining eight pedigrees, sequencing of the whole of the 2B domain of hHb6 identified no mutations, despite the fact that in all three families in whom linkage analysis has been performed, data are suggestive of or consistent with linkage to 12q13 Table 1. The 298 bp PCR fragment contains a cutting site for the restriction enzyme BseR1 that is lost following the G to A transversion. The resultant RFLP was used to confirm segregation of the mutation with the phenotype in part of one pedigree Figure 3, as well as to confirm its absence in 36 normal controls (data not shown). The RFLP method we have developed will be of value in screening isolated cases or pedigrees for this common pathogenic mutation. Both the common mutant lysine residue, and the charge-conservative aspartic acid substitution found at the same codon in one other family (Winter et al., 1997bWinter H. Rogers M.A. Langbein L. et al.Mutations in the hair cortex keratin hHb6 cause the inherited hair disease monilethrix.Nature Genet. 1997 b; 16: 372-374Crossref PubMed Scopus (156) Google Scholar), replace a glutamic acid residue that occupies an internal d position in the heptad repeats of the α-helix. The d position is often occupied by glutamic acid, but relatively rarely by lysine residues, whereas the a position is often taken by lysine, but rarely by glutamic acid residues. This suggests that lysine in the d position interferes with coiled coil formation, and hence possibly with intermediate filament assembly. Whereas pathogenic mutations at the corresponding residue occur in epidermal keratin genes (see above), the precise mechanism of intermediate filament disruption may differ in hair keratins. It has been suggested (Parry and Hard, α-keratin, 1995Parry DaD Hard, α-keratin I.F. A structural model lacking a head-to-tail molecular overlap but having hybrid features of both epidermal and vimentin IF.Proteins: Structure, Function Genet. 1995; 22: 267-272Crossref PubMed Scopus (24) Google Scholar,Parry, 1996Parry DaD Hard α-keratin intermediate filaments: an alternative interpretation of the low-angle equatorial X-ray diffraction pattern, and the axial disposition of putative disulphide bonds in the intra- and inter-protofilamentous networks.Int J Biol Macromol. 1996; 19: 45-50Crossref PubMed Scopus (39) Google Scholar) that unlike epidermal keratins, hair keratin molecules in the IF, although maintaining the major overlaps characteristic of all IF, lack the head-to-tail overlap between similarly directed molecules. The hard α-keratins are believed instead to be stabilized by numerous intermolecular disulfide bonds. Because of these differences in filament assembly, the mutant lysine residue in the d position of the type II chain may actually increase stability in monilethrix hair by interacting with an aspartate residue in the e position of the type I keratin (Parry D.A.D., Massey University, Palmerston North, New Zealand, personal communication). Many keratin disorders arise from defects in either partner of a keratin pair (McLean and Lane, 1995McLean Whi Lane E.B. Intermediate filaments in disease.Curr Opin Cell Biol. 1995; 7: 118-125Crossref PubMed Scopus (209) Google Scholar), but no families of monilethrix have as yet been mapped to the type I keratin locus, suggesting that type II keratin defects may have a particular relevance to the disease. Certain epidermal keratins seem similarly to be uniquely associated with particular phenotypes, e.g., keratin 9 with epidermolytic palmoplantar keratoderma (Bonifas et al., 1994Bonifas J.M. Matsuruma K. Chen M.A. et al.Mutations of keratin 9 in 2 families with palmoplantar epidermolytic hyperkeratosis.J Invest Dermatol. 1994; 103: 474-477Crossref PubMed Scopus (68) Google Scholar;Reis et al., 1994Reis A. Hennies H-C Langbein L. et al.Keratin 9 gene mutations in epidermolytic palmoplantar keratoderma (EPPK).Nature Genet. 1994; 6: 174-179Crossref PubMed Scopus (223) Google Scholar), keratin 2e with ichthyosis bullosa of Siemens (Kremer et al., 1994Kremer H. Zeeuwen P. McLean Whi et al.Ichthyosis bullosa of Siemens is caused by mutations in the keratin 2e gene.J Invest Dermatol. 1994; 103: 286-289Crossref PubMed Scopus (86) Google Scholar;McLean et al., 1994McLean Whi Morley S.M. Lane E.B. et al.Ichthyosis bullosa of Siemens – a disease involving keratin 2e.J Invest Dermatol. 1994; 103: 277-281Crossref PubMed Scopus (92) Google Scholar;Rothnagel et al., 1994Rothnagel J.A. Traupe H. Wojcik S. et al.Mutations in the rod domain of keratin 2e in patients with ichthyosis bullosa of Siemens.Nature Genet. 1994; 7: 485-490Crossref PubMed Scopus (124) Google Scholar;Jones et al., 1997Jones D.O. Watts C. Mills C. Sharpe G.R. Marks R. Bowden P.E. A new keratin mutation in ichthyosis bullosa of Siemens.J Invest Dermatol. 1997; 108: 354-356Abstract Full Text PDF PubMed Scopus (26) Google Scholar), and keratin 17 with pachyonychia congenita type II (McLean et al., 1995McLean Whi Rugg E.L. Lunny D.P. et al.Keratin 16 and keratin 17 mutations cause pachyonychia congenita.Nature Genet. 1995; 9: 273-278Crossref PubMed Scopus (280) Google Scholar;Smith et al., 1997Smith Fjd Corden L.D. Rugg E.L. et al.Missense mutations in keratin 17 cause either pachyonychia congenita type 2 or a phenotype resembling steatocystoma multiplex.J Invest Dermatol. 1997; 108: 220-223Abstract Full Text PDF PubMed Scopus (122) Google Scholar). In situ studies indicate that hHb1 and hHb3 have similar expression patterns in the emerging cortex, but hHb6 mRNA starts and ends relatively higher in the emerging hair shaft (Rogers et al., 1997Rogers M.A. Langbein L. Praetzel S. Moll I. Krieg T. Winter H. Schweizer J. Sequences and differential expression of three novel human type II hair keratins.Differentiation. 1997; 61: 187-194Crossref PubMed Scopus (50) Google Scholar); however, although mutations in hHb6 appear to be commonest, they are apparently not the only cause of monilethrix. As noted above, since our work was carried out,Winter et al., 1997aWinter H. Rogers M.A. Gebhardt M. et al.A new mutation in the type II hair cortex keratin hHb1 involved in the inherited hair disorder monilethrix.Hum Genet. 1997 a; 101: 165-169Crossref PubMed Scopus (89) Google Scholar have reported a similar mutation, E403K, in the corresponding codon of hHb1 in a family with monilethrix. It thus appears that the monilethrix phenotype is not unique to mutations in hHb6, but is a general phenomenon of defects in cortical keratins. There is a high degree of homology between hHb6 and hHb3 and particularly hHb1, even in the V domains and intron sequences (Rogers et al., 1997Rogers M.A. Langbein L. Praetzel S. Moll I. Krieg T. Winter H. Schweizer J. Sequences and differential expression of three novel human type II hair keratins.Differentiation. 1997; 61: 187-194Crossref PubMed Scopus (50) Google Scholar). The fact that fragility occurs, despite this apparent redundancy of expressed structural genes, suggests that these keratins have distinct roles. The mechanism by which keratin filament defects cause the clinical phenomena of monilethrix remains obscure. The regular periodicity of beading is not diurnal and is asynchronous between follicles (Comaish, 1969Comaish S. Autoradiographic studies of hair growth and rhythm in monilethrix.Br J Dermatol. 1969; 81: 443-447Crossref PubMed Scopus (15) Google Scholar;De Berker and Dawber, 1992De Berker D. Dawber RpR Variations in the beading configuration in monilethrix.Pediatric Dermatol. 1992; 9: 19-21Crossref PubMed Scopus (12) Google Scholar), suggesting an intrinsic follicular clock. It has been suggested that the narrowing could be due to compression by the inner root sheath, but dystrophic change occurs below this level (Ito et al., 1990Ito M. Hasimoto K. Katsumi K. Sato Y. Pathogenesis of monilethrix: computer stereography and electron microscopy.J Invest Dermatol. 1990; 95: 186-194Abstract Full Text PDF PubMed Google Scholar). It seems more likely that narrowing is intrinsic to the hair due to a periodic reduction of effective hair formation. We have previously suggested (Healy et al., 1995Healy E. Holmes S.C. Belgaid C.E. Stephenson A.M. McLean Whi Rees J.L. Munro C. A gene for monilethrix is closely linked to the type II keratin gene cluster at 12q13.Hum Mol Genet. 1995; 4: 2399-2402Crossref PubMed Scopus (51) Google Scholar) that the periodicity results from a feedback loop in which cytolysis of dystrophic cortical keratin IF evokes a cytokine response that directly or indirectly modulates keratin gene expression, in a manner that counteracts the defect. Restoration of IF integrity would result in a reduction of cytolysis and cytokine release and hence complete the cycle. Although the recurrent G to A transversion at position 413 occurs frequently in monilethrix patients, we could not correlate this with a specific clinical phenotype. The same mutation was found in affected patients within families who might have severe dystrophic alopecia, limited involvement of only follicular keratosis, or be obligate carriers with no discernible phenotype (Birch-Machin et al., 1997Birch-Machin M.A. Healy E. Turner R. et al.Mapping of monilethrix to the type II keratin gene cluster at chromosome 12q13 in three new families, including one with variable expressivity.Br J Dermatol. 1997; 137: 339-343Crossref PubMed Scopus (25) Google Scholar). The variable severity of monilethrix within families and with time suggests that other factors also affect gene or disease expression. Amongst these influences are likely to be sex hormones, because both puberty and pregnancy are capable of inducing changes in hair growth (Alexander and Grant, 1958Alexander Jod Grant P. Monilethrix: report of 3 cases with extensive family history.Scot Med J. 1958; 3: 356-360PubMed Google Scholar). Usually patient’s hair growth improves during hormonal active episodes, although a worsening has also been documented (Tietze, 1995Tietze K. Monilethrix. Genetik, Klinik, Begleitsymptomatik und Krankheitsverlauf. Inaugural Thesis, Medical Faculty of the University of Münster, Germany1995Google Scholar). The prospects for improved management of monilethrix may depend on elucidation of these mechanisms, which allow normalization of hair growth despite the continued presence of the keratin gene defect. We are grateful to the families for their willing co-operation, and to referring physicians. We are grateful to Prof. E.B. Lane, Dr. W.H.I. McLean, and Dr. D.A.D. Parry for informative discussion. E.H. is an MRC Training Fellow. Supported in part by the North-Eastern Skin Research Fund (J.L.R.), the “Verein der Freunde und Förderer der Universität zu Köln e.V.” (B.P.K.), and the “Köln Fortune” program of the Medical Center of the University of Cologne (B.P.K.).