Dyskeratosis congenita

Oral Oncology (2006) 42, 331–336

http://intl.elsevierhealth.com/journals/oron/

REVIEW

Dyskeratosis congenita T.P.B. Handley a, J.A. McCaul b, G.R. Ogden a b

a,*

Unit of Oral Surgery and Medicine, University of Dundee, Park Place, Dundee DD1 4HR, UK Department of Oral and Maxillofacial Surgery, Monklands Hospital, Airdre, UK

Received 4 June 2005; accepted 6 June 2005

KEYWORDS

Summary Dyskeratosis congenita is an inherited disorder that usually presents in males, consisting of the triad of leukoplakia of the mucous membranes, nail dystrophy and skin pigmentation. Whilst most cases are X-linked, autosomal dominant and recessive forms have been reported. The significance of the condition lies in premature mortality arising from either bone marrow failure or malignant change within the areas of leukoplakia. Various mucocutaneous and non-mucocutaneous manifestations have been reported. The syndrome arises from an inherited defect within the DKC1 gene that codes for the protein dyskerin in the X-linked recessive form of the disorder, whereas mutations in the RNA component of telomerase (TERC) result in the autosomal dominant form of the condition. The identification of a white patch within the mouth of a child in the absence of any other obvious cause should arouse suspicion of this rare condition. Greater understanding of the molecular biology surrounding this syndrome should lead to improvements in diagnosis, monitoring of disease progression and therapy. c 2005 Elsevier Ltd. All rights reserved.

Oral mucosa; Oral cancer; Dyskeratosis congenita; Dyskerin; DKC1 gene; Review

 Introduction

Dyskeratosis congenita (DC) is a rare, inherited, disorder, with premature ageing, bone marrow failure and malignancy. It consists of the triad of nail dystrophy increased skin pigmentation and mucosal leukoplakia.1–4 The syndrome often proves fatal due to progressive bone marrow failure (or malig-

* Corresponding author. Tel.: +44 01382 635989; fax: +44 01382 425783. E-mail address: [email protected] (G.R. Ogden).



nant change within areas of mucosal leukoplakia). A vast number of associated anomalies have been reported and are reviewed below, together with an update on recent understanding of the molecular basis of the disorder in its various inherited forms.

Clinical presentation Mucocutaneous features The mucocutaneous features are the most consistent feature of DC. Reticulated skin hyperpigmentation affecting the neck, face, chest and arms is

1368-8375/$ - see front matter c 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.oraloncology.2005.06.007

332 the most common finding occurring in approximately 90% of patients.5 This can be in either a localised or florid generalised form and the degree of pigmentation has been observed to increase with age. A variety of other skin changes which have been reported in the literature include cutaneous atrophy, hyperhidrosis of the palms and soles, telangiectasia, cracking, fissuring, bullae formation, loss of dermal ridges,6 hair tufts with keratotic plugs on the limbs and keratinised basal cell papillomas.7 Dystrophic changes in the nails are similarly prevalent affecting 90% of patients.5 This has been observed to be more severe in the fingers than in the toes, and can vary quite considerably in severity between digits.6 Dystrophic changes usually begin with longitudinal ridging and splitting and may progress to complete nail loss.8 Leukoplakia, which is the third feature of the classic clinical triad, has been reported in 80% of affected patients.5 This can occur on any mucosal surface, but has been most frequently reported affecting the oral mucosa. The specific intraoral sites previously published include, lingual mucosa,9–12 buccal mucosa13 and the palate,14 with the tongue being the most frequently affected.15 The other sites reported include the urethra, glans penis, vagina and anorectal region.6 DC patients have a recognised increased risk of malignancy from pre-existing mucosal leukoplakia,16 reaching an incidence of approximately 35% with a peak in the third decade of life.6,9 Given that it is difficult to predict which lesions will undergo malignant change, markers for such instability have been sought. These include ultrastructural change,17 p53 expression18 and cytokeratin profiles.19 Evidence for increased cellular activity revealed increased numbers of mitochondria, nucleoli and the retention of complex cell to cell contact at a time when most cells would be terminally differentiated. When DC is complicated by malignant disease the prognosis is generally considered to be poor. Malignant disease reported in DC include, solid tumours of the tongue, buccal mucosa, nasopharynx, rectum, cervix, vagina, skin, oesophagus and pancreas.6 Haematological malignancy has also been reported including myelodysplasia5 and Hodgkin’s 20 lymphoma. There is a wide age variation in the initial presentation of clinical signs of DC. Nail dystrophy, leukoplakia and skin hyperpigmentation tend to appear in the first decade of life,4 with median ages of onset of six, seven and eight years, respectively.5

T.P.B. Handley et al.

The non-mucocutaneous features These features include bone marrow failure, pulmonary disease, ophthalmic, skeletal, dental, genitourinary, gastrointestinal and neurological abnormalities. One of the most common features of this disease is bone marrow failure resulting in peripheral cytopenias. It has been shown that 85% of DC patients have a peripheral cytopenia of one or more lineages, with approximately 75% of these patients developing pancytopenia.5 In 80% of these patients the age of onset for the development of pancytopenia is less than 20 years of age, with half of them developing pancytopenia before 10 years. It has been estimated that 80–90% of patients will have developed bone marrow failure by the age of 30,5,21 and approaching 94% by the age of 40 years.5 Bone marrow failure is reported as the principal cause of death in 70% of patients with DC, as a consequence of bleeding or opportunistic infections with cytomegalovirus, pneumocystis carinii or candida.8 In some patients, the development of bone marrow abnormalities may appear before the classical cutaneous manifestations, resulting in the initial diagnosis of idiopathic aplastic anaemia.5,22–24 DC patients also have features, which overlap with Fanconi’s anaemia, which is also characterised by bone marrow failure and a predisposition to malignancy. Therefore all patients with unexplained aplastic anaemia should be investigated for DC. Pulmonary complications are reported to develop in approximately 20% of DC patients, resulting in reduced diffusion capacity with or without a restrictive defect.5 The mortality rate from these pulmonary complications has been estimated at between 10% and 15%.5 Postmortem studies on two subjects who had died suddenly from acute pulmonary failure, showed abnormalities of pulmonary vasculature and abnormally high levels of pulmonary fibrosis.5 Pulmonary complications in the past may have been overlooked and may well provide the answer as to why there is such a high incidence of fatal pulmonary complications following bone marrow transplantation. This further reinforces the importance of pulmonary shielding during radiotherapy and avoiding agents associated with pulmonary toxicity such as busulphan. Other non-mucocutaneous abnormalities associated with DC include ophthalmic abnormalities such as epiphoria secondary to nasolacrimal duct obstruction, conjunctivitis, blepharitis, pterygium formation, ectropion, loss of eyelashes, strabismus, cataracts and optic atrophy. These ophthalmic abnormalities have been observed in

Dyskeratosis congenita approximately half of DC cases, with epiphoria being the most common ophthalmic finding.25 The skeletal anomalies, are reportedly seen in approximately 20% of cases,26,27 and include mandibular hypoplasia, osteoporosis, abnormal bone trabeculation, avascular necrosis and scoliosis. Manifestations in the oral cavity other than leukoplakia, include hyperpigmentation of the buccal mucosa, severe periodontal disease,28 hypocalcified teeth6 and taurodontism.29,30 The genitourinary abnormalities include hypoplastic testes, hypospadias, phimosis, urethral stenosis and horseshoe kidneys.8 Gastrointestinal abnormalities reported in the literature include, developmental oesophageal webs in the postcricoid region resulting in dysphagia6,14,28 which have been associated with malignant transformation,32 hepatomegally and cirrhosis. Other abnormalities, which have been reported, include altered mental status and learning difficulties, microcephaly, intracranial calcifications, alopecia, hair greying, deafness and amyloidosis1–4 peripheral neuropathy and choanal atresia.33,31 In 1995 at the Hammersmith Hospital in London the DC registry was established,21 and has been extremely important in confirming all of the previously reported observations, helping to identify new features, as well as being key in the identification of the DKC1 gene. As of November 2002 the registry had information on 297 patients (227 males and 70 females) from 38 countries,34 and has been able to distinguish DC into three different genetic subtypes; X-linked recessive, autosomal dominant and autosomal recessive.21 The precise incidence of DC is as yet unknown, however the prevalence has been estimated to be approximately 1 in 1,000,000.

Genetic and molecular basis of DC X-linked recessive form A male sex preponderance of 85% is seen in DC,8 and in most cases the pattern of inheritance is consistent with the X-linked recessive trait. Linkage analysis allowed location of the gene for DC to Xq28.35,36 Positional cloning of the mutated gene at that locus followed and this is now known as DKC1 (formerly termed NAP57).36–41 This identification has allowed genetic testing for DC which can be used to provide antenatal diagnosis in X linked families, identify carriers and confirm a diagnosis in suspected individuals.42–46 The DKC1 gene is expressed in all tissues of the body, indicat-

333 ing a vital ‘house keeping’ role in the human cell and correlating well with the multisystem phenotype of DC.34 The DKC1 gene encodes dyskerin, a nucleolar protein, which is an essential structural component of small nucleolar RNA–protein complexes (snoRNPs) and of the mammalian telomerase enzyme complex. It is part of the H/ACA class of small nucleolar RNAs and is responsible for pseudouridylation of specific residues of ribosomal RNA.47 Apart from this catalytic activity, dyskerin also has a structural role and is vital for appropriate maturation and maintenance of box H/ACA snoRNAs. These processes are essential for ribosome biogenesis and hence led to the initial suggestion that X linked DC was due to defective protein synthesis consequent to ‘ribosomopathy’.48 Shortly following the discovery of the DKC1 gene, a box H/ACA motif was identified in the essential RNA component of the human telomerase reverse transcriptase enzyme complex.49 The dyskerin protein associates with the enzyme complex and appears to be necessary for its stability and function. This enzyme maintains chromosome end caps during cell replication which otherwise shorten due to the end replication problem.50 Laboratory mice bred as knock outs for the mouse telomerase RNA component (mTERC) shorten their long telomeres through rounds of cell division. Over succeeding generations these mice develop critically short telomeres and develop features very similar to the human DC phenotype, including chromosomal instability, premature aging and increased radiosensitivity.51 Analysis of telomere restriction fragments from DC patients has shown very shortened telomeres which led to the hypothesis that telomeric attrition due to dysfunctional telomerase is responsible for the clinical features of DC.49,52 In one study of DC patients site-specific pseudouridylation and RNA processing was not defective,49 and similar results have recently been obtained for the peripheral blood mononuclear cells of one X-linked patient.54 This concept was not widely accepted initially but received further support with the discovery of hTERC mutations in three families with an autosomal dominant form of DC.55 Some controversy as to which of these dyskerin functions is the most significant in DC remains. Recent work producing an DKC1m knockout mouse has been reported.56 These animals developed all of the clinical features seen in human DC, thus confirming their utility as a good model of the disease. Interestingly, the DC phenotype appeared within one generation and so did not require progressive generational telomere shortening to produce

334 characteristic clinical features. This model therefore seems to tip the balance of significance of dyskerin functions back toward rRNA processing.

Autosomal dominant dyskeratosis congenita This form of DC has been associated with a milder spectrum of clinical manifestation.55 Through linkage analysis in one large family it was demonstrated that the gene for autosomal dominant DC (AD-DC) is on chromosome 3q.57 This is in fact the same locus as that to which the gene for the human telomerase RNA component (hTERC) had previously been mapped, and thus led to hTERC mutation analysis. It was subsequently demonstrated that autosomal dominant DC is due to mutations in the hTERC gene, and these mutations have been found in several of its domains.57 hTERC and telomerase reverse transcriptase form the core of the active telomerase enzyme complex, which is responsible for synthesising telomeric DNA repeats which form part of the nucleoprotein complexes that cap the ends of chromosomes.58,59 In DC shortened telomeres can lead to chromosome end to end joining and breakage, fusion and bridge cycles. The ensuing genomic instability leads to widespread cell death and occasionally to the development of malignancy.58,59 The consequences of hTERC mutations and their affect on telomerase activity as well as on the structure of telomerase RNA have been studied. The results of these studies demonstrated a reduction in telomerase activity either through impaired hTERC RNA accumulation or stability, or by directly reducing the catalytic activity of telomerase.60–64 The currently held belief is therefore that AD-DC arises from an abnormality in telomerase activity,65 due both to loss of normal tissue telomerase activity and exacerbated by telomeric shortening in utero when the telomerase enzyme is still widely active in dividing cells before 20 weeks of gestation. This defect is seen first in high turnover tissues and explains the clinical manifestation of disease affecting the haemopoietic system and mucosa and skin. Recent in vivo evidence has been found that a mutant hTERC carrying a GC to AG double substitution does not behave as a dominant negative for telomere maintenance.53 This mutated hTERC telomerase enzyme complex actually functions as a weakly active telomerase enzyme, which is defective in telomere elongation.53 There has been wide speculation regarding the role of telomerase beyond telomere length maintenance in maintaining cell viability.66 Clearly in the context of this form

T.P.B. Handley et al. of AD-DC it is telomere length maintenance which is the crucial molecular and chromosomal defect. Notably, in autosomal dominant DC clinical features tend to emerge at an earlier age and with an increased number and severity of the clinical features associated with DC with each successive generation. This has been hypothesised to be due to successive telomere shortening over generations in a fashion analogous to the pattern seen in the hTERC knockout mouse.51,64

Autosomal recessive dyskeratosis congenita The genetic basis of this form of DC is at present unknown and further studies in this area are required.

A severe variant of DC Hoyeraal–Hreidarsson syndrome (HH) is a severe multisystem disorder that can present in the neonatal period and infancy. It is characterised by severe growth retardation of perinatal onset, bone marrow failure, immunodeficiency, cerebellar hypoplasia and microcephaly.67,68 These affected infants die before the characteristic mucocutaneous features develop, and the overlap of these features with DC has led to the analysis of the DKC1 gene in these patients. This demonstrated that some of the male HH patients are a severe variant of DC, where death occurs before diagnostic features of DC develop. In addition to this, identification of several mutations in the catalytic domain of dyskerin responsible for pseudouridylation have now been identified in patients with HH syndrome.42,69–71

The diagnosis of dyskeratosis congenita When all the classical mucocutaneous features of DC are present then the diagnosis is relatively straight forward. In some cases diagnosis may not be so easy, for example with the presentation of aplastic anaemia without any mucocutaneous features. Now that the mutated genes in X linked recessive and autosomal dominant DC subtypes are known, DKC1 and hTERC, respectively, genetic analysis can confirm the diagnosis in the majority of DC patients. It is appropriate to screen for the DKC1 gene if the patient is male with two of the following features of the disease: abnormal skin pigmentation, nail dystrophy, leukoplakia, bone

Dyskeratosis congenita marrow failure. Any patient presenting with idiopathic aplastic anaemia should undergo chromosomal breakage analysis with mitomycin-C or diepoxybutane to exclude Fanconi’s anaemia. If this chromosomal breakage analysis is normal (i.e., negative for Fanconi’s anaemia) then progression to analyse hTERC gene is appropriate. Although rare, Dyskeratosis congenita should be considered and excluded whenever a child presents with oral leukoplakia. The last 10 years have seen significant improvements in our understanding of the condition. Identification of the gene responsible offers the potential for (i) improving diagnosis, (ii) monitoring progression of the disease once diagnosed and (iii) novel forms of therapy for this inherited disorder.

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