Dyskeratosis congenita

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Dyskeratosis congenita
Other namesZinsser-Cole-Engman syndrome, [1] [2] : 570 
X-linked recessive.svg
Dyskeratosis congenita is inherited in an X-linked recessive manner
Specialty Medical genetics   OOjs UI icon edit-ltr-progressive.svg

Dyskeratosis congenita (DKC), also known as Zinsser-Engman-Cole syndrome, is a rare progressive congenital disorder with a highly variable phenotype. [3] The entity was classically defined by the triad of abnormal skin pigmentation, nail dystrophy, and leukoplakia of the oral mucosa, and MDS/AML, but these components do not always occur. [3] DKC is characterized by short telomeres. Some of the manifestations resemble premature ageing (similar to progeria) and cognitive impairment can be a feature. The disease initially mainly affects the skin, but a major consequence is progressive bone marrow failure which occurs in over 80%, causing early mortality. [3]

Contents

Presentation

DKC can be characterized by cutaneous pigmentation, premature graying, dystrophy of the nails, leukoplakia of the oral mucosa, continuous lacrimation due to atresia of the lacrimal ducts, often thrombocytopenia, anemia, testicular atrophy in the male carriers, and predisposition to cancer. [4] [5] Many of these symptoms are characteristic of geriatrics, and those carrying the more serious forms of the disease often have significantly shortened lifespans. Also, liver abnormalities are associated with this syndrome, Nodular Regenerative Hypoplasia of the liver, although rare, it is one of many manifestations of liver disorders short telomeres can cause. [6]

Predisposition to cancer

Susceptibility to cancer seems counterintuitive because in many known cancers reactivation of telomerase is actually a required step for malignancy to evolve (see telomere). In a disease where telomerase is affected, it does not seem to follow that cancer would be a complication to result. The authors note the paradoxical nature of cancer predisposition in individuals who seem to lack one of the required components for cancer to form. It is thought [7] that without functional telomerase, chromosomes will likely be attached together at their ends through the non-homologous end joining pathway. If this proves to be a common enough occurrence, malignancy even without telomerase present is possible. Myelodysplastic Syndrome is associated with this syndrome usually presenting as a Hypoplastic Bone Marrow that can resemble Aplastic Anemia, but can be differentiated with >10% dysplasia in affected cell lines, sometimes not possible though because of the Hypoplastic marrow reducing blood cells to be observed, genetic clones are usually not present more often than not with Hypoplastic Myelodysplastic Disorder associated with Dyskeratosis Congenita.

Genetics

Of the components of the telomerase RNA component (TERC), one of key importance is the box H/ACA domain. This H/ACA domain is responsible for maturation and stability of TERC and therefore of telomerase as a whole. The mammalian H/ACA ribonucleoprotein contains four protein subunits: dyskerin, Gar1, Nop10, and Nhp2. Mutations in Nop10, [8] Nhp2 [9] and dyskerin1 [10] have all been shown to lead to DKC-like symptoms.

X-linked

The best characterized form of dyskeratosis congenita is a result of one or more mutations in the long arm of the X chromosome in the gene DKC1. [7] [10] This results in the X-linked recessive form of the disease wherein the major protein affected is dyskerin. Of the five mutations described by Heiss and colleagues in Nature Genetics, [10] four were single nucleotide polymorphisms all resulting in the change of highly conserved amino acids. One case was an in-frame deletion resulting in the loss of a leucine residue, also conserved in mammals. In three of the cases, the specific amino acids affected (phenylalanine, proline, glycine) are found in the same locus in humans as they are in yeast ( S. Cerevisiae ) and the brown rat (R. Norvegicus). [10] This establishes the sequence conservation and importance of dyskerin within the eukaryotes. The relevant nature of dyskerin throughout most species is to catalyze the post-transcriptional pseudouridylation of specific uridines found in non-coding RNAs, such as ribosomal RNA (rRNA). Cbf5, the yeast analog of human dyskerin, is indeed known to be associated with the processing and maturation of rRNA. [7] In humans, this role can be attributed to dyskerin. [10] Thus, the X-linked form of this disease may result in specific issues related to dysfunctional RNA and perhaps a graver phenotype. Within the vertebrates, as opposed to single celled eukaryotes, dyskerin is a key component of the telomerase RNA component (TERC) in the form of the H/ACA motif. [11] This X-linked variety, like the Nop10 and Nhp2 mutations, demonstrates shortened telomeres as a result of lower TERC concentrations. [12]

Autosomal dominant

3 genes: TERC, TERT, TINF2 The evidence supporting the importance of the H/ACA domain in human telomerase is abundant. At least one study [13] has shown that these mutations affect telomerase activity by negatively affecting pre-RNP assembly and maturation of human telomerase RNA. Nonetheless, mutations that directly affect the telomerase RNA components would presumably exist and should also cause premature aging or DKC-like symptoms. Indeed, three families with mutations in the human TERC gene have been studied with intriguing results. [7] In two of these families, two family-specific single nucleotide polymorphisms were present while in the other there persisted a large-scale deletion (821 base pairs of DNA) on chromosome 3 which includes 74 bases coding for a section of the H/ACA domain. These three different mutations result in a mild form of dyskeratosis congenita which uniquely follows an autosomal dominant pattern of inheritance. Premature graying, early dental loss, predisposition to skin cancer, as well as shortening of telomere length continue to be characteristic of this disease. [14]

Autosomal recessive

6 genes: The true phenotype of DKC individuals may depend upon which protein has incurred a mutation. One documented autosomal recessive mutation [8] in a family that carries DKC has been found in NOP10. Specifically, the mutation is a change of base from cytosine to thymine in a highly conserved region of the NOP10 sequence. This mutation, on chromosome 15, results in an amino acid change from arginine to tryptophan. Homozygous recessive individuals show the symptoms of dyskeratosis congenita in full. As compared to age-matched normal individuals, those suffering from DKC have telomeres of a much shorter length. Furthermore, heterozygotes, those who have one normal allele and one coding for the disease, also show relatively shortened telomeres. The cause of this was determined to be a reduction in TERC levels in those with the Nop10 mutation. With TERC levels down, telomere maintenance, especially in development, would be presumed to suffer accordingly. This would lead to the telomere shortening described. [8]

NHP2 mutations are similar in characterization to NOP10. These mutations are also autosomal recessive with three specific single-nucleotide polymorphisms being recognized which result in dyskeratosis congenita. Also, like NOP10, individuals with these NHP2 mutations have a reduction in the amount of telomerase RNA component (TERC) present in the cell. Again, it can be presumed that a reduction in TERC results in aberrant telomere maintenance and thus shortened telomeres. Those homozygous recessive for mutations in NHP2 do show shorter telomeres when compared with age-matched normal individuals. [9]

Pathophysiology

Dyskeratosis congenita is a disorder of poor telomere maintenance [7] mainly due to a number of gene mutations that give rise to abnormal ribosome function, termed ribosomopathy. Specifically, the disease is related to one or more mutations which directly or indirectly affect the vertebrate telomerase RNA component (TERC). [15]

Telomerase is a reverse transcriptase which maintains a specific repeat sequence of DNA, the telomere, during development. Telomeres are placed by telomerase on both ends of linear chromosomes as a way to protect linear DNA from general forms of chemical damage and to correct for the chromosomal end-shortening that occurs during normal DNA replication. [16] This end-shortening is the result of the eukaryotic DNA polymerases having no mechanism for synthesizing the final nucleotides present on the end of the "lagging strand" of double stranded DNA. DNA polymerase can only synthesize new DNA from an old DNA strand in the 5'→3' direction. Given that DNA has two strands that are complementary, one strand must be 5'→3' while the other is 3'→5'. This inability to synthesize in the 3'→5' directionality is compensated with the use of Okazaki fragments, short pieces of DNA that are synthesized 5'→3' from the 3'→5' as the replication fork moves. As DNA polymerase requires RNA primers for DNA binding in order to commence replication, each Okazaki fragment is thus preceded by an RNA primer on the strand being synthesized. When the end of the chromosome is reached, the final RNA primer is placed upon this nucleotide region, and it is inevitably removed. Unfortunately once the primer is removed, DNA polymerase is unable to synthesize the remaining bases. [16] [17]

Sufferers of DKC have been shown to have a reduction in TERC levels invariably affecting the normal function of telomerase which maintains these telomeres. [7] [8] [10] With TERC levels down, telomere maintenance during development suffers accordingly. In humans, telomerase is inactive in most cell types after early development (except in extreme cases such as cancer). [11] Thus, if telomerase is not able to efficiently affect the DNA in the beginning of life, chromosomal instability becomes a grave possibility in individuals much earlier than would be expected. [18]

A study shows that proliferative defects in DC skin keratinocytes are corrected by expression of the telomerase reverse transcriptase, TERT, or by activation of endogenous telomerase through expression of papillomavirus E6/E7 of the telomerase RNA component, TERC. [19]

Diagnosis

Since the disease has a wide variety of symptoms due to involvement of multiple systems of the body, diagnostic testing depends on the clinical findings in each individual patient. Commonly used tests include a complete blood count (CBC), bone marrow examination, leukocyte telomere length test (e.g. Flow FISH), pulmonary function test, and genetic testing. [20] [21]

Management

The mainstay of treatment in dyskeratosis congenita is hematopoietic stem cell transplantation, best outcome with sibling donor. Short term therapy in initial stages is with anabolic steroids [oxymetholone, danazol] or with erythropoietin-like hormones or with granulocyte-colony stimulating factor [filgrastim) all these therapies are directed to cope with effects of bone marrow failure which manifests as low red and white blood cell counts. These medications help to increase the blood components and make up for the deficiencies caused due to bone marrow failure. Dyskeratosis Congenita in regards to stem cell transplantation have to be very carefully treated with low intensity radiation/chemo to avoid potentially catastrophic effects of Host versus graft disease and toxicity to other organs affected by short telomeres which makes them very sensitive to any radiation especially the lungs, and liver. [22]

Prognosis

DC is associated with shorter life expectancy, but many live to at least age 60. [23] Main cause of mortality in these patients are related to bone marrow failure. Nearly 80% of the patients of dyskeratosis congenita develop bone marrow failure. [24]

Research

Recent research has used induced pluripotent stem cells to study disease mechanisms in humans, and discovered that the reprogramming of somatic cells restores telomere elongation in dyskeratosis congenita (DKC) cells despite the genetic lesions that affect telomerase. The reprogrammed DKC cells were able to overcome a critical limitation in TERC levels and restored function (telomere maintenance and self-renewal). Therapeutically, methods aimed at increasing TERC expression could prove beneficial in DKC. [25]

See also

Related Research Articles

<span class="mw-page-title-main">Telomere</span> Region of repetitive nucleotide sequences on chromosomes

A telomere is a region of repetitive nucleotide sequences associated with specialized proteins at the ends of linear chromosomes. Telomeres are a widespread genetic feature most commonly found in eukaryotes. In most, if not all species possessing them, they protect the terminal regions of chromosomal DNA from progressive degradation and ensure the integrity of linear chromosomes by preventing DNA repair systems from mistaking the very ends of the DNA strand for a double-strand break.

<span class="mw-page-title-main">Telomerase</span> Telomere-restoring protein active in the most rapidly dividing cells

Telomerase, also called terminal transferase, is a ribonucleoprotein that adds a species-dependent telomere repeat sequence to the 3' end of telomeres. A telomere is a region of repetitive sequences at each end of the chromosomes of most eukaryotes. Telomeres protect the end of the chromosome from DNA damage or from fusion with neighbouring chromosomes. The fruit fly Drosophila melanogaster lacks telomerase, but instead uses retrotransposons to maintain telomeres.

<span class="mw-page-title-main">Werner syndrome</span> Medical condition

Werner syndrome (WS) or Werner's syndrome, also known as "adult progeria", is a rare, autosomal recessive disorder which is characterized by the appearance of premature aging.

<span class="mw-page-title-main">Fanconi anemia</span> Medical condition

Fanconi anemia (FA) is a rare, autosomal recessive, genetic disease resulting in impaired response to DNA damage in the FA/BRCA pathway. Although it is a very rare disorder, study of this and other bone marrow failure syndromes has improved scientific understanding of the mechanisms of normal bone marrow function and development of cancer. Among those affected, the majority develop cancer, most often acute myelogenous leukemia (AML), MDS, and liver tumors. 90% develop aplastic anemia by age 40. About 60–75% have congenital defects, commonly short stature, abnormalities of the skin, arms, head, eyes, kidneys, and ears, and developmental disabilities. Around 75% have some form of endocrine problem, with varying degrees of severity. 60% of FA is FANC-A, 16q24.3, which has later onset bone marrow failure.

In genetics, anticipation is a phenomenon whereby as a genetic disorder is passed on to the next generation, the symptoms of the genetic disorder become apparent at an earlier age with each generation. In most cases, an increase in the severity of symptoms is also noted. Anticipation is common in trinucleotide repeat disorders, such as Huntington's disease and myotonic dystrophy, where a dynamic mutation in DNA occurs. All of these diseases have neurological symptoms. Prior to the understanding of the genetic mechanism for anticipation, it was debated whether anticipation was a true biological phenomenon or whether the earlier age of diagnosis was related to heightened awareness of disease symptoms within a family.

<span class="mw-page-title-main">Haploinsufficiency</span> Concept in genetics

Haploinsufficiency in genetics describes a model of dominant gene action in diploid organisms, in which a single copy of the wild-type allele at a locus in heterozygous combination with a variant allele is insufficient to produce the wild-type phenotype. Haploinsufficiency may arise from a de novo or inherited loss-of-function mutation in the variant allele, such that it yields little or no gene product. Although the other, standard allele still produces the standard amount of product, the total product is insufficient to produce the standard phenotype. This heterozygous genotype may result in a non- or sub-standard, deleterious, and (or) disease phenotype. Haploinsufficiency is the standard explanation for dominant deleterious alleles.

Small nuclear RNA (snRNA) is a class of small RNA molecules that are found within the splicing speckles and Cajal bodies of the cell nucleus in eukaryotic cells. The length of an average snRNA is approximately 150 nucleotides. They are transcribed by either RNA polymerase II or RNA polymerase III. Their primary function is in the processing of pre-messenger RNA (hnRNA) in the nucleus. They have also been shown to aid in the regulation of transcription factors or RNA polymerase II, and maintaining the telomeres.

<span class="mw-page-title-main">GATA1</span> Protein-coding gene in humans

GATA-binding factor 1 or GATA-1 is the founding member of the GATA family of transcription factors. This protein is widely expressed throughout vertebrate species. In humans and mice, it is encoded by the GATA1 and Gata1 genes, respectively. These genes are located on the X chromosome in both species.

In molecular biology, small nucleolar RNAs (snoRNAs) are a class of small RNA molecules that primarily guide chemical modifications of other RNAs, mainly ribosomal RNAs, transfer RNAs and small nuclear RNAs. There are two main classes of snoRNA, the C/D box snoRNAs, which are associated with methylation, and the H/ACA box snoRNAs, which are associated with pseudouridylation. SnoRNAs are commonly referred to as guide RNAs but should not be confused with the guide RNAs that direct RNA editing in trypanosomes or the guide RNAs (gRNAs) used by Cas9 for CRISPR gene editing.

<span class="mw-page-title-main">Y RNA</span>

Y RNAs are small non-coding RNAs. They are components of the Ro60 ribonucleoprotein particle which is a target of autoimmune antibodies in patients with systemic lupus erythematosus. They are also reported to be necessary for DNA replication through interactions with chromatin and initiation proteins. However, mouse embryonic stem cells lacking Y RNAs are viable and have normal cell cycles.

<span class="mw-page-title-main">Telomerase reverse transcriptase</span> Catalytic subunit of the enzyme telomerase

Telomerase reverse transcriptase is a catalytic subunit of the enzyme telomerase, which, together with the telomerase RNA component (TERC), comprises the most important unit of the telomerase complex.

<span class="mw-page-title-main">Telomerase RNA component</span> NcRNA found in eukaryotes

Telomerase RNA component, also known as TR, TER or TERC, is an ncRNA found in eukaryotes that is a component of telomerase, the enzyme used to extend telomeres. TERC serves as a template for telomere replication by telomerase. Telomerase RNAs differ greatly in sequence and structure between vertebrates, ciliates and yeasts, but they share a 5' pseudoknot structure close to the template sequence. The vertebrate telomerase RNAs have a 3' H/ACA snoRNA-like domain.

<span class="mw-page-title-main">Dyskerin</span> Protein

H/ACA ribonucleoprotein complex subunit 4 is a protein that in humans is encoded by the gene DKC1.

<span class="mw-page-title-main">Poly(A)-specific ribonuclease</span> Protein-coding gene in the species Homo sapiens

Poly(A)-specific ribonuclease (PARN), also known as polyadenylate-specific ribonuclease or deadenylating nuclease (DAN), is an enzyme that in humans is encoded by the PARN gene.

<span class="mw-page-title-main">Shwachman–Diamond syndrome</span> Medical condition

Shwachman–Diamond syndrome (SDS), or Shwachman–Bodian–Diamond syndrome, is a rare congenital disorder characterized by exocrine pancreatic insufficiency, bone marrow dysfunction, skeletal and cardiac abnormalities and short stature. After cystic fibrosis (CF), it is the second most common cause of exocrine pancreatic insufficiency in children. It is associated with the SBDS gene and has autosomal recessive inheritance.

<span class="mw-page-title-main">Cerebroretinal microangiopathy with calcifications and cysts</span> Medical condition

Cerebroretinal microangiopathy with calcifications and cysts (CRMCC) is a rare genetic disorder, which affects multiple organs. Its hallmarks are widespread progressive calcifications, cysts and abnormalities of the white matter of the brain, usually occurring together with abnormalities of the blood vessels of the retina. Additional features include poor prenatal growth, preterm birth, anemia, osteopenia and bone fractures, and gastrointestinal bleeding. It is caused by compound heterozygous mutations in the conserved telomere maintenance component 1 (CTC1) gene, but its exact pathophysiology is still not well understood.

<span class="mw-page-title-main">Telomeric repeat–containing RNA</span> Long non-coding RNA transcribed from telomeres

Telomeric repeat–containing RNA (TERRA) is a long non-coding RNA transcribed from telomeres - repetitive nucleotide regions found on the ends of chromosomes that function to protect DNA from deterioration or fusion with neighboring chromosomes. TERRA has been shown to be ubiquitously expressed in almost all cell types containing linear chromosomes - including humans, mice, and yeasts. While the exact function of TERRA is still an active area of research, it is generally believed to play a role in regulating telomerase activity as well as maintaining the heterochromatic state at the ends of chromosomes. TERRA interaction with other associated telomeric proteins has also been shown to help regulate telomere integrity in a length-dependent manner.

Ribosomopathies are diseases caused by abnormalities in the structure or function of ribosomal component proteins or rRNA genes, or other genes whose products are involved in ribosome biogenesis.

<span class="mw-page-title-main">WRAP53</span> Protein-coding gene in the species Homo sapiens

WRAP53 is a gene implicated in cancer development. The name was coined in 2009 to describe the dual role of this gene, encoding both an antisense RNA that regulates the p53 tumor suppressor and a protein involved in DNA repair, telomere elongation and maintenance of nuclear organelles Cajal bodies.

<span class="mw-page-title-main">Hoyeraal–Hreidarsson syndrome</span> Medical condition

Hoyeraal–Hreidasson syndrome is a very rare multisystem X-linked recessive disorder characterized by excessively short telomeres and is considered a severe form of dyskeratosis congenita. Being an X-linked disorder, Hoyeraal–Hreidasson syndrome primarily affects males. Patients typically present in early childhood with cerebellar hypoplasia, immunodeficiency, progressive bone marrow failure, and intrauterine growth restriction. The primary cause of death in Hoyeraal–Hreidasson syndrome is bone marrow failure, but mortality from cancer and pulmonary fibrosis is also significant.

References

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  2. ↑ James W, Berger T, Elston D (2005). Andrews' Diseases of the Skin: Clinical Dermatology (10th ed.). Saunders. ISBN   0-7216-2921-0..
  3. 1 2 3 Online Mendelian Inheritance in Man (OMIM): 127550
  4. ↑ Kirwan M, Dokal I (April 2009). "Dyskeratosis congenita, stem cells and telomeres". Biochimica et Biophysica Acta (BBA) - Molecular Basis of Disease. 1792 (4): 371–379. doi:10.1016/j.bbadis.2009.01.010. PMC   2686081 . PMID   19419704.
  5. ↑ Dokal I, Tummala H, Vulliamy T (August 2022). "Inherited bone marrow failure in the pediatric patient". Blood. 140 (6): 556–570. doi:10.1182/blood.2020006481. PMC   9373017 . PMID   35605178.
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  15. ↑ Dorgaleleh S, Naghipoor K, Hajimohammadi Z, Dastaviz F, Oladnabi M (February 2022). "Molecular insight of dyskeratosis congenita: Defects in telomere length homeostasis". Journal of Clinical and Translational Research. 8 (1): 20–30. PMC   8791241 . PMID   35097237.
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