Katalin Szakszon

KAT6B is a tumor suppressor histone H3 lysine 23 acetyltransferase undergoing genomic loss in small cell lung cancer

Recent efforts to sequence human cancer genomes have highlighted that point mutations in genes involved in the epigenetic setting occur in tumor cells. Small cell lung cancer (SCLC) is an aggressive tumor with poor prognosis, where little is known about the genetic events related to its development. Herein, we have identified the presence of homozygous deletions of the candidate histone acetyltransferase KAT6B, and the loss of the corresponding transcript, in SCLC cell lines and primary tumors. Furthermore, we show, in vitro and in vivo, that the depletion of KAT6B expression enhances cancer growth, while its restoration induces tumor suppressor-like features. Most importantly, we demonstrate that KAT6B exerts its tumor-inhibitory role through a newly defined type of histone H3 Lys23 acetyltransferase activity.

Phenotype and genotype in 103 patients with tricho-rhino-phalangeal syndrome.

Tricho-rhino-phalangeal syndrome (TRPS) is characterized by craniofacial and skeletal abnormalities, and subdivided in TRPS I, caused by mutations in TRPS1, and TRPS II, caused by a contiguous gene deletion affecting (amongst others) TRPS1 and EXT1. We performed a collaborative international study to delineate phenotype, natural history, variability, and genotype-phenotype correlations in more detail. We gathered information on 103 cytogenetically or molecularly confirmed affected individuals. TRPS I was present in 85 individuals (22 missense mutations, 62 other mutations), TRPS II in 14, and in 5 it remained uncertain whether TRPS1 was partially or completely deleted. Main features defining the facial phenotype include fine and sparse hair, thick and broad eyebrows, especially the medial portion, a broad nasal ridge and tip, underdeveloped nasal alae, and a broad columella. The facial manifestations in patients with TRPS I and TRPS II do not show a significant difference. In the limbs the main findings are short hands and feet, hypermobility, and a tendency for isolated metacarpals and metatarsals to be shortened. Nails of fingers and toes are typically thin and dystrophic. The radiological hallmark are the cone-shaped epiphyses and in TRPS II multiple exostoses. Osteopenia is common in both, as is reduced linear growth, both prenatally and postnatally. Variability for all findings, also within a single family, can be marked. Morbidity mostly concerns joint problems, manifesting in increased or decreased mobility, pain and in a minority an increased fracture rate. The hips can be markedly affected at a (very) young age. Intellectual disability is uncommon in TRPS I and, if present, usually mild. In TRPS II intellectual disability is present in most but not all, and again typically mild to moderate in severity. Missense mutations are located exclusively in exon 6 and 7 of TRPS1. Other mutations are located anywhere in exons 4-7. Whole gene deletions are common but have variable breakpoints. Most of the phenotype in patients with TRPS II is explained by the deletion of TRPS1 and EXT1, but haploinsufficiency of RAD21 is also likely to contribute. Genotype-phenotype studies showed that mutations located in exon 6 may have somewhat more pronounced facial characteristics and more marked shortening of hands and feet compared to mutations located elsewhere in TRPS1, but numbers are too small to allow firm conclusions.

De novo mutations of the gene encoding the histone acetyltransferase KAT6B in two patients with Say-Barber/Biesecker/Young-Simpson syndrome

The Say‐Barber/Biesecker/Young‐Simpson (SBBYS) type of the blepharophimosis–mental retardation syndrome group (Ohdo‐like syndromes) is a multiple congenital malformation syndrome characterized by vertical narrowing and shortening of the palpebral fissures, ptosis, intellectual disability, hypothyroidism, hearing impairment, and dental anomalies. Mutations of the gene encoding the histone‐acetyltransferase KAT6B have been recently identified in individuals affected by SBBYS syndrome. SBBYS syndrome‐causing KAT6B mutations cluster in a ∼1,700 basepair region in the 3′ part of the large exon 18, while mutations located in the 5′ region of the same exon have recently been identified to cause the genitopatellar syndrome (GPS), a clinically distinct although partially overlapping malformation‐intellectual disability syndrome. Here, we present two children with clinical features of SBBYS syndrome and de novo truncating KAT6B mutations, including a boy who was diagnosed at the age of 4 months. Our results confirm the implication of KAT6B mutations in typical SBBYS syndrome and emphasize the importance of genotype–phenotype correlations at the KAT6B locus where mutations truncating the KAT6B protein at the amino‐acid positions ∼1,350–1,920 cause SBBYS syndrome.

Thirteen New Patients with Guanidinoacetate Methyltransferase Deficiency and Functional Characterization of Nineteen Novel Missense Variants in the GAMT Gene

Guanidinoacetate methyltransferase deficiency (GAMT‐D) is an autosomal recessively inherited disorder of creatine biosynthesis. Creatine deficiency on cranial proton magnetic resonance spectroscopy, and elevated guanidinoacetate levels in body fluids are the biomarkers of GAMT‐D. In 74 patients, 50 different mutations in the GAMT gene have been identified with missense variants being the most common. Clinical and biochemical features of the patients with missense variants were obtained from their physicians using a questionnaire. In 20 patients, 17 missense variants, 25% had a severe, 55% a moderate, and 20% a mild phenotype. The effect of these variants on GAMT enzyme activity was overexpressed using primary GAMT‐D fibroblasts: 17 variants retained no significant activity and are therefore considered pathogenic. Two additional variants, c.22C>A (p.Pro8Thr) and c.79T>C (p.Tyr27His) (the latter detected in control cohorts) are in fact not pathogenic as these alleles restored GAMT enzyme activity, although both were predicted to be possibly damaging by in silico analysis. We report 13 new patients with GAMT‐D, six novel mutations and functional analysis of 19 missense variants, all being included in our public LOVD database. Our functional assay is important for the confirmation of the pathogenicity of identified missense variants in the GAMT gene.

POLD1 Germline Mutations in Patients Initially Diagnosed with Werner Syndrome.

Segmental progeroid syndromes are rare, heterogeneous disorders characterized by signs of premature aging affecting more than one tissue or organ. A prototypic example is the Werner syndrome (WS), caused by biallelic germline mutations in the Werner helicase gene (WRN). While heterozygous lamin A/C (LMNA) mutations are found in a few nonclassical cases of WS, another 10%–15% of patients initially diagnosed with WS do not have mutations in WRN or LMNA. Germline POLD1 mutations were recently reported in five patients with another segmental progeroid disorder: mandibular hypoplasia, deafness, progeroid features syndrome. Here, we describe eight additional patients with heterozygous POLD1 mutations, thereby substantially expanding the characterization of this new example of segmental progeroid disorders. First, we identified POLD1 mutations in patients initially diagnosed with WS. Second, we describe POLD1 mutation carriers without clinically relevant hearing impairment or mandibular underdevelopment, both previously thought to represent obligate diagnostic features. These patients also exhibit a lower incidence of metabolic abnormalities and joint contractures. Third, we document postnatal short stature and premature greying/loss of hair in POLD1 mutation carriers. We conclude that POLD1 germline mutations can result in a variably expressed and probably underdiagnosed segmental progeroid syndrome.

Complete recovery from psychosis upon miglustat treatment in a juvenile Niemann–Pick C patient

Niemann-Pick disease type C is a rare lipid trafficking disorder characterized by the accumulation of cholesterol and glycosphingolipids in the brain and viscera. Perinatal, early infantile, late infantile, juvenile and adult forms are distinguished based on the age of manifestation. In the juvenile form, patients in their early years are usually, but not always, symptom free, but present with neurodegeneration later in their lives. These include clumsiness, ataxia, seizures, motor and intellectual decline. Psychiatric manifestations may occur at any stage of the disease. These manifestations include schizophrenia, presenile dementia, depression or psychosis. In 2009, miglustat was approved for the therapy of the disease. We present a case of a patient with juvenile Niemann-Pick C disease whose psychosis was reversed completely by miglustat treatment. Based on our clinical experience we suggest considering Niemann-Pick C in cases of therapy-resistant psychosis and encourage the introduction of miglustat in Niemann-Pick C patients even in the most advanced cases, with respect to psychiatric illness.

New mutations in the ATM gene and clinical data of 25 AT patients

Ataxia telangiectasia (AT) is an autosomal recessive disorder characterized by cerebellar degeneration, immunodeficiency, oculocutaneous telangiectasias, chromosomal instability, radiosensitivity, and cancer predisposition. The gene mutated in the patients, ATM, encodes a member of the phosphatidylinositol 3-kinase family proteins. The ATM protein has a key role in the cellular response to DNA damage. Truncating and splice site mutations in ATM have been found in most patients with the classical AT phenotype. Here we report of our extensive ATM mutation screening on 25 AT patients from 19 families of different ethnic origin. Previously unknown mutations were identified in six patients including a new homozygous missense mutation, c.8110T>C (p.Cys2704Arg), in a severely affected patient. Comprehensive clinical data are presented for all patients described here along with data on ATM function generated by analysis of cell lines established from a subset of the patients.

Blepharophimosis mental retardation syndrome Say-Barber/Biesecker/Young-Simpson type – New findings with neuroimaging†

We report on a female patient with blepharophimosis mental retardation syndrome of Say/Barber/Biesecker/Young‐Simpson (SBBYS) type. Main findings in her were marked developmental delay, blepharophimosis, ptosis, cleft palate, external auditory canal stenosis, small and malformed teeth, hypothyroidism, hearing impairment, and joint limitations. We performed diffusion tensor magnetic resonance imaging (MRI) and tractography of the brain which showed inappropriate myelination and disturbed white matter integrity. Cytogenetic analysis, subtelomeric fluorescence in situ hybridization and comparative genomic hybridization failed to identify an abnormality. It remains uncertain whether the MRI findings are specific to the present patient or form part of the SBBYS syndrome.

Detection of subtelomeric chromosomal rearrangements in idiopathic mental retardation

A kromoszomak szubtelomerikus regioi genben gazdag teruletek, atrendeződesuk hagyomanyos kromoszomaanalizissel nem detektalhato. Mivel a mentalis retardaciok kozel 7%-aert felelősek, kimutatasuk diagnosztikai szempontbol jelentős, es lehetőseget nyujt az ismetlődes megakadalyozasara is. A kimutatasukra alkalmas modszerek egyike a szubtelomerikus fl uoreszcencia in situ hibridizacio. Otvenkilenc idiopathias mentalisan retardalt beteg kozul 35 kozepes/sulyos ertelmi fogyatekost valasztottunk ki a nemzetkozi irodalomban ajanlott kriteriumok alapjan. Kozuluk 6 beteg eseteben mutattunk ki szubtelomerikus aberraciot, 5 familiaris (ket csalad), egy de novo esetnek bizonyult. Huszonkilenc betegben szubtelomerikus kromoszomaatrendeződest nem igazoltunk. A 6 beteg kozul kettőben 8pter deleciot es 12pter duplikaciot, haromban 21qter deleciot es 10pter duplikaciot azonositottunk kiegyensulyozatlan transzlokacio formajaban. Egy betegnel de novo keletkezett 3qter deleciot detektaltunk. Az elteresek eredetenek tisztazasa soran 12 egeszseges csaladtag kozul ot bizonyult kiegyensulyozott transzlokaciohordozonak. Az irodalmi adatokkal osszhangban megallapitottuk, hogy a fenotipust a delecio es a duplikacio merete, valamint transzlokaciok eseten az erintett partner kromoszomak egyuttesen hatarozzak meg.Subtelomeric regions of chromosomes are rich in genes; their rearrangements cannot be identified by traditional chromosome analysis. Since these subtelomeric aberrations are responsible for about 7% of cases with mental retardation, their detection is important both from the diagnostic point of view and to prevent recurrence in the family. Subtelomeric chromosomal alterations can be detected by fluorescence in situ hybridization. Based on international criteria, 35 out of 59 patients with mental retardation have been selected. Subtelomeric rearrangements were revealed in 6 patients (5 familial cases, 1 new onset) whereas the subtelomeric FISH result was normal in 29 cases. Deletion of 8pter and duplication of 12pter were detected in 2 patients, while a deletion of 21qter and duplication of the 10pter due to an unbalanced translocation were found in 3 other cases. Finally, a new onset deletion of 3qter was observed in 1 patient. In order to clarify the origin of chromosome aberrations, 12 healthy family members were also examined, 5 of them carried balanced translocations. We concluded that the phenotype is mostly influenced by the size of regions involved in deletion/duplication and - in case of translocations - by the associated chromosomal abnormalities.

A recurrent synonymous KAT6B mutation causes Say-Barber-Biesecker/Young-Simpson syndrome by inducing aberrant splicing.

Mutations of the histone acetyltransferase‐encoding KAT6B gene cause the Say‐Barber‐Biesecker/Young‐Simpson (SBBYS) type of blepharophimosis‐“mental retardation” syndromes and the more severe genitopatellar syndrome. The SBBYS syndrome‐causing mutations are clustered in the large exon 18 of KAT6B and almost exclusively lead to predicted protein truncation. An atypical KAT6B mutation, a de novo synonymous variant located in exon 16 (c.3147G>A, p.(Pro1049Pro)) was previously identified in three unrelated patients. This exonic mutation was predicted in silico to cause protein truncation through aberrant splicing. Here, we report three additional unrelated children with typical SBBYS syndrome and the KAT6B c.3147G>A mutation. We show on RNA derived from patient blood that the mutation indeed induces aberrant splicing through the use of a cryptic exonic splice acceptor site created by the sequence variant. Our results thus identify the synonymous variant c.3147G>A as a splice site mutation and a mutational hot spot in SBBYS syndrome.