TREACHER COLLINS SYNDROME: A CLINICAL AND MOLECULAR STUDY BASED ON A LARGE SERIES OF PATIENTS

By Vincent et al

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Presented by Joyce Wang’ombe

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CONTENT

• Introduction
• Aim of the study
• Materials and methods
• Results
• Discussion
• Critique

INTRODUCTION

• Treacher Collins syndrome (TCS) is also known as Franceschetti-Zwahlen-Klein syndrome
• It is a rare genetic condition affecting the way the face develops
• It belongs to a larger group of disorders called mandibulofacial dysostosis (MFD)
• Treacher Collins syndrome is the most common among Mandibulofacial dysostosis occurring in 1 in 50, 000 births

INTRODUCTION

INTRODUCTION

• TCS is characterized by typical bilateral facial features such as:

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• Malar and mandibular hypoplasia
• Downward slanting palpebral fissures
• Coloboma of the lower eyelid
• Microtia
• Conductive hearing loss

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• Intellectual disability is rarely reported and no visceral or skeletal malformations have been reported

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INTRODUCTION

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Treacher Collins syndrome features in children

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INTRODUCTION

• Treacher Collins syndrome is genetically heterogeneous and three genes are involved:
• TCOF1 and POLR1D- autosomal dominant
• POLR1D and POLR1C – autosomal recessive
• TCOF1 is the major gene involved
• It encodes Treacle, a nucleolar phosphoprotein involved in production of ribosomal RNA
• This protein is active during early embryonic development in structures that become bones and other tissues of the face

INTRODUCTION

• POLR1D and POLR1C encode subunits of RNA polymerase 1 and 3 respectively
• Heterozygous intragenic deletions in TCOF1 have been described as rare causes of TCS
• No correlations between phenotypic variability and location of mutations in TCOF1 have been established

INTRODUCTION

• They also investigated another gene EFTUD2 which had been identified in patients with Mandibulofacial dysostosis and microcephaly or type Guion- Almeida
• Guion-Almeida is characterized by severe microcephaly, intellectual disability, choanal, eosophageal and aural atresia, cleft palate and congenital heart defects
• In another study it was investigated that mutation in this gene caused MFD without microcephaly

AIM OF THE STUDY

• They investigated clinical features in patients assessed for Treacher Collins syndrome and performed molecular screening of the four genes: TCOF1, POLR1D, POLR1C, and EFTUD2; in order to establish genotype- phenotype correlations

MATERIALS AND METHODS

• Clinical data and DNA samples were obtained from 146 patients: 71 males and 75 females
• The age range was from birth to 40 years old
• 76% of patients were from France, Guadeloupe and Reunion islands
• 24% from Belgium, Hungary, Lithuania, Morocco, Portugal, Spain and Switzerland
• The physician had to complete a form identifying 19 clinical features

MATERIALS AND METHODS

• The patients were also put into two groups namely: typical and atypical Treacher Collins syndrome (TCS)
• Typical TCS patients presented with the characteristic facial ‘gestalt’, including mandibular and malar hypoplasia, downward slanting palpebral fissures, lower eyelid coloboma and microtia
• Atypical TCS patients were defined by the presence of intellectual disability, microcephaly, anomaly of the limbs, and organ malformation (except cardiac)

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MATERIALS AND METHODS

• In addition they also came up with a severity score for the clinical features
• Mildly affected ≤8, and severely affected ≥9

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MATERIALS AND METHODS

• The patients were screened for the 4 genes using different methods:
• Direct sequencing by Sanger methods
• Multiplex ligation-dependent probe amplification analysis
• Array-comparative genomic hybridization
• Denaturing high pressure liquid chromatography
• A statistical analysis using the chi-square test was performed to establish any relation between two qualitative variables

MATERIALS AND METHODS

• They first investigated variants in the TCOF1 gene by Sanger sequencing
• In the absence of a point mutation, they looked for large rearrangements by Multiplex ligation-dependent probe amplification (MLPA)
• In the absence of molecular anomaly in the TCOF1 gene they investigated the POLR1D and POLR1C genes by Sanger sequencing and MLPA
• If these three genes were normal or if an abnormality was detected by MLPA, they analyzed the patients by array-CGH
• If microcephaly, intellectual disability or esophageal atresia were described, they analyzed the EFTUD2 gene by Sanger sequencing as first line

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RESULTS

RESULTS�

Frequency of clinical findings in patients with mutations in TCOF1, POLR1D , and EFTUD2.

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RESULTS �

• In patients with mutations in TCOF1, 7 had cardiac malformation
• Intellectual disability was present in one patient
• One patient had nail hypoplasia
• Mean severity score for the phenotype in TCOF1 of 7.5
• In POLR1D, mutations one patient had severe scoliosis
• Mean severity score for the phenotype in POLR1D of 7

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CLINICAL DATA

RESULTS�

• Results for all the patients
• They found 101/146 patients positive for TCOF1 and POLR1D and no mutation in POLR1C
• Out of the 11 analyzed for EFTUD2 gene, 4 patients had molecular abnormalities
• 92 patients had mutations in TCOF1 gene and 47 were novel
• 85/92 were predicted to lead to a premature stop codon as result of frameshift mutation

MOLECULAR STUDIES

RESULTS�

• Similar to Teber et al. findings, frameshift mutation was the most frequent with 58/92 patients
• The common 5bp deletion in exon 24 in previous studies occurred in 7/92 of the patients
• 2 patients exhibited large deletions encompassing several genes including TCOF1
• Among the 92 patients with molecular abnormalities in TCOF1, 27/92 were familial cases, 47/92 were sporadic cases, and 29/92 were de novo mutations

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MOLECULAR STUDIES

RESULTS�

• Extreme intrafamilial variability was observed with either mutations in TCOF1 and POLR1D mutations
• There was an inherited mutation in 2 out of 6 patients referred to as sporadic in TCOF1 and 4 in POLR1D
• In Familial cases 18 mutations were inherited from the mother and 6 from the father
• One father had somatic mosaicism; about 30% mosaicism

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MOLECULAR STUDIES

RESULTS�

• Two coding domains were distinguished: LisH exon 1-2 domain and Treacle domain exon 2 -24
• 4 mutations were found in LisH domain and 89/92 were found in the Treacle domain except for a hotspot in exon 24 which had 17/92 of the mutations
• There were 9 patients with anomalies in POLR1D

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MOLECULAR STUDIES

RESULTS�

• No significant association between any of the clinical features and the two genes studied was noted
• There was no significant association between the genotype and the degree of severity
• No specific association occurred when the phenotype was scored as typical or atypical in TCOF1 mutations
• In TCOF1 the severity of the TCS phenotype was not significantly different whatever the mutation

PHENOTYPE GENOTYPE CORRELATION

RESULTS�

• Regarding mutations in LisH or treacle domain there was no association with the clinical features or degree of severity
• Among the 7 patients with cardiac defect, the mutations were all located in the TCOF1 gene
• There were two large deletions identified encompassing TCOF1 and other genes in patients that had typical TCS and Intellectual disability

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PHENOTYPE GENOTYPE CORRELATION

RESULTS �

• 19 SNPs were identified
• One SNP specifically studied (rs28372960) was present in 6 patients but there was no correlation of it with the clinical features or severity of the disease
• All patients with EFTUD2 gene had microcephaly, malar and mandibular hypoplasia, deafness, downward slanting palpebral fissures and microtia
• One patient had esophageal atresia and a complex cardiac malformation
• Patients with mutations in POLR1D had mild features and no life threatening complications

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PHENOTYPE GENOTYPE CORRELATION

Front or lateral view of four patients from the series.

(a) Patient with a heterozygous missense mutation in TCOF1 

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(b) Patient with an intragenic deletion of TCOF1

(c) Patient with a missense mutation in POLR1D 

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(d) Patient with complete deletion of POLR1D 

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RESULTS �

DISCUSSION

• In this study they analyzed clinical data including additional features that had not been previously described and molecular data of 3 genes involved in the syndrome
• A high rate of congenital cardiac defects in patients with mutations in TCOF1 8% were observed
• Intellectual disability(ID) was observed in only one patient hence confirming that (ID) is not a feature of TCS
• Due to extreme intrafamilial variation observed, this study suggests verifying inheritance of the mutation even for asymptomatic parents

DISCUSSION

• The fact that the high rate of SNPs was detected and no association with any of the 19 clinical features suggests the implication of other factors
• In the study by Teber et al. the severity score was based on the overall facial features.
• This study suggested evaluating the severity based on functional impairment and level of intervention required
• Among the 45 Patients with no mutations in TCOF1, POLR1D and POLR1C, 6 patients had typical Treacher Collins syndrome features suggesting implications of additional genes

CRITIQUE

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• Detailed supplementary data that made it easier to understand the paper

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• There was a lot of repeated details in the results and in the discussion.
• The case of somatic mosaicism was not clear in the graph
• The total number patients kept on changing with the clinical features observed

POSITIVE

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NEGATIVE

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