Risks and Recommendations in Prenatally Detected De Novo Balanced Chromosomal Rearrangements from Assessment of Long-Term Outcomes.

The 6%-9% risk of an untoward outcome previously established by Warburton for prenatally detected de novo balanced chromosomal rearrangements (BCRs) does not account for long-term morbidity. We performed long-term follow-up (mean 17 years) of a registry-based nationwide cohort of 41 individuals carrying a prenatally detected de novo BCR with normal first trimester screening/ultrasound scan. We observed a significantly higher frequency of neurodevelopmental and/or neuropsychiatric disorders than in a matched control group (19.5% versus 8.3%, p = 0.04), which was increased to 26.8% upon clinical follow-up. Chromosomal microarray of 32 carriers revealed no pathogenic imbalances, illustrating a low prognostic value when fetal ultrasound scan is normal. In contrast, mate-pair sequencing revealed disrupted genes (ARID1B, NPAS3, CELF4), regulatory domains of known developmental genes (ZEB2, HOXC), and complex BCRs associated with adverse outcomes. Seven unmappable autosomal-autosomal BCRs with breakpoints involving pericentromeric/heterochromatic regions may represent a low-risk group. We performed independent phenotype-aware and blinded interpretation, which accurately predicted benign outcomes (specificity = 100%) but demonstrated relatively low sensitivity for prediction of the clinical outcome in affected carriers (sensitivity = 45%-55%). This sensitivity emphasizes the challenges associated with prenatal risk prediction for long-term morbidity in the absence of phenotypic data given the still immature annotation of the morbidity genome and poorly understood long-range regulatory mechanisms. In conclusion, we upwardly revise the previous estimates of Warburton to a morbidity risk of 27% and recommend sequencing of the chromosomal breakpoints as the first-tier diagnostic test in pregnancies with a de novo BCR.

BBS Proteins Affect Ciliogenesis and Are Essential for Hedgehog Signaling, but Not for Formation of iPSC-Derived RPE-65 Expressing RPE-Like Cells.

Bardet-Biedl syndrome (BBS) is a ciliopathy characterized by retinal dystrophy, renal cysts, obesity and polydactyly. BBS genes have been implicated in ciliogenesis, hedgehog signaling and retinal pigment epithelium maturation. BBS1 and BBS5 are members of the BBSome, implicated in cilia transport of proteins, and BBS10 is a member of the chaperonin-complex, mediating BBSome assembly. In this study, involvement of BBS1, BBS5 and BBS10 in ciliogenesis and hedgehog signaling were investigated in BBS-defective patient fibroblasts as well as in RPE-hTERT cells following siRNA-mediated knockdown of the BBS genes. Furthermore, the ability of BBS1-defective induced pluripotent stem-cells (iPSCs) to differentiate into RPE cells was assessed. We report that cells lacking functional BBS5 or BBS10 have a reduced number of primary cilia, whereas cells lacking functional BBS1 display shorter primary cilia compared to wild-type cells. Hedgehog signaling was substantially impaired and Smoothened, a component of hedgehog signaling, was trapped inside the cilia of the BBS-defective cells, even in the absence of Smoothened agonist. Preliminary results demonstrated the ability of BBS1-defective iPSC to differentiate into RPE-65 expressing RPE-like cells. The BBS1-/--defective RPE-like cells were less pigmented, compared to RPE-like cells differentiated from control iPSCs, indicating an impact of BBS1 on RPE maturation.

Mosaic MECP2 variants in males with classical Rett syndrome features, including stereotypical hand movements.

Rett syndrome is rarely suspected in males because of the X-linked dominant inheritance. In the literature, only six male patients have been reported with methyl-CpG-binding protein 2 (MECP2) mosaicism. Next-generation sequencing (NGS) methods have enabled better detection of somatic mosaicism compared to conventional Sanger sequencing; however, mosaics can still be difficult to detect. We present clinical and molecular findings in two males mosaic for a pathogenic MECP2 variant. Both have been reexamined using deep sequencing of DNA isolated from four different cell tissues (blood, muscle, fibroblasts and oral mucosa). Deep sequencing of the different tissues revealed that the variants were present in all tissues. In one patient, the molecular diagnosis could only be established by reexamination after a normal whole exome sequencing, and the other case is an example of reverse genetic diagnostics. Rett syndrome should be considered in males with neurodevelopmental delay and stereotypical hand movements. Subsequent to clinical diagnosis males should be investigated with NGS-based technologies of MECP2 with high read depth and a low threshold for variant calls. If the initial analysis on full blood derived DNA fails to confirm the suspicion, we recommend repeating the analysis on another tissue, preferentially fibroblasts to increase the diagnostic yield.

A Register-Based Study of Diseases With an Autosomal Recessive Origin in Small Children in Denmark According to Maternal Country of Origin.

BACKGROUND: Compared with children born of Danish mothers, the mortality of children, born and living in Denmark, is significantly increased in those with a mother from Afghanistan, Iraq, Pakistan, Somalia, and Turkey. Consanguinity has been suggested to account for part of this disparity. Since information on consanguinity is lacking, this suggestion is difficult to test. With an indirect approach, we addressed this question by comparing the risk of diseases with autosomal recessive inheritance in children born in Denmark of Danish-born women and of women born in these five countries, respectively. METHODS: All children born in Denmark (1994-2010) were followed until 5 years of age or end-of-study period for the risk of hospitalisation with diseases of autosomal recessive aetiology, and therefore considered consanguinity-related. Diagnoses of autosomal recessive diseases were identified using two different methods: a literature review of consanguinity-associated diseases and a search in the Online Catalogue of Human Genes and Genetic Disorders. Risks were also calculated for diseases with known non-autosomal recessive aetiology (considered non-consanguinity-related). We estimated adjusted hazard ratios for the diseases in children of foreign-born women compared with children of Danish-born women. RESULTS: Compared with offspring of Danish-born women, the risk of a consanguinity-related disease was significantly increased in children of foreign-born women, although the absolute risk was low. The risk of non-consanguinity-related diseases did not differ between the groups compared. CONCLUSIONS: The findings support the hypothesis that consanguinity accounts for some, however a minor part, of the disparity in child mortality among migrants in Denmark.

Low bone turnover phenotype in Rett syndrome: results of biochemical bone marker analysis.

BACKGROUND: Patients with Rett syndrome (RTT) are at risk of having low bone mass and low-energy fractures. METHODS: We characterized bone metabolism by both bone formation and resorption markers in blood in a RTT population of 61 girls and women and 122 well-matched healthy controls. Levels of N-terminal propeptides of collagen type 1 (P1NP), C-terminal telopeptide cross links (CTX), osteocalcin (OC), and bone-specific alkaline phosphatase (B-ALP) were compared between RTT patients and controls in regression models adjusted for BMI, vitamin D status, volumetric bone mineral apparent density of the lumbar spine (vBMAD spine), and femoral neck (vBMAD neck). We examined biochemical bone marker levels overall and stratified to persons younger than age 25 y or equal to or older than age 25 y. RESULTS: The RTT patients had reduced levels of all biochemical bone markers (P < 0.05), which remained significant in persons younger than 25 y (P ≤ 0.001) regarding P1NP, CTX, and OC. Bone marker levels were not significantly associated to methyl-CpG-binding protein 2 (MECP2) mutation group, walking ability, or previous low-energy fractures. CONCLUSION: Our findings of a low bone turnover state in girls with RTT suggest critical attention to medical treatment of low bone mass in young RTT patients.

Chromosomal rearrangements in Tourette syndrome: implications for identification of candidate susceptibility genes and review of the literature.

Tourette syndrome (TS) is a childhood-onset complex neurobiological disorder characterized by a combination of persistent motor and vocal tics and frequent presence of other neuropsychiatric comorbidities. TS shares the fate of other complex disorders, where the genetic etiology is largely unknown, and identification of susceptibility genes through linkage and association studies has been complicated due to inherent difficulties such as no clear mode of inheritance, genetic heterogeneity, and apparently incomplete penetrance. Positional cloning through mapping of disease-related chromosome rearrangements has been an efficient tool for the cloning of disease genes in several Mendelian disorders and in a number of complex disorders. Through cytogenetic investigation of 205 TS patients, we identified three possibly disease-associated chromosome rearrangements rendering this approach relevant in chasing TS susceptibility genes.

Segregation of a 4p16.3 duplication with a characteristic appearance, macrocephaly, speech delay and mild intellectual disability in a 3-generation family.

Microscopically visible rearrangements of chromosome 4p includes the two well known abnormalities: partial trisomy 4p, and deletions of the Wolf-Hirschhorn critical regions 1 and 2 (WHSCR 1 and WHSCR2, respectively), which cause well-defined phenotypes including minor anomalies, and developmental delay/intellectual disability. In contrast small duplications of 4p are rare but with the advent of microarray techniques a few cases have been reported in recent years. Here we describe a 3 Mb duplication at 4p16.3 segregating with a characteristic phenotype, macrocephaly, speech delay and mild intellectual disability in a three generation family.

Microduplication of 15q13.3 and Xq21.31 in a family with Tourette syndrome and comorbidities.

Tourette syndrome (TS) is a childhood onset neurodevelopmental disorder. Although it is widely accepted that genetic factors play a significant role in TS pathogenesis the etiology of this disorder is largely unknown. Identification of rare copy number variations (CNVs) as susceptibility factors in several neuropsychiatric disorders such as attention deficit-hyperactivity disorder (ADHD), autism and schizophrenia, suggests involvement of these rare structural changes also in TS etiology. In a male patient with TS, ADHD, and OCD (obsessive compulsive disorder) we identified two microduplications (at 15q13.3 and Xq21.31) inherited from a mother with subclinical ADHD. The 15q duplication included the CHRNA7 gene; while two genes, PABPC5 and PCDH11X, were within the Xq duplication. The Xq21.31 duplication was present in three brothers with TS including the proband, but not in an unaffected brother, whereas the 15q duplication was present only in the proband and his mother. The structural variations observed in this family may contribute to the observed symptoms, but further studies are necessary to investigate the possible involvement of the described variations in the TS etiology.

Dominant optic atrophy in Denmark - report of 15 novel mutations in OPA1, using a strategy with a detection rate of 90%.

BACKGROUND: Investigation of the OPA1 mutation spectrum in autosomal dominant optic atrophy (ADOA) in Denmark. METHODS: Index patients from 93 unrelated ADOA families were assessed for a common Danish founder mutation (c.2826_2836delinsGGATGCTCCA) inOPA1. If negative, direct DNA sequencing of the coding sequence and multiplex ligation-dependent probe amplification (MLPA) were performed. Results from MLPA analysis have been previously reported. Haplotype analysis was carried out analysing single nucleotide polymorphisms (SNP). Retrospective clinical data were retrieved from medical files. RESULTS: Probably causative mutations were identified in 84 out of 93 families (90%) including 15 novel mutations. Three mutations c.983A > G, c.2708_2711delTTAG and c.2826_2836delinsGGATGCTCCA, were responsible for ADOA in10, 11 and 28 families, respectively, corresponding to 11%, 12% and 30%. A common haplotype in nine of ten c.983A > G families suggests that they descend from a single founder. The c.2708_2711delTTAG mutation was present on at least two haplotypes and has been repeatedly reported in various ethnic groups,thus represents a mutational hotspot. Clinical examinations of index patients with the two latter mutations demonstrated large inter- and intra-familial variations apparently. CONCLUSIONS: Genetic testing for OPA1mutations assist in the diagnosis. We have identified mutations in OPA1 in 90% of families including 15 novel mutations. Both DNA sequencing and MLPA analysis are necessary to achieve a high detection rate. More than half of the affected families in Denmark are represented by three common mutations, at least two of which are due to a founder effect, which may account for the high prevalence of ADOA in Denmark.

Partial duplication of 13q31.3-q34 and deletion of 13q34 associated with diaphragmatic hernia as a sole malformation in a fetus.

Partial duplications and deletions of chromosome 13 are rare and the phenotypic expressions of both aneuploidies are highly variable. Here we report on a fetus diagnosed prenatally with partial trisomy of 13q and a diaphragmatic hernia as a sole malformation. The parents had decided to terminate the pregnancy after the finding of diaphragmatic hernia by ultrasound scan, which was also confirmed by autopsy of the fetus. Subsequently chromosome analysis, fluorescence in situ hybridization (FISH), and array comparative genomic hybridization (array CGH) was carried out on fetal tissue. The chromosome analysis revealed additional material on chromosome 13, which was shown to be from the same chromosome, by FISH analysis. Array CGH demonstrated a partial duplication and a small deletion at the distal long arm of chromosome 13. The parents had normal karyotypes. This is the first case of a de novo pure partial duplication of 13q31.3-q34 and distal deletion of 13q34 with a phenotype apparently only involving a diaphragmatic hernia and three lung lobes on both sides. Microarray analysis was useful in refining the chromosomal imbalance and suggesting a candidate region for diaphragmatic hernia.

Deletions and rearrangements of the H19/IGF2 enhancer region in patients with Silver-Russell syndrome and growth retardation.

Silver-Russell syndrome (SRS) is characterised by prenatal and postnatal growth retardation, dysmorphic facial features, and body asymmetry. In 35-60% of SRS cases the paternally methylated imprinting control region (ICR) upstream of the H19 gene (H19-ICR) is hypomethylated, leading to downregulation of IGF2 and bi-allelic expression of H19. H19 and IGF2 are reciprocally imprinted genes on chromosome 11p15. The expression is regulated by the imprinted methylation of the ICR, which modulates the transcription of H19 and IGF2 facilitated by enhancers downstream of H19. A promoter element of IGF2, IGF2P0, is differentially methylated equivalently to the H19-ICR, though in a small number of SRS cases this association is disrupted--that is, hypomethylation affects either H19-ICR or IGF2P0. Three pedigrees associated with hypomethylation of IGF2P0 in the probands are presented here, two with paternally derived deletions, and one with a balanced translocation of inferred paternal origin. They all have a breakpoint within the H19/IGF2 enhancer region. One proband has severe growth retardation, the others have SRS. This is the first report of paternally derived structural chromosomal mutations in 11p15 causing SRS. These cases define a novel aetiology of the growth retardation in SRS, namely, dissociation of IGF2 from its enhancers.

The effect of casein glycomacropeptide versus free synthetic amino acids for early treatment of phenylketonuria in a mice model.

INTRODUCTION: Management of phenylketonuria (PKU) is mainly achieved through dietary control with limited intake of phenylalanine (Phe) from food, supplemented with low protein (LP) food and a mixture of free synthetic (FS) amino acids (AA) (FSAA). Casein glycomacropeptide (CGMP) is a natural peptide released in whey during cheese making by the action of the enzyme chymosin. Because CGMP in its pure form does not contain Phe, it is nutritionally suitable as a supplement in the diet for PKU when enriched with specific AAs. Lacprodan® CGMP-20 (= CGMP) used in this study contained only trace amounts of Phe due to minor presence of other proteins/peptides. OBJECTIVE: The aims were to address the following questions in a classical PKU mouse model: Study 1, off diet: Can pure CGMP or CGMP supplemented with Large Neutral Amino Acids (LNAA) as a supplement to normal diet significantly lower the content of Phe in the brain compared to a control group on normal diet, and does supplementation of selected LNAA results in significant lower brain Phe level?. Study 2, on diet: Does a combination of CGMP, essential (non-Phe) EAAs and LP diet, provide similar plasma and brain Phe levels, growth and behavioral skills as a formula which alone consist of FSAA, with a similar composition?. MATERIAL AND METHODS: 45 female mice homozygous for the Pahenu2 mutation were treated for 12 weeks in five different groups; G1(N-CGMP), fed on Normal (N) casein diet (75%) in combination with CGMP (25%); G2 (N-CGMP-LNAA), fed on Normal (N) casein diet (75%) in combination with CGMP (19,7%) and selected LNAA (5,3% Leu, Tyr and Trp); G3 (N), fed on normal casein diet (100%); G4 (CGMP-EAA-LP), fed on CGMP (70,4%) in combination with essential AA (19,6%) and LP diet; G5 (FSAA-LP), fed on FSAA (100%) and LP diet. The following parameters were measured during the treatment period: Plasma AA profiles including Phe and Tyr, growth, food and water intake and number of teeth cut. At the end of the treatment period, a body scan (fat and lean body mass) and a behavioral test (Barnes Maze) were performed. Finally, the brains were examined for content of Phe, Tyr, Trp, dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), serotonin (5-HT) and 5-hydroxyindole-acetic acid (5-HIAA), and the bone density and bone mineral content were determined by dual-energy x-ray absorptiometry. RESULTS: Study 1: Mice off diet supplemented with CGMP (G1 (N-CGMP)) or supplemented with CGMP in combination with LNAA (G2 (N-CGMP-LNAA)) had significantly lower Phe in plasma and in the brain compared to mice fed only casein (G3 (N)). Extra LNAA (Tyr, Trp and Leu) to CGMP did not have any significant impact on Phe levels in the plasma and brain, but an increase in serotonin was measured in the brain of G2 mice compared to G1. Study 2: PKU mice fed with mixture of CGMP and EAA as supplement to LP diet (G4 (CGMP-EAA-LP)) demonstrated lower plasma-Phe levels but similar brain- Phe levels and growth as mice fed on an almost identical combination of FSAA (G5 (FSAA-LP)). CONCLUSION: CGMP can be a relevant supplement for the treatment of PKU.

Genomic deletions in OPA1 in Danish patients with autosomal dominant optic atrophy.

BACKGROUND: Autosomal dominant optic atrophy (ADOA, Kjer disease, MIM #165500) is the most common form of hereditary optic neuropathy. Mutations in OPA1 located at chromosome 3q28 are the predominant cause for ADOA explaining between 32 and 89% of cases. Although deletions of OPA1 were recently reported in ADOA, the frequency of OPA1 genomic rearrangements in Denmark, where ADOA has a high prevalence, is unknown. The aim of the study was to identify copy number variations in OPA1 in Danish ADOA patients. METHODS: Forty unrelated ADOA patients, selected from a group of 100 ADOA patients as being negative for OPA1 point mutations, were tested for genomic rearrangements in OPA1 by multiplex ligation probe amplification (MLPA). When only one probe was abnormal results were confirmed by additional manually added probes. Segregation analysis was performed in families with detected mutations when possible. RESULTS: Ten families had OPA1 deletions, including two with deletions of the entire coding region and eight with intragenic deletions. Segregation analysis was possible in five families, and showed that the deletions segregated with the disease. CONCLUSION: Deletions in the OPA1 gene were found in 10 patients presenting with phenotypic autosomal dominant optic neuropathy. Genetic testing for deletions in OPA1 should be offered for patients with clinically diagnosed ADOA and no OPA1 mutations detected by DNA sequencing analysis.

Interstitial deletion of 14q24.3-q32.2 in a male patient with plagiocephaly, BPES features, developmental delay, and congenital heart defects.

Distal interstitial deletions of chromosome 14 involving the 14q24-q23.2 region are rare, and only been reported so far in 20 patients. Ten of these patients were analyzed both clinically and genetically. Here we present a de novo interstitial deletion of chromosome 14q24.3-q32.2 in a male patient with developmental delay, language impairment, plagiocephaly, BPES features (blepharophimosis, ptosis, epicanthus), and congenital heart defect. The deletion breakpoints were fine mapped using fluorescence in situ hybridization (FISH) and the size of the deletion is estimated to be approximately 23 Mb. Based on genotype-phenotype comparisons of the 10 previously published patients and the present case, we suggest that the shortest regions for deletion overlap may include candidate genes for speech impairment, mental retardation, and hypotonia.

Two new cases with microdeletion of 17q23.2 suggest presence of a candidate gene for sensorineural hearing loss within this region.

Microdeletion of the 17q23.2 region has very recently been suggested as a new emerging syndrome based on the finding of 8 cases with common phenotypes including mild-to-moderate developmental delay, heart defects, microcephaly, postnatal growth retardation, and hand, foot, and limb abnormalities. In this report, we describe two new 17q23.2 deletion patients with mild intellectual disability and sensorineural hearing loss. They both had submicroscopic deletions smaller than the common deleted region for the 8 previously described 17q23.2 microdeletion cases. TBX4 was previously suggested as the responsible gene for the heart or limb defects observed in 17q23.2 deletion patients, but the present cases do not have these features despite deletion of this gene. The finding of sensorineural hearing loss in 5 of the 10 cases, including the present cases, with a microdeletion at17q23.2, strongly suggests the presence of a candidate gene for hearing loss within this region. We screened 41 patients with profound sensorineural hearing loss for mutations of TBX2 and detected no mutations.

Patients with Rett syndrome sustain low-energy fractures.

We present the first case-control study addressing both fracture occurrence and fracture mechanisms in Rett syndrome (RTT). Two previous studies have shown increased fracture risk in RTT. This was also our hypothesis regarding the Danish RTT population. Therefore, we investigated risk factors associated with low-energy trauma and the association to methyl-CpG-binding protein 2 (MECP2) mutations. A total of 61 female patients with RTT and 122 healthy controls matched according to age and pubertal/menopause status were examined by questionnaires, bone biochemical markers in blood, and clinical and x-ray evaluations. National register search on fracture diagnoses was done to obtain complete fracture histories. Our results showed that patients with RTT sustained significantly more low-energy fractures from early age compared with controls, even though overall fracture occurrence apparently was not increased. Low-energy fractures were significantly associated with less mobility and lack of ambulation. Associations with MECP2 mutations or epilepsy were not demonstrated, contrary to previous findings. Blood biochemistry indicated a possible need for D vitamin supplementation in RTT. Our study casts light on fracture occurrence in RTT and points to a need for future research in bone development and fracture risk to establish directions for improved prevention and treatment of low-energy fractures in RTT.

Bardet-Biedl syndrome in Denmark--report of 13 novel sequence variations in six genes.

Bardet-Biedl syndrome (BBS) is an autosomal recessive disease characterized by retinal dystrophy, polydactyly, obesity, learning disabilities, renal involvement, and male hypogenitalism. BBS is genetically heterogeneous with mutations of 14 genes, accounting for approximately 70% of cases. Triallelic inheritance has been suggested in about 5% of cases. Forty-nine unrelated BBS patients were screened for mutations by DHPLC analysis in BBS1, BBS2, BBS4, BBS6/MKKS, BBS10, and BBS12. The selected genes either account for more than 5% of the mutational load or are commonly reported in triallelic inheritance. Eight patients with only one or no BBS mutation were further investigated by single nucleotide polymorphism (SNP) analysis. In total, mutations were detected in 44 patients. Twenty percent had two mutations in BBS1, 18% in BBS2, 4% in BBS9, 43% in BBS10, and 2% in BBS12. Five patients were heterozygous for a sequence variation in BBS6/MKKS. We found eight patients with three sequence variations in two genes, which could be explained by triallelic inheritance, by the prevalence of heterozygous carriers or the third sequence variations representing rare polymorphisms. All changes found in a second BBS gene were amino acid substitutions. Genotype-phenotype correlations suggest a milder phenotype for BBS1 compared to BBS2 and BBS10, which we ascribe to the hypomorphic p.Met390Arg-mutation.

Screening for copy number alterations in loci associated with autism spectrum disorders by two-color multiplex ligation-dependent probe amplification.

The autism spectrum disorder (ASD) is a heterogenous condition characterized by impaired socialization and communication in association with stereotypic behaviors. ASD is highly heritable and heterogeneous with a complex genetic etiology. Recurrent submicroscopic deletions or duplications have been identified in a subgroup of individuals with ASD using array technology. Adequate genetic testing for these genomic imbalances have not yet been widely implemented in the diagnostic setting due to lack of feasible and cost-effective methods as well as difficulties to interpret the clinical significance of these small copy number variants (CNVs). We developed a multiplex ligation-dependent probe amplification (MLPA) assay to investigate its usefulness for detection of copy number alterations (CNAs) in autistic patients. This test proved to be easy to perform, fast, cost-effective, and suitable for reliable detection of multiple loci in a single reaction. We screened 148 autistic patients for 15 different loci covering 26 genes and found a 15q11-13 interstitial duplication that had escaped detection by conventional karyotyping in 1.3% of the patients. Synthetic probe MLPA allows for a flexible analysis of a continuously increasing number of CNAs associated with autism. Our result show that MLPA assay is an easy and cost-effective method for the identification of selected CNAs in diagnostic laboratories.

Bi-Allelic Pathogenic Variations in MERTK Including Deletions Are Associated with an Early Onset Progressive Form of Retinitis Pigmentosa.

Bi-allelic pathogenic variants in MERTK cause retinitis pigmentosa (RP). Since deletions of more than one exon have been reported repeatedly for MERTK, CNV (copy number variation) analysis of next-generation sequencing (NGS) data has proven important in molecular genetic diagnostics of MERTK. CNV analysis was performed on NGS data of 677 individuals with inherited retinal diseases (IRD) and confirmed by quantitative RT-PCR analysis. Clinical evaluation was based on retrospective records. Clinical re-examination included visual field examination, dark adaption, scotopic and photopic full-field electroretinograms (ffERG), multifocal ERG (mfERG) and optic coherence tomography (OCT). Fourteen variants were detected in MERTK in six individuals, three of which were deletions of more than one exon. Clinical examinations of five out of six individuals revealed a severe phenotype with early-onset generalized retinal dystrophy with night blindness and progressive visual field loss; however, one individual had a milder phenotype. Three individuals had hearing impairments. We show that deletions represent a substantial part of the causative variants in MERTK and emphasize that CNV analysis should be included in the molecular genetic diagnostics of IRDs.