CAPRIN1 haploinsufficiency causes a neurodevelopmental disorder with language impairment, ADHD and ASD.

We describe an autosomal dominant disorder associated with loss-of-function variants in the Cell cycle associated protein 1 (CAPRIN1; MIM*601178). CAPRIN1 encodes a ubiquitous protein that regulates the transport and translation of neuronal mRNAs critical for synaptic plasticity, as well as mRNAs encoding proteins important for cell proliferation and migration in multiple cell types. We identified 12 cases with loss-of-function CAPRIN1 variants, and a neurodevelopmental phenotype characterized by language impairment/speech delay (100%), intellectual disability (83%), attention deficit hyperactivity disorder (82%) and autism spectrum disorder (67%). Affected individuals also had respiratory problems (50%), limb/skeletal anomalies (50%), developmental delay (42%) feeding difficulties (33%), seizures (33%) and ophthalmologic problems (33%). In patient-derived lymphoblasts and fibroblasts, we showed a monoallelic expression of the wild-type allele, and a reduction of the transcript and protein compatible with a half dose. To further study pathogenic mechanisms, we generated sCAPRIN1+/- human induced pluripotent stem cells via CRISPR-Cas9 mutagenesis and differentiated them into neuronal progenitor cells and cortical neurons. CAPRIN1 loss caused reduced neuronal processes, overall disruption of the neuronal organization and an increased neuronal degeneration. We also observed an alteration of mRNA translation in CAPRIN1+/- neurons, compatible with its suggested function as translational inhibitor. CAPRIN1+/- neurons also showed an impaired calcium signalling and increased oxidative stress, two mechanisms that may directly affect neuronal networks development, maintenance and function. According to what was previously observed in the mouse model, measurements of activity in CAPRIN1+/- neurons via micro-electrode arrays indicated lower spike rates and bursts, with an overall reduced activity. In conclusion, we demonstrate that CAPRIN1 haploinsufficiency causes a novel autosomal dominant neurodevelopmental disorder and identify morphological and functional alterations associated with this disorder in human neuronal models.

FOXG1 variants can be associated with milder phenotypes than congenital Rett syndrome with unassisted walking and language development.

Since 2008, FOXG1 haploinsufficiency has been linked to a severe neurodevelopmental phenotype resembling Rett syndrome but with earlier onset. Most patients are unable to sit, walk, or speak. For years, FOXG1 sequencing was only prescribed in such severe cases, limiting insight into the full clinical spectrum associated with this gene. Next-generation sequencing (NGS) now enables unbiased diagnostics. Through the European Reference Network for Rare Malformation Syndromes, Intellectual and Other Neurodevelopmental Disorders, we gathered data from patients with heterozygous FOXG1 variants presenting a mild phenotype, defined as able to speak and walk independently. We also reviewed data from three previously reported patients meeting our criteria. We identified five new patients with pathogenic FOXG1 missense variants, primarily in the forkhead domain, showing varying nonspecific intellectual disability and developmental delay. These features are not typical of congenital Rett syndrome and were rarely associated with microcephaly and epilepsy. Our findings are consistent with a previous genotype-phenotype analysis by Mitter et al. suggesting the delineation of five different FOXG1 genotype groups. Milder phenotypes were associated with missense variants in the forkhead domain. This information may facilitate prognostic assessments in children carrying a FOXG1 variant and improve the interpretation of new variants identified with genomic sequencing.

YWHAE loss of function causes a rare neurodevelopmental disease with brain abnormalities in human and mouse.

PURPOSE: Miller-Dieker syndrome is caused by a multiple gene deletion, including PAFAH1B1 and YWHAE. Although deletion of PAFAH1B1 causes lissencephaly unambiguously, deletion of YWHAE alone has not clearly been linked to a human disorder. METHODS: Cases with YWHAE variants were collected through international data sharing networks. To address the specific impact of YWHAE loss of function, we phenotyped a mouse knockout of Ywhae. RESULTS: We report a series of 10 individuals with heterozygous loss-of-function YWHAE variants (3 single-nucleotide variants and 7 deletions <1 Mb encompassing YWHAE but not PAFAH1B1), including 8 new cases and 2 follow-ups, added with 5 cases (copy number variants) from literature review. Although, until now, only 1 intragenic deletion has been described in YWHAE, we report 4 new variants specifically in YWHAE (3 splice variants and 1 intragenic deletion). The most frequent manifestations are developmental delay, delayed speech, seizures, and brain malformations, including corpus callosum hypoplasia, delayed myelination, and ventricular dilatation. Individuals with variants affecting YWHAE alone have milder features than those with larger deletions. Neuroanatomical studies in Ywhae-/- mice revealed brain structural defects, including thin cerebral cortex, corpus callosum dysgenesis, and hydrocephalus paralleling those seen in humans. CONCLUSION: This study further demonstrates that YWHAE loss-of-function variants cause a neurodevelopmental disease with brain abnormalities.

A Solve-RD ClinVar-based reanalysis of 1522 index cases from ERN-ITHACA reveals common pitfalls and misinterpretations in exome sequencing.

PURPOSE: Within the Solve-RD project (https://solve-rd.eu/), the European Reference Network for Intellectual disability, TeleHealth, Autism and Congenital Anomalies aimed to investigate whether a reanalysis of exomes from unsolved cases based on ClinVar annotations could establish additional diagnoses. We present the results of the "ClinVar low-hanging fruit" reanalysis, reasons for the failure of previous analyses, and lessons learned. METHODS: Data from the first 3576 exomes (1522 probands and 2054 relatives) collected from European Reference Network for Intellectual disability, TeleHealth, Autism and Congenital Anomalies was reanalyzed by the Solve-RD consortium by evaluating for the presence of single-nucleotide variant, and small insertions and deletions already reported as (likely) pathogenic in ClinVar. Variants were filtered according to frequency, genotype, and mode of inheritance and reinterpreted. RESULTS: We identified causal variants in 59 cases (3.9%), 50 of them also raised by other approaches and 9 leading to new diagnoses, highlighting interpretation challenges: variants in genes not known to be involved in human disease at the time of the first analysis, misleading genotypes, or variants undetected by local pipelines (variants in off-target regions, low quality filters, low allelic balance, or high frequency). CONCLUSION: The "ClinVar low-hanging fruit" analysis represents an effective, fast, and easy approach to recover causal variants from exome sequencing data, herewith contributing to the reduction of the diagnostic deadlock.

IQSEC2 disorder: A new disease entity or a Rett spectrum continuum?

IQSEC2 mutations are associated with IQSEC2-related intellectual disability (ID). Phenotypic spectrum has been better defined in the last few years by the increasing number of reported cases although the genotype-phenotype relationship for IQSEC2 remains overall complex. As for IQSEC2-related ID a wide phenotypic diversity has been described in Rett syndrome (RTT). Several patients harboring IQSEC2 mutations present with clinical symptoms similar to RTT and some cases meet most of the criteria for classic RTT. With the aim of establishing a genotype-phenotype correlation, we collected data of 16 patients harboring IQSEC2 point mutations (15 of them previously unreported) and of five novel patients carrying CNVs encompassing IQSEC2. Most of our patients surprisingly shared a moderate-to-mild phenotype. The similarities in the clinical course between our mild cases and patients with milder forms of atypical RTT reinforce the hypothesis that also IQSEC2 mutated patients may lay under the wide clinical spectrum of RTT and thus IQSEC2 should be considered in the differential diagnosis. Our data confirm that position, type of variant and gender are crucial for IQSEC2-associated phenotype delineation.

New Candidates for Autism/Intellectual Disability Identified by Whole-Exome Sequencing.

Intellectual disability (ID) is characterized by impairments in the cognitive processes and in the tasks of daily life. It encompasses a clinically and genetically heterogeneous group of neurodevelopmental disorders often associated with autism spectrum disorder (ASD). Social and communication abilities are strongly compromised in ASD. The prevalence of ID/ASD is 1-3%, and approximately 30% of the patients remain without a molecular diagnosis. Considering the extreme genetic locus heterogeneity, next-generation sequencing approaches have provided powerful tools for candidate gene identification. Molecular diagnosis is crucial to improve outcome, prevent complications, and hopefully start a therapeutic approach. Here, we performed parent-offspring trio whole-exome sequencing (WES) in a cohort of 60 mostly syndromic ID/ASD patients and we detected 8 pathogenic variants in genes already known to be associated with ID/ASD (SYNGAP1, SMAD6, PACS1, SHANK3, KMT2A, KCNQ2, ACTB, and POGZ). We found four de novo disruptive variants of four novel candidate ASD/ID genes: MBP, PCDHA1, PCDH15, PDPR. We additionally selected via bioinformatic tools many variants in unknown genes that alone or in combination can contribute to the phenotype. In conclusion, our data confirm the efficacy of WES in detecting pathogenic variants of known and novel ID/ASD genes.

17p13.3 microdeletion including YWHAE and CRK genes: towards a clinical characterization.

INTRODUCTION: The short arm of chromosome 17 is characterized by a high density of low copy repeats, creating the opportunity for non-allelic homologous recombination to occur. Microdeletions of the 17p13.3 region are responsible for neuronal migration disorders including isolated lissencephaly sequence and Miller-Dieker syndrome. CASE REPORT: We describe the case of a 4-year and 2-month-old female with peculiar somatic traits and neurodevelopmental delay. At the age of 6 months, she started to present with infantile spasms syndrome; therefore, we administered vigabatrin followed by two cycles of adrenocorticotropic hormone, with good response. The coexistence of epileptic activity, neuropsychological delay, brain imaging abnormalities, and peculiar somatic features oriented us towards the hypothesis of a genetic etiology that could explain her clinical picture. Array CGH identified a 730 Kb deletion in the p13.3 region of the short arm of chromosome 17 including eleven genes, among these are YWHAE and CRK. DISCUSSION: Microdeletions of the 17p13.3 region involving only YWHAE and CRK, sparing PAFAH1B1, result in neurodevelopmental delay, growth retardation, craniofacial dysmorphisms, and mild structural brain abnormalities. Differently from the previously described patients carrying YWHAE and CRK deletions, the main complaint of our patient was represented by seizures. The absence of clear neuronal migration defects and mutations of the PAFAH1B1 gene in our patient underlines the central role of additional genes located in the 17p13.3 chromosomal region in the pathogenesis of epilepsy and helps to expand the phenotype of 17p13.3 microdeletion syndrome.

Clinical Features at Onset and Genetic Characterization of Pediatric and Adult Patients with TNF-α Receptor-Associated Periodic Syndrome (TRAPS): A Series of 80 Cases from the AIDA Network.

This study explores demographic, clinical, and therapeutic features of tumor necrosis factor receptor-associated periodic syndrome (TRAPS) in a cohort of 80 patients recruited from 19 Italian referral Centers. Patients' data were collected retrospectively and then analyzed according to age groups (disease onset before or after 16 years) and genotype (high penetrance (HP) and low penetrance (LP) TNFRSF1A gene variants). Pediatric- and adult-onset were reported, respectively, in 44 and 36 patients; HP and LP variants were found, respectively, in 32 and 44 cases. A positive family history for recurrent fever was reported more frequently in the pediatric group than in the adult group (p < 0.05). With reference to clinical features during attacks, pericarditis and myalgia were reported more frequently in the context of adult-onset disease than in the pediatric age (with p < 0.01 and p < 0.05, respectively), while abdominal pain was present in 84% of children and in 25% of adults (p < 0.01). Abdominal pain was significantly associated also to the presence of HP mutations (p < 0.01), while oral aphthosis was more frequently found in the LP variant group (p < 0.05). Systemic amyloidosis occurred in 25% of subjects carrying HP variants. As concerns laboratory features, HP mutations were significantly associated to higher ESR values (p < 0.01) and to the persistence of steadily elevated inflammatory markers during asymptomatic periods (p < 0.05). The presence of mutations involving a cysteine residue, abdominal pain, and lymphadenopathy during flares significantly correlated with the risk of developing amyloidosis and renal impairment. Conversely, the administration of colchicine negatively correlated to the development of pathologic proteinuria (p < 0.05). Both NSAIDs and colchicine were used as monotherapy more frequently in the LP group compared to the HP group (p < 0.01). Biologic agents were prescribed to 49 (61%) patients; R92Q subjects were more frequently on NSAIDs monotherapy than other patients (p < 0.01); nevertheless, they required biologic therapy in 53.1% of cases. At disease onset, the latest classification criteria for TRAPS were fulfilled by 64/80 (80%) patients (clinical plus genetic items) and 46/80 (57.5%) patients (clinical items only). No statistically significant differences were found in the sensitivity of the classification criteria according to age at onset and according to genotype (p < 0.05). This study describes one of the widest cohorts of TRAPS patients in the literature, suggesting that the clinical expression of this syndrome is more influenced by the penetrance of the mutation rather than by the age at onset itself. Given the high phenotypic heterogeneity of the disease, a definite diagnosis should rely on both accurate working clinical assessment and complementary genotype.

PIK3CA-CDKN2A clonal evolution in metastatic breast cancer and multiple points cell-free DNA analysis.

BACKGROUND: Daily experience tells us that breast cancer can be controlled using standard protocols up to the advent of a relapse. Now new frontiers in precision medicine like liquid biopsy of cell free DNA (cfDNA) give us the possibility to understand cancer evolution and pick up the key mutation on specific cancer driver gene. However, tight schedule of standardized protocol may impair the use of personalized experimental drugs in a timely therapeutic window. MAIN BODY: Here, using a combination of deep next generation sequencing and cfDNA liquid biopsy, we demonstrated that it is possible to monitor cancer relapse over time. We showed for the first time the exact correspondence from the increasing clonal expansion and clinical worsening of metastatic breast cancer. CONCLUSION: Thanks to liquid biopsy may be possible to introduce new experimental drugs in the correct therapeutic window which would lead in the near future to an effective treatment which otherwise remains challenging.

Urine-derived podocytes-lineage cells: A promising tool for precision medicine in Alport Syndrome.

Alport Syndrome (ATS) is a rare genetic disorder caused by collagen IV genes mutations, leading to glomerular basement membrane damage up to end-stage renal disease. Podocytes, the main component of the glomerular structure, are the only cells able to produce all the three collagens IV alpha chains associated with ATS and thus, they are key players in ATS pathogenesis. However, podocytes-targeted therapeutic strategies have been hampered by the difficulty of non-invasively isolating them and transcripts-based diagnostic approaches are complicated by the inaccessibility of other COL4 chains-expressing cells. We firstly isolated podocyte-lineage cells from ATS patients' urine samples, in a non-invasive way. RT-PCR analysis revealed COL4A3, COL4A4, and COL4A5 expression. Transcripts analysis on RNA extracted from patient's urine derived podocyte-lineage cells allowed defining the pathogenic role of intronic variants, namely one mutation in COL4A3 (c.3882+5G>A), three mutations in COL4A4 (c.1623+2T>A, c.3699_3706+1del, c.2545+143T>A), and one mutation in COL4A5 (c.3454+2T>C). Therefore, our cellular model represents a novel tool, essential to unequivocally prove the effect of spliceogenic intronic variants on transcripts expressed exclusively at a glomerular level. This process is a key step for providing the patient with a definite molecular diagnosis and with a proper recurrence risk. The established system also opens up the possibility of testing personalized therapeutic approaches on disease-relevant cells.

FANCM c.5791C>T nonsense mutation (rs144567652) induces exon skipping, affects DNA repair activity and is a familial breast cancer risk factor.

Numerous genetic factors that influence breast cancer risk are known. However, approximately two-thirds of the overall familial risk remain unexplained. To determine whether some of the missing heritability is due to rare variants conferring high to moderate risk, we tested for an association between the c.5791C>T nonsense mutation (p.Arg1931*; rs144567652) in exon 22 of FANCM gene and breast cancer. An analysis of genotyping data from 8635 familial breast cancer cases and 6625 controls from different countries yielded an association between the c.5791C>T mutation and breast cancer risk [odds ratio (OR) = 3.93 (95% confidence interval (CI) = 1.28-12.11; P = 0.017)]. Moreover, we performed two meta-analyses of studies from countries with carriers in both cases and controls and of all available data. These analyses showed breast cancer associations with OR = 3.67 (95% CI = 1.04-12.87; P = 0.043) and OR = 3.33 (95% CI = 1.09-13.62; P = 0.032), respectively. Based on information theory-based prediction, we established that the mutation caused an out-of-frame deletion of exon 22, due to the creation of a binding site for the pre-mRNA processing protein hnRNP A1. Furthermore, genetic complementation analyses showed that the mutation influenced the DNA repair activity of the FANCM protein. In summary, we provide evidence for the first time showing that the common p.Arg1931* loss-of-function variant in FANCM is a risk factor for familial breast cancer.

Microduplication of the ARID1A gene causes intellectual disability with recognizable syndromic features.

PURPOSE: To determine whether duplication of the ARID1A gene is responsible for a new recognizable syndrome. METHODS: We describe four patients with a 1p36.11 microduplication involving ARID1A as identified by array-comparative genomic hybridization . We performed comparative transcriptomic analysis of patient-derived fibroblasts using RNA sequencing and evaluated the impact of ARID1A duplication on the cell cycle using fluorescence-activated cell sorting. Functional relationships between differentially expressed genes were investigated with ingenuity pathway analysis (IPA). RESULTS: Combining the genomic data, we defined a small (122 kb), minimally critical region that overlaps the full ARID1A gene. The four patients shared a strikingly similar phenotype that included intellectual disability and microcephaly. Transcriptomic analysis revealed the deregulated expression of several genes previously linked to microcephaly and developmental disorders as well as the involvement of signaling pathways relevant to microcephaly, among which the polo-like kinase (PLK) pathway was especially notable. Cell-cycle analysis of patient-derived fibroblasts showed a significant increase in the proportion of cells in G1 phase at the expense of G2-M cells. CONCLUSION: Our study reports a new microduplication syndrome involving the ARID1A gene. This work is the first step in clarifying the pathophysiological mechanism that links changes in the gene dosage of ARID1A with intellectual disability and microcephaly.Genet Med advance online publication 01 December 2016.

MECP2 missense mutations outside the canonical MBD and TRD domains in males with intellectual disability.

Methyl-CpG-binding protein 2 (MeCP2) is a nuclear protein highly expressed in neurons that is involved in transcriptional modulation and chromatin remodeling. Mutations in MECP2 in females are associated with Rett syndrome, a neurological disorder characterized by a normal neonatal period, followed by the arrest of development and regression of acquired skills. Although it was initially thought that MECP2 pathogenic mutations in males were not compatible with life, starting from 1999 about 60 male patients have been identified and their phenotype varies from severe neonatal encephalopathy to mild intellectual disability. Targeted next-generation sequencing of a panel of intellectual disability related genes was performed on two unrelated male patients, and two missense variants in MECP2 were identified (p.Gly185Val and p.Arg167Trp). These variants lie outside the canonical methyl-CpG-binding domain and transcription repression domain domains, where the pathogenicity of missense variants is more difficult to establish. In both families, variants were found in all affected siblings and were inherited from the asymptomatic mother, showing skewed X-chromosome inactivation. We report here the first missense variant located in AT-hook domain 1 and we underline the importance of MECP2 substitutions outside the canonical MeCP2 domains in X-linked intellectual disability.

Evidence of digenic inheritance in Alport syndrome.

BACKGROUND: Alport syndrome is a clinically heterogeneous, progressive nephropathy caused by mutations in collagen IV genes, namely COL4A3 and COL4A4 on chromosome 2 and COL4A5 on chromosome X. The wide phenotypic variability and the presence of incomplete penetrance suggest that a simple Mendelian model cannot completely explain the genetic control of this disease. Therefore, we explored the possibility that Alport syndrome is under digenic control. METHODS: Using massively parallel sequencing, we identified 11 patients who had pathogenic mutations in two collagen IV genes. For each proband, we ascertained the presence of the same mutations in up to 12 members of the extended family for a total of 56 persons studied. RESULTS: Overall, 23 mutations were found. Individuals with two pathogenic mutations in different genes had a mean age of renal function deterioration intermediate with respect to the autosomal-dominant form and the autosomal-recessive one, in line with molecule stoichiometry of the disruption of the type IV collagen triple helix. CONCLUSIONS: Segregation analysis indicated three possible digenic segregation models: (i) autosomal inheritance with mutations on different chromosomes, resembling recessive inheritance (five families); (ii) autosomal inheritance with mutations on the same chromosome resembling dominant inheritance (two families) and (iii) unlinked autosomal and X-linked inheritance having a peculiar segregation (four families). This pedigree analysis provides evidence for digenic inheritance of Alport syndrome. Clinical geneticists and nephrologists should be aware of this possibility in order to more accurately assess inheritance probabilities, predict prognosis and identify other family members at risk.

Potentially Treatable Disorder Diagnosed Post Mortem by Exome Analysis in a Boy with Respiratory Distress.

We highlight the importance of exome sequencing in solving a clinical case of a child who died at 14 months after a series of respiratory crises. He was the half-brother of a girl diagnosed at 7 years with the early-onset seizure variant of Rett syndrome due to CDKL5 mutation. We performed a test for CDKL5 in the boy, which came back negative. Driven by the mother's compelling need for a diagnosis, we moved forward performing whole exome sequencing analysis. Surprisingly, two missense mutations in compound heterozygosity were identified in the RAPSN gene encoding a receptor-associated protein with a key role in clustering and anchoring nicotinic acetylcholine receptors at synaptic sites. This gene is responsible for a congenital form of myasthenic syndrome, a disease potentially treatable with cholinesterase inhibitors. Therefore, an earlier diagnosis in this boy would have led to a better clinical management and prognosis. Our study supports the key role of exome sequencing in achieving a definite diagnosis in severe perinatal diseases, an essential step especially when a specific therapy is available.

Recurrent duplications of 17q12 associated with variable phenotypes.

The ability to identify the clinical nature of the recurrent duplication of chromosome 17q12 has been limited by its rarity and the diverse range of phenotypes associated with this genomic change. In order to further define the clinical features of affected patients, detailed clinical information was collected in the largest series to date (30 patients and 2 of their siblings) through a multi-institutional collaborative effort. The majority of patients presented with developmental delays varying from mild to severe. Though dysmorphic features were commonly reported, patients do not have consistent and recognizable features. Cardiac, ophthalmologic, growth, behavioral, and other abnormalities were each present in a subset of patients. The newly associated features potentially resulting from 17q12 duplication include height and weight above the 95th percentile, cataracts, microphthalmia, coloboma, astigmatism, tracheomalacia, cutaneous mosaicism, pectus excavatum, scoliosis, hypermobility, hypospadias, diverticulum of Kommerell, pyloric stenosis, and pseudohypoparathryoidism. The majority of duplications were inherited with some carrier parents reporting learning disabilities or microcephaly. We identified additional, potentially contributory copy number changes in a subset of patients, including one patient each with 16p11.2 deletion and 15q13.3 deletion. Our data further define and expand the clinical spectrum associated with duplications of 17q12 and provide support for the role of genomic modifiers contributing to phenotypic variability.

Interstitial 22q13 deletions not involving SHANK3 gene: a new contiguous gene syndrome.

Phelan-McDermid syndrome (22q13.3 deletion syndrome) is a contiguous gene disorder resulting from the deletion of the distal long arm of chromosome 22. SHANK3, a gene within the minimal critical region, is a candidate gene for the major neurological features of this syndrome. We report clinical and molecular data from a study of nine patients with overlapping interstitial deletions in 22q13 not involving SHANK3. All of these deletions overlap with the largest, but not with the smallest deletion associated with Phelan-McDermid syndrome. The deletion sizes and breakpoints varied considerably among our patients, with the largest deletion spanning 6.9 Mb and the smallest deletion spanning 2.7 Mb. Eight out of nine patients had a de novo deletion, while in one patient the origin of deletion was unknown. These patients shared clinical features common to Phelan-McDermid syndrome: developmental delay (11/12), speech delay (11/12), hypotonia (9/12), and feeding difficulties (7/12). Moreover, the majority of patients (8/12) exhibited macrocephaly. In the minimal deleted region, we identified two candidate genes, SULT4A1 and PARVB (associated with the PTEN pathway), which could be associated in our cohort with neurological features and macrocephaly/hypotonia, respectively. This study suggests that the haploinsufficiency of genes in the 22q13 region beside SHANK3 contributes to cognitive and speech development, and that these genes are involved in the phenotype associated with the larger Phelan-McDermid syndrome 22q13 deletions. Moreover, because the deletions in our patients do not involve the SHANK3 gene, we posit the existence of a new contiguous gene syndrome proximal to the smallest terminal deletions in the 22q13 region.

Targeted next-generation sequencing appoints c16orf57 as clericuzio-type poikiloderma with neutropenia gene.

Next-generation sequencing is a straightforward tool for the identification of disease genes in extended genomic regions. Autozygosity mapping was performed on a five-generation inbred Italian family with three siblings affected with Clericuzio-type poikiloderma with neutropenia (PN [MIM %604173]), a rare autosomal-recessive genodermatosis characterised by poikiloderma, pachyonychia, and chronic neutropenia. The siblings were initially diagnosed as affected with Rothmund-Thomson syndrome (RTS [MIM #268400]), with which PN shows phenotypic overlap. Linkage analysis on all living subjects of the family identified a large 16q region inherited identically by descent (IBD) in all affected family members. Deep sequencing of this 3.4 Mb region previously enriched with array capture revealed a homozygous c.504-2 A>C mismatch in all affected siblings. The mutation destroys the invariant AG acceptor site of intron 4 of the evolutionarily conserved C16orf57 gene. Two distinct deleterious mutations (c.502A>G and c.666_676+1del12) identified in an unrelated PN patient confirmed that the C16orf57 gene is responsible for PN. The function of the predicted C16orf57 gene is unknown, but its product has been shown to be interconnected to RECQL4 protein via SMAD4 proteins. The unravelled clinical and genetic identity of PN allows patients to undergo genetic testing and follow-up.