Hadia Hijazi

Xq22 deletions that encompass PLP1 (Xq22‐PLP1‐DEL) are notable for variable expressivity of neurological disease traits in females ranging from a mild late‐onset form of spastic paraplegia type 2 (MIM# 312920), sometimes associated with skewed X‐inactivation, to an early‐onset neurological disease trait (EONDT) of severe developmental delay, intellectual disability, and behavioral abnormalities. Size and gene content of Xq22‐PLP1‐DEL vary and were proposed as potential molecular etiologies underlying variable expressivity in carrier females where two smallest regions of overlap (SROs) were suggested to influence disease. We ascertained a cohort of eight unrelated patients harboring Xq22‐PLP1‐DEL and performed high‐density array comparative genomic hybridization and breakpoint‐junction sequencing. Molecular characterization of Xq22‐PLP1‐DEL from 17 cases (eight herein and nine published) revealed an overrepresentation of breakpoints that reside within repeats (11/17, ~65%) and the clustering of ~47% of proximal breakpoints in a genomic instability hotspot with characteristic non‐B DNA density. These findings implicate a potential role for genomic architecture in stimulating the formation of Xq22‐PLP1‐DEL. The correlation of Xq22‐PLP1‐DEL gene content with neurological disease trait in female cases enabled refinement of the associated SROs to a single genomic interval containing six genes. Our data support the hypothesis that genes contiguous to PLP1 contribute to EONDT.

We describe a 4-year-old boy with developmental delay who was found to carry by clinical grade (CG) molecular cytogenetics (MCs) a chromosome Xq26 microduplication. The report prompted a referral of the patient for possible X-linked acrogigantism (X-LAG), a well-defined condition (MIM300942) due to chromosomal microduplication of a nearby region. The patient was evaluated clinically and investigated for endocrine abnormalities related to X-LAG and not only did he not have acrogigantism, but his growth parameters and other hormones were all normal. We then performed high definition MCs and the duplication copy number variant (CNV) was confirmed to precisely map outside the X-LAG critical region and definitely did not harbor the X-LAG candidate gene, GPR101. The patient’s phenotype resembled that of other patients with Xq26 CNVs. The case is instructive for the need for high definition MCs when CG MCs’ results are inconsistent with the patient’s phenotype. It is also useful for further supporting the contention that GPR101 is the gene responsible for X-LAG.

Embryonic lethality is a recognized phenotypic expression of individual gene mutations in model organisms. However, identifying embryonic lethal genes in humans is challenging, especially when the phenotype is manifested at the preimplantation stage. In an ongoing effort to exploit the highly consanguineous nature of the Saudi population to catalog recessively acting embryonic lethal genes in humans, we have identified two families with a female-limited infertility phenotype. Using autozygosity mapping and whole exome sequencing, we map this phenotype to a single mutation in TLE6, a maternal effect gene that encodes a member of the subcortical maternal complex in mammalian oocytes. Consistent with the published phenotype of mouse Tle6 mutants, embryos from female patients who are homozygous for the TLE6 mutation fail to undergo early cleavage, with resulting sterility. The human mutation abrogates TLE6 phosphorylation, a step that is reported to be critical for the PKA-mediated progression of oocyte meiosis II. Furthermore, the TLE6 mutation impairs its binding to components of the subcortical maternal complex. In this first report of a human defect in a member of the subcortical maternal subcritical maternal complex, we show that the TLE6 mutation is gender-specific and leads to the earliest known human embryonic lethality phenotype.

Cutis Marmorata Telangiectatica Congenita (CMTC) is a congenital localized or generalized vascular anomaly, usually sporadic in occurrence. It can be associated with other cutaneous or systemic manifestations. About 300 cases have been reported. The molecular etiology remains largely unknown. The main purpose of this study is to delineate the molecular basis for a syndromic CMTC phenotype in a consanguineous Saudi family. Clinical phenotyping including detailed neurological imaging, followed by autozygosity mapping and trio whole exome sequencing (WES) are also studied. We have identified a homozygous truncating mutation in ARL6IP6 as the likely cause of a syndromic form of CMTC associated with major dysmorphism, developmental delay, transient ischemic attacks and cerebral vascular malformations. This gene was previously implicated by genome wide association study (GWAS) as a susceptibility locus to ischemic stroke in young adults. We identify ARL6IP6 as a novel candidate gene for a syndromic form of CMTC. This suggests that ischemic stroke or transient ischemic attacks (TIA) may represent, at least in some cases, the mild end of a phenotypic spectrum that has at its severe end autosomal recessive CMTC. This finding contributes to a growing appreciation of the continuum of Mendelian and common complex diseases.

Background: Klippel-Feil anomaly (KFA) can be seen in a number of syndromes. We describe an apparently novel syndromic association with KFA. Methods: Clinical phenotyping of two consanguineous families followed by combined autozygome/exome analysis. Results: Two patients from two apparently unrelated families shared a strikingly similar phenotype characterised by KFA, myopathy, mild short stature, microcephaly, and distinctive facies. They shared a single founder autozygous interval in which whole exome sequencing revealed a truncating mutation in MYO18B. There was virtually complete loss of the transcript in peripheral blood, indicative of nonsense-mediated decay. Electron microscopy of muscle confirms abnormal myosin filaments with accompanying myopathic changes. Conclusions: Deficiency of MYO18B is linked to a novel developmental disorder which combines KFA with myopathy. This suggests a widespread developmental role for this gene in humans, as observed for its murine ortholog.

Several umbilical abnormalities have been linked to and utilized to aid in the clinical diagnosis of certain syndromes. For instance, umbilical skin redundancy has long been recognized as a core feature of Rieger syndrome although its association with other disorders is unknown. In this article, we report for the first time the occurrence of this distinct clinical sign in association with two other syndromes: Morquio syndrome and FKBP14‐related Ehlers–Danlos syndrome (EDS). Our observation is clinically significant because patients with Morquio syndrome are often diagnosed only after they develop typical skeletal manifestations, which reduces the efficacy of available enzyme replacement therapy, so the umbilical sign we report here can facilitate a much earlier diagnosis. In addition, the extreme rarity of FKBP14‐related Ehlers–Danlos syndrome (EDS) can greatly delay the diagnosis of this condition unless it is recognized in the differential diagnosis of redundant umbilical skin as we argue in this report.

Background: Epileptic encephalopathy is a broad clinical category that is highly heterogeneous genetically. Objective: To describe a multiplex extended consanguineous family that defines a molecularly novel subtype of early infantile epileptic encephalopathy. Methods: Autozygosity mapping and exome sequencing for the identification of the causal mutation. This was followed by expression analysis of the candidate gene. Results: In an extended multigenerational family with six affected individuals, a single novel disease locus was identified on chromosome 12p13.31-p13.2. Within that locus, the only deleterious novel exomic variant was a homozygous truncating mutation in NECAP1, encoding a clathrin-accessory protein. The mutation was confirmed to trigger nonsense-mediated decay. Consistent with previous reports, we show that NECAP1 is highly enriched in the central nervous system. Conclusions: NECAP1 is known to regulate clathrin-mediated endocytosis in synapses. The mutation we report here links for the first time this trafficking pathway in early infantile epileptic encephalopathy.

An extremely rare pellagra‐like condition has been described, which was partially responsive to niacin and associated with a multisystem involvement. The condition was proposed to represent a novel autosomal recessive entity but the underlying mutation remained unknown for almost three decades. The objective of this study was to identify the causal mutation in the pellagra‐like condition and investigate the mechanism by which niacin confers clinical benefit. Autozygosity mapping and exome sequencing were used to identify the causal mutation, and comet assay on patient fibroblasts before and after niacin treatment to assess its effect on DNA damage. We identified a single disease locus that harbors a novel mutation in ERCC5, thus confirming that the condition is in fact xeroderma pigmentosum/Cockayne syndrome (XP/CS) complex. Importantly, we also show that the previously described dermatological response to niacin is consistent with a dramatic protective effect against ultraviolet‐induced DNA damage in patient fibroblasts conferred by niacin treatment. Our findings show the power of exome sequencing in reassigning previously described novel clinical entities, and suggest a mechanism for the dermatological response to niacin in patients with XP/CS complex. This raises interesting possibilities about the potential therapeutic use of niacin in XP.