Citations

Pathogenic variants in KMT5B , a lysine methyltransferase, are associated with global developmental delay, macrocephaly, autism, and congenital anomalies (OMIM # 617788). Given the relatively recent discovery of this disorder, it has not been fully characterized. Deep phenotyping of the largest ( n = 43) patient cohort to date identified that hypotonia and congenital heart defects are prominent features that were previously not associated with this syndrome. Both missense variants and putative loss-of-function variants resulted in slow growth in patient-derived cell lines. KMT5B homozygous knockout mice were smaller in size than their wild-type littermates but did not have significantly smaller brains, suggesting relative macrocephaly, also noted as a prominent clinical feature. RNA sequencing of patient lymphoblasts and Kmt5b haploinsufficient mouse brains identified differentially expressed pathways associated with nervous system development and function including axon guidance signaling. Overall, we identified additional pathogenic variants and clinical features in KMT5B -related neurodevelopmental disorder and provide insights into the molecular mechanisms of the disorder using multiple model systems.

Sonic hedgehog signaling regulates processes of embryonic development across multiple tissues, yet factors regulating context-specific Shh signaling remain poorly understood. Exome sequencing of families with polymicrogyria (disordered cortical folding) revealed multiple individuals with biallelic deleterious variants in TMEM161B , which encodes a multi-pass transmembrane protein of unknown function. Tmem161b null mice demonstrated holoprosencephaly, craniofacial midline defects, eye defects, and spinal cord patterning changes consistent with impaired Shh signaling, but were without limb defects, suggesting a CNS-specific role of Tmem161b. Tmem161b depletion impaired the response to Smoothened activation in vitro and disrupted cortical histogenesis in vivo in both mouse and ferret models, including leading to abnormal gyration in the ferret model. Tmem161b localizes non-exclusively to the primary cilium, and scanning electron microscopy revealed shortened, dysmorphic, and ballooned ventricular zone cilia in the Tmem161b null mouse, suggesting that the Shh-related phenotypes may reflect ciliary dysfunction. Our data identify TMEM161B as a regulator of cerebral cortical gyration, as involved in primary ciliary structure, as a regulator of Shh signaling, and further implicate Shh signaling in human gyral development.

Abstract Qualitative disorders of platelets are often missed at clinical evaluation. Hermansky-Pudlak (HPS) syndrome is a rare genetic metabolic disorder with subtype specific clinical associations most prevalent in Puerto Rico with strong link to consanguinity. HPS is usually associated with albinism, visual impairment and a qualitative platelet dysfunction due to absence of dense granules. Ceroid accumulation can be associated with inflammatory bowel disease, pulmonary fibrosis and kidney disease. Ten variants have been described with types 1, 2 and 4 associated with severe disease whilst type 3, 5 and 6 is associated with mild disease. Little is known about types 7, 8, 9 and 10. A Caucasian adult female presented with a history of intermittent episodes of severe bleeding. She carried the diagnosis of probable von Willebrand's disease at presentation. This particular patient did not respond to cryoprecipitate infusion but bleeding stopped secondary to infusion of normal platelets, a clue to a platelet storage disorder. Hence she was re-investigated for a bleeding disorder and initial coagulation testing identified abnormal platelet aggregation and amplitude pattern to epinephrine. She was subsequently evaluated with platelet transmission electron microscopy and platelet flow cytometry at the Mayo Clinic Hematopathology. Platelet TEM showed complete absence of dense granules and a normal flow cytometry. HPS was suspected and initial genetic studies identified a variant genetic abnormality. Further studies were done at the NIH. Classical clinical features of HPS like nystagmus, and ocular albinism was not identified at initial neurological and ophthalmologic examination. But more detailed evaluation revealed subclinical oculocutaneous albinism. Genome wide SNP analysis showed regions of homozygosity including HPS 1 and HPS 6; deletions were not identified in these genes. Full length HPS1 transcript was amplified by PCR of genomic DNA. Targeted next-generation sequencing identified a novel homozygous missense variant [c.383T>C (p.V128A)] in HPS6. Reduced HPS6 mRNA levels were found in the patient's skin fibroblasts compared to cells from patients with HPS-1 and normal control cells. HPS6 protein expression in the patient's cells was also low and approximately 60% lower than that of normal cells. HPS is a rare platelet storage disorder that can often be missed. Platelet aggregation tests need to be followed up by platelet TEM and genetic testing to definitively diagnose this condition. Further work up has defined a new missense variant by SNP analysis, next generation sequencing and fibroblast culture. It is important to identify the subtype of HPS, because certain subtypes of HPS (HPS 1, 2 and 4) have clinical manifestations like progressive pulmonary fibrosis. Identification of the platelet storage disorder also helps in management of the bleeding diathesis. Disclosures No relevant conflicts of interest to declare.

Heřmanský–Pudlák syndrome (HPS), a rare autosomal recessive disorder, manifests with oculocutaneous albinism and a bleeding diathesis. However, severity of disease can be variable and is typically related to the genetic subtype of HPS; HPS type 6 (HPS‐6) is an uncommon subtype generally associated with mild disease. A Caucasian adult female presented with a history of severe bleeding; ophthalmologic examination indicated occult oculocutaneous albinism. The patient was diagnosed with a platelet storage pool disorder, and platelet whole mount electron microscopy demonstrated absent delta granules. Genome‐wide SNP analysis showed regions of homozygosity that included the HPS1 and HPS6 genes. Full length HPS1 transcript was amplified by PCR of genomic DNA. Targeted next‐generation sequencing identified a novel homozygous missense variant in HPS6 (c.383 T > C; p.V128A); this was associated with significantly reduced HPS6 mRNA and protein expression in the patient's fibroblasts compared to control cells. These findings highlight the variable severity of disease manifestations in patients with HPS, and illustrate that HPS can be diagnosed in patients with excessive bleeding and occult oculocutaneous albinism. Genetic analysis and platelet electron microscopy are useful diagnostic tests in evaluating patients with suspected HPS.Clinical Trial registration:Registrar: ClinicalTrials.govWebsite: www.clinicaltrials.govRegistration Numbers: NCT00001456 and NCT00084305.