{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,8,1]],"date-time":"2026-08-01T04:58:35Z","timestamp":1785560315330,"version":"3.56.0"},"reference-count":34,"publisher":"MDPI AG","issue":"15","license":[{"start":{"date-parts":[[2026,7,30]],"date-time":"2026-07-30T00:00:00Z","timestamp":1785369600000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"name":"Servicio de Salud de Castilla-La Mancha","award":["CGP180358"],"award-info":[{"award-number":["CGP180358"]}]},{"award":["CGP180358"],"award-info":[{"award-number":["CGP180358"]}],"id":[{"id":"https:\/\/ror.org\/03k8zj440","id-type":"ROR","asserted-by":"publisher"}]},{"name":"Asociaci\u00f3n para la difusi\u00f3n de Avances Oncol\u00f3gicos","award":["CAT190273"],"award-info":[{"award-number":["CAT190273"]}]}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Int. J. Mol. Sci.","IJMS"],"abstract":"<jats:p>Plasma biochemistry often presents significant limitations in diagnosing borderline metabolic disorders, particularly within complex neurodevelopmental phenotypes. Here, we present the clinical genomic evaluation of a six-year patient presenting with early-onset hypotonia and severe gastrointestinal complications whose newborn screening panel did not evaluate biotinidase (BTD) activity. While initial baseline plasma biochemistry yielded borderline residual BTD function (46% of the population mean), targeted sequencing identified a novel, compound heterozygous hypomorphic variant (p.Thr459Met) in trans with the common p.Asp424His allele. In vitro functional validation confirmed that p.Thr459Met induces severe protein misfolding and intracellular retention, impairing enzyme secretion. Biotin supplementation triggered a documented and favorable therapeutic improvement, establishing this borderline enzymatic background as an actionable metabolic vulnerability unmasked by chronic gastrointestinal stressors. This study underscores the critical value of functional genomic characterization over static enzymatic biomarkers to identify highly treatable metabolic components within heterogeneous clinical landscapes.<\/jats:p>","DOI":"10.3390\/ijms27156847","type":"journal-article","created":{"date-parts":[[2026,7,30]],"date-time":"2026-07-30T16:15:00Z","timestamp":1785428100000},"page":"6847","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":0,"title":["Characterization of a Novel BTD Hypomorphic Variant in a Patient with Complex Neurodevelopmental Delay: Resolving Actionable Metabolic Vulnerabilities Beyond Borderline Plasma Biochemistry"],"prefix":"10.3390","volume":"27","author":[{"ORCID":"https:\/\/orcid.org\/0009-0007-5987-6583","authenticated-orcid":false,"given":"Claudia","family":"Toledo-Pacheco","sequence":"first","affiliation":[{"name":"Servicio de Gen\u00e9tica, Hospital Universitario de Toledo (HUT), 45007 Toledo, Spain"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Minerva","family":"Montero-Hern\u00e1ndez","sequence":"additional","affiliation":[{"name":"Instituto de Biomedicina, Universidad de Castilla-La Mancha (UCLM), 02008 Albacete, Spain"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Mar\u00eda Pilar","family":"L\u00f3pez-Garrido","sequence":"additional","affiliation":[{"name":"Instituto de Biomedicina, Universidad de Castilla-La Mancha (UCLM), 02008 Albacete, Spain"},{"name":"Facultad de Medicina CR, Universidad de Castilla-La Mancha (UCLM), 13071 Ciudad Real, Spain"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-5246-3793","authenticated-orcid":false,"given":"Carles","family":"de Diego-Bogu\u00f1\u00e1","sequence":"additional","affiliation":[{"name":"Servicio de Gen\u00e9tica, Hospital Universitario de Toledo (HUT), 45007 Toledo, Spain"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Francisco","family":"S\u00e1nchez-S\u00e1nchez","sequence":"additional","affiliation":[{"name":"Instituto de Biomedicina, Universidad de Castilla-La Mancha (UCLM), 02008 Albacete, Spain"},{"name":"Facultad de Medicina AB, Universidad de Castilla-La Mancha (UCLM), 02008 Albacete, Spain"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"1968","published-online":{"date-parts":[[2026,7,30]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"78","DOI":"10.1016\/S0022-3476(05)81649-X","article-title":"Partial biotinidase deficiency: Clinical and biochemical features","volume":"116","author":"Levy","year":"1990","journal-title":"J. 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