1- To validate non standard diagnostic testing like RNAseq when standard methodologies fail to reveal an answer. 

2- To use transcriptome analysis on skin fibroblast for proof of principle for the study of brain malformation disorders.

Establishing genetic diagnostic confirmation is crucial for patients with rare disease as it can guide the patient management, clarify expected prognosis, recurrence risk and best possible options for family planning.

Despite the rapid evolution in the genetic diagnostic field, the mendelian diagnostic yield remains uncertain in approximately 50% of cases. Clinical expert to guide other modalities like whole genome sequence and RNA seq appeared promising when standard methodologies fail to provide an answer.

We are presenting a case of 3-year-old boy of non-consanguineous parent. He was born after an uneventful pregnancy at full term. He presented to medical attention at 6 months with repetitive abnormal movement suggestive of infantile spasm, EEG confirmed hypsarrhythmia and he was managed with Vigabatrin. He had developmental regression in the context of medically refractory epilepsy. His physical examination at 6 month was notable for weight and length on 25^th percentile and head circumference 41 cm (5^th percentile), brachycephaly, hypertelorism, depressed nasal bridge with axial hypotonia and exaggerated deep tendon reflex.

His brain MRI showed lissencephaly with posterior- anterior gradient and subcortical band heterotropia, suggestive of LIS mutation. Further investigations included negative epilepsy panel, microarray and PAFAH1B1 deletion/duplication.  

Based on genotype tissue expression of the suspected genetic abnormality, we performed skin biopsy for fibroblast transcriptome analysis which revealed multiple splicing disruptions in gene PAFAH1B1 (old name LIS1) and MCPH1 pathogenic variant were detected.

This case study demonstrates that LIS1 is expressed in skin fibroblast and that RNAseq analysis is promising modality to elucidate novel mutations and confirm genetic diagnosis.