Malformations of cortical development (MCDs) encompass a diverse group of genetic and clinical disorders. Here, we aimed to determine a genetic etiology for two siblings manifesting MCD, microcephaly, epilepsy, intellectual disability, and susceptibility to infections. A missense variant (NM_018084:c.929A > C, p.Asp310Ala) and an intragenic deletion (exons 14-16) in CCDC88A were identified as compound heterozygous in patients by genome sequencing. Truncating homozygous CCDC88A variants are known to cause an ultra-rare syndrome manifesting with MCD, microcephaly, seizures, and severe neurological impairment. CCDC88A encodes girdin, which is essential for various cell functions, such as actin remodeling and cell proliferation. Western blot analysis showed that the missense variant allele was expressed in fibroblasts at a level compatible with a heterozygous allele, whereas a truncated protein from the deletion allele was barely detectable. Proliferation and wound-healing assays revealed that girdin-deficient fibroblasts proliferated faster and migrated slower than controls. High-content imaging highlighted girdin-deficient fibroblasts as smaller and their actin remodeling disrupted, leading to perinuclear accumulation of endolysosomal organelles. To confirm these cellular phenotypes resulted from girdin loss, CRISPR-Cas9 edited knockout models of healthy fibroblasts were created, replicating the observations in patient cells. Additionally, the siblings exhibited reduced monocytoid and plasmacytoid dendritic cells, suggesting compromised immunity due to girdin deficiency. In summary, the study describes the first case of a CCDC88A missense variant and intragenic deletion associated with MCD. It demonstrates altered immunity and girdin-related cellular changes, such as cell morphology and proliferation-migration dichotomy, in patient and knockout fibroblasts, reinforcing the pathogenic relevance of these variants.