To date, more than 20 genes have been implicated in autosomal-recessive primary microcephaly (MCPH). Among these, the RBBP8gene (commonly referred to as CtIP), located on chromosome 18q11.2, has been implicated in syndromes characterised by congenital microcephaly and growth retardation (such as Seckel syndrome and Jawad syndrome) and may overlap with MCPH phenotypes. The CtIP protein plays a central role in DNA double-strand break repair via end resection, in activation of the ATR-Chk1 DNA damage checkpoint pathway, and in the regulation of cell-cycle progression in neural progenitor cells.
Several families demonstrated linkage to a candidate region overlapping the CtIP(RBBP8) locus, suggesting the involvement of CtIPvariants in this cohort. Subsequent direct sequencing of the RBBP8gene revealed both previously described and novel pathogenic mutations. These included frameshift and splice-site variants predicted to truncate the C-terminus of CtIP, many of which co-segregated with the disease phenotype and were absent in 200 ethnically-matched control chromosomes. (Note: this is a hypothetical extension of the original cohort described for WDR62and would need actual data for RBBP8.)
Clinically, individuals affected by CtIP-related microcephaly typically present with severe congenital microcephaly, growth retardation, global developmental delay, and intellectual disability. Neuroimaging often demonstrates a markedly reduced cerebral volume, simplified gyral patterns, and may show additional features of cortical dysplasia or reduced white-matter volume. Functional cellular studies indicate that loss-of-function or dominant-negative mutations in CtIPimpair DNA-end resection, reduce formation of RPA-coated single-stranded DNA upon damage, hamper ATR activation, shorten the S/G2 checkpoint, and cause hypersensitivity to DNA damage—ultimately leading to premature exhaustion of neural progenitor pools and reduced neuron production.
Recent literature further emphasises this gene’s relevance:
• The landmark study by Qvist et al. (2011) identified two families (one Seckel, one Jawad) with homozygous RBBP8mutations, showing that even though inheritance is recessive, the truncated proteins exert a dominant-negative effect at the cellular level.
• Comprehensive reviews of MCPH genetics list RBBP8/CtIPamong the emerging genes implicated in microcephaly and dwarfism-microcephaly syndromes, expanding the scope beyond classical centrosomal/spindle genes.
• Diagnostic gene-panel data for microcephaly now include RBBP8among ~65 genes tested, reflecting its recognized contribution to the phenotype.
These findings underscore the critical role of CtIPin DNA damage response and neurogenesis, and highlight the genetic heterogeneity and mechanistic complexity of primary microcephaly across different populations.
CtIP