A CPS1 carrier-screening certificate for exa-cel CRISPR specificity: public CHANGE-seq calibration, HPRC carrier-census scope, and the FDA PMR#2 denominator framework

For exagamglogene autotemcel (exa-cel), the clinically actionable variant-equity question is carrier screening for the CPS1/rs114518452 off-target liability, not treatment efficacy. The approved BCL11A enhancer SpCas9-NGG guide is the GATA1-core-proximal guide in a local NGG tiling, and rs114518452 creates a CPS1 NGG off-target with CFD_alt=0.947368. Five source-bound anchors define the carrier-screening certificate: the rs114518452 AFR/NFE allele-frequency ratio is 309.9; the single-site carrier-object ratio is 302.9; the 46-site worst-case burden ratio is 45.3 and is kept separate from the single-site 4.556% AFR allele-frequency axis; independent PRJNA733110 reprocessing gives CPS1 ALT-allele indels of 8.84–10.75%; and a public GSE149363 CHANGE-seq calibration maps the CPS1 CFD score into the high-detectability tail (CFD 0.8: 53/55 rows detected, p=0.964, Wilson 95% CI 0.877–0.990; exact CPS1 bin [0.90,0.95): 6/6 detected, Wilson lower 0.610). A 232-haplotype HPRC validated-CFD census adds a public pangenome scope check, but its 30-haplotype AFR stratum is underpowered for excluding a 4.556% allele-frequency positive control (P[zero]=0.247). The public-data ceiling is therefore a regulatory denominator and screening/detection framework for PMR#2. It cannot demonstrate prospective carrier-HSPC safety, treatment benefit, or population-level equity outcomes; the remaining evidence gap is rs114518452-stratified CD34^+ HSPC editing with carrier and non-carrier donors, on-target HbF induction, and WT SpCas9 versus HiFi-Cas9 arms.