Inherited human Apollo deficiency causes severe bone marrow failure and developmental defects

Abstract

Inherited bone marrow failure syndromes (IBMFS) represent a group of disorders typified by impaired production of one or several blood cell types. The telomere biology disorders dyskeratosis congenita (DC) and its severe variant Høyeraal-Hreidarsson (HH) syndrome are rare IBMFS characterized by bone marrow failure, developmental defects, and various premature aging complications associated with critically short telomeres. Here we identified biallelic variants in the gene encoding the 5'-to-3' DNA exonuclease Apollo/SNM1B in three unrelated patients presenting with a DC/HH phenotype consisting of early onset hypocellular bone marrow failure, B and NK lymphopenia, developmental anomalies, microcephaly and/or intrauterine growth retardation. All three patients carry a homozygous or compound heterozygous (in combination with a null-allele) missense variant affecting the same residue L142 (L142F or L142S) located in the catalytic domain of Apollo. Apollo-deficient cells from patients exhibited spontaneous chromosome instability and impaired DNA repair that was complemented by CRISPR/Cas9-mediated gene correction. Furthermore, patients' cells showed signs of telomere fragility that were however not associated with global reduction of telomere length. Unlike patients' cells, human Apollo KO HT1080-cell lines showed strong telomere dysfunction accompanied by excessive telomere shortening, suggesting that the L142S and L142F Apollo variants are hypomorphic. Collectively, these findings define human Apollo as a genome caretaker and identify biallelic Apollo variants as a genetic cause of a hitherto unrecognized severe IBMFS combining clinical hallmarks of DC/HH with normal telomere length.

A novel telomere biology disease‐associated gastritis identified through a whole exome sequencing‐driven approach

“…Here, we identified pathogenic variants in POT1 and DCLRE1B (alias symbol APOLLO) in two unusual severe gastritides thought to be of unknown etiology despite extensive clinical workup. Unlike other telomere complex genes which cause shortening of telomere length, these genes are associated with long and normal telomere length, respectively [7][8][9]. Data regarding involvement of POT1 and DCLRE1B genes in telomeropathies are only recently emerging with proposals that these genes should be included on cancer-risk predisposing multigene panel tests as well as genetic tests for telomere biology dysfunction [10,11].…”

Identification of Adult Patients With Classical Dyskeratosis Congenita or Cryptic Telomere Biology Disorder by Telomere Length Screening Using Age-modified Criteria

“…[39][40][41] Moreover, recent discoveries identified even candidate genes for patients with TBD features and normal TL (Apollo) or even increased TL (POT1). 10,42 Therefore, the decoding of previously unknown genomic alterations that interfere with telomere maintenance and telomeres/telomerases regulatory functions is of utmost importance to improve understanding of TBDs and close the gap of yet unknown TBD causes.…”

“… 7 However, genetic testing fails to properly diagnose an underlying TBD in at least 20% of patients, 8 simply because not all TBD-causing genes are known to date as exemplified by the very recent discovery of alterations in candidate genes such as Apollo or RPA1 . 9 , 10 In addition, the functional consequences of yet undefined variants found in TBD-genes can more accurately be determined once the in vivo read out, that is, shortened TL result, is available. In addition, rare families with cases of phenocopy (reviewed in Revy et al 11 ), that is, where screened relatives from yet unknown patients with genetic TBD have short telomeres despite of being genetically normal, will obviously be missed by next generation sequencing (NGS) screening alone.…”

DNA-PK and the TRF2 iDDR inhibit MRN-initiated resection at leading-end telomeres

“…In amniotes, the 5' exonuclease Apollo/DCLRE1B/SNM1B has evolved a YxLxP motif in its C-terminus that allows it to bind to the TRFH domain of TRF2 through the interaction with a region surrounding F120 [7][8][9][10][11] . TRF2-bound Apollo is thought to initiate 5' end resection at leading-end telomeres to allow subsequent long-range resection by Exo1 When Apollo is deleted or prevented from binding to TRF2 (e.g., when TRF2-F120A is used to complement deletion of TRF2), leading-end telomeres do not regain their normal 3' overhangs and are vulnerable to end joining [13][14][15][16][17] . Such leading-end telomere fusions are abolished by deletion of Ku70, which initially suggested that they are mediated by c-NHEJ 13,15 .…”

“…When Apollo is deleted or prevented from binding to TRF2 (e.g., when TRF2-F120A is used to complement deletion of TRF2), leading-end telomeres do not regain their normal 3' overhangs and are vulnerable to end joining [13][14][15][16][17] . Such leading-end telomere fusions are abolished by deletion of Ku70, which initially suggested that they are mediated by c-NHEJ 13,15 .…”