Submission Details

Spermatogenic failure 46 (SPGF46) is characterized by male infertility due to morphologic abnormalities of the flagella (MMAF), including flagella that are absent, short, coiled, angulated, and/or of irregular caliber (Liu et al., 2020, PMID: 32619401 and Yang et al., 2020, PMID: 32681648).

Mutations in DNAH8 were first reported in 2020 in association with SPGF46 (OMIM: 619095, MONDO:0033673) involving morphologic abnormalities of the flagella (Liu et al., 2020, PMID: 32619401 and Yang et al., 2020, PMID: 32681648). DNAH8 belongs to the axonemal outer dynein arm DHC group and is expressed in sperm flagella but not in the lungs nor the Fallopian tubes (Whitfield et al., 2019, PMID:31178125, Uhlen et al., 2015, PMID: 25613900 and Liu et al., 2020, PMID: 32619401).

Only one case reported as a Primary Ciliary Dyskinesia patient had a homozygous nonsense mutation in DNAH8 (Watson et al., 2014, PMID: 24307375). The gender/sex, fertility status, and clinical features of the patient were not detailed (Watson et al., 2014, PMID: 24307375).

Per criteria outlined by the ClinGen Lumping and Splitting Working Group we found different molecular mechanisms. Therefore, the following disease entities have been split into two entities: SPGF46 (OMIM: 619095, MONDO:0033673) and Primary Ciliary Dyskinesia.

Evidence from 7 probands in 4 publications were included in this curation (Liu et al., 2020, PMID: 32619401, Yang et al., 2020, PMID: 32681648, Zhou et al., 2021, PMID: 33611675 and Weng et al., 2021, PMID: 33704367) representing 14 unique variants (6 missense, 3 frameshift and 2 splicing). The maximum score for genetic evidence (12 pts.) has been reached.

This gene-disease association is supported by experimental evidence. Abundant data confirms the expression of DNAH8 mRNA (Uhlen et al., 2015, PMID: 25613900) and protein (Whitfield et al., 2019, PMID:31178125, Uhlen et al., 2015, PMID: 25613900 and Liu et al., 2020, PMID: 32619401) in sperm flagella. DNAH8 immunostaining signal was absent from the sperm flagella from subjects harboring bi-allelic DNAH8 mutations (Liu et al., 2020, PMID: 32619401). Moreover, sperm from these patients (patient’s cells) present abnormal morphology and motility including absent, short, coiled, and irregular-caliber flagella (Liu et al., 2020, PMID: 32619401).

In mice (Mus musculus), the t haplotype (t complex) is a structural rearrangement naturally occurring in the proximal third of chromosome 17. A t-containing chromosome 17 presents four large nonoverlapping inversion polymorphisms spanning nearly the entire t complex region (Pilder, Olds-Clarke, Phillips, & Silver, 1993, PMID: 8405685). The t complex encompasses two Dynein light chains coding genes (Tctex1 and Tctex2) and the mice homologous of the human DNAH8 gene (DNAHC8). Mice harboring a homozygous t-complex are sterile and exhibit, through C2+ induced arrest, a characteristic sperm tail curvature phenotype called “curlicue”. Compared to the wild-type, the t complex version of DNAHC8 carries seventeen (17) missense mutations at highly conserved positions and may be capable of substantially altering normal DNAHC8 function (null allele) (Samant et al., 2005, PMID: 16054618).

A series of backcrosses have generated null-DNAHC8 mice. Homozygous null-DNAHC8 mice are sterile and present sperm with no flagella (like in humans). Heterozygotes (t/null-DNAHC8) mice are also sterile but exhibit the “curlicue” sperm flagella phenotype (Pilder, Olds-Clarke, Phillips, & Silver, 1993, PMID: 8405685). All these results argue that DNAHC8 plays a key role in the process of generating the sperm flagella and suggests that the “curlicue” phenotype is a multigenic phenomenon.

DNAH8-knockout mice (DNAH8-/-) were generated through the use of CRISPR-Cas9 technology (Liu et al., 2020, PMID: 32619401). Diminished sperm movement was observed in DNAH8-/- male mice. Hematoxylin and eosin staining showed significantly higher rates of abnormal flagella in the knockout mice, including absent, short, coiled, bent, and/or irregularly shaped flagella, which reminds the phenotype observed in MMAF-affected men with biallelic DNAH8 mutations (Liu et al., 2020, PMID: 32619401).

In summary, there is strong evidence to support the relationship between DNAH8 and autosomal recessive Spermatogenic failure 46. Three years must elapse from the first proposal of the association to reach a definitive classification without any valid contradictory evidence. We will re-evaluate this gene-disease relationship at that time to determine if an upgraded classification of definitive is warranted. This classification was approved by the ClinGen Motile Ciliopathy GCEP on June 9, 2022 (SOP Version 8).