HIPK4 is a novel gene associated with teratozoospermia and male infertility

This study identifies biallelic loss-of-function variants in the HIPK4 gene as a novel autosomal-recessive cause of teratozoospermia and male infertility, characterized by 100% abnormal sperm head morphology, while also highlighting the genetic heterogeneity within a single family where distinct mutations in HIPK4 and DNAH17 underlie different sperm defects.

The Gist

The Big Picture: A Family Mystery

Imagine a family with four brothers who are all struggling to become fathers. They visit a fertility clinic, and the doctors find that their sperm looks "weird." Some have bent tails, some have no tails, and others have misshapen heads.

Usually, when a whole family has the same problem, doctors assume they all have the same broken instruction manual (gene) passed down from their parents. But in this study, the researchers (led by Dr. Koser and colleagues) discovered something surprising: The brothers didn't all have the same problem.

It was like a house where four rooms were broken, but two rooms had a broken door, and the other two rooms had a broken window. They looked like the same house, but the damage came from two different causes.

The Two Different Culprits

The researchers used a high-tech "gene scanner" (Exome Sequencing) to look at the brothers' DNA. They found two distinct genetic errors causing the infertility:

1. The "Broken Engine" (Gene: DNAH17)

• Who: One of the older brothers (M865).
• The Problem: His sperm tails (flagella) are the engines that make the sperm swim. The gene DNAH17 is responsible for building the "propellers" on these engines.
• The Analogy: Imagine a boat. The DNAH17 gene builds the outboard motor. In this brother, the motor is completely missing or broken. Without a motor, the boat (sperm) can't move forward, even if the hull (head) looks okay.
• The Result: His sperm can't swim (asthenozoospermia) and the tails are bent or coiled (MMAF).

2. The "Mold Maker" (Gene: HIPK4)

• Who: The other three brothers.
• The Problem: These brothers have a broken gene called HIPK4. This gene acts like a mold or a 3D printer for the sperm's head.
• The Analogy: Think of a sperm cell as a dart. The tail is the flight stabilizer, but the head is the pointy tip that needs to be perfectly shaped to hit the target (the egg). The HIPK4 gene is the factory machine that shapes that tip.
  • In these brothers, the factory machine has a "Start" button that is broken. Instead of making a full, perfect dart, the machine starts printing 35 steps too late.
  • The Result: The darts come out misshapen. Some are round, some are tiny, some are tapered, and some are just blobs. They look like "potato chips" instead of darts. This is called teratozoospermia.

How They Proved It

The scientists didn't just guess; they ran experiments:

• Microscope Magic: They looked at the sperm under powerful electron microscopes. The brother with the DNAH17 issue had tails with missing internal gears. The brothers with the HIPK4 issue had heads that were all wrong shapes.
• The Lab Test: They took the broken HIPK4 gene and put it into a petri dish with human cells. The cells tried to make the protein, but because the "Start" button was broken, they made a tiny, unstable version of the protein that fell apart quickly. This confirmed the gene was indeed broken.

The Good News: Hope for Fathers

The most exciting part of the story is the solution.

• Even though the sperm heads are misshapen, the brothers with the HIPK4 problem can still have children.
• One of the brothers successfully used a procedure called ICSI (Intracytoplasmic Sperm Injection).
• The Analogy: If natural swimming is like trying to throw a dart at a target from across the room, ICSI is like someone picking up the dart and gently placing it right next to the target. Since the sperm can't swim well or look right, doctors help them by injecting the sperm directly into the egg.
• One of the brothers in the study actually became a father this way!

Why This Matters

1. Genetic Detective Work: This study shows that even within the same family, different brothers can have different genetic causes for infertility. You can't just assume everyone has the same problem; you have to test everyone individually.
2. New Diagnosis: Before this, we didn't know that the HIPK4 gene was a major cause of misshapen sperm heads in humans. Now, doctors can test for this specific gene.
3. Treatment: Knowing the cause helps doctors choose the right treatment. If you know the sperm heads are broken because of HIPK4, you know that ICSI is a very good option.

Summary

This paper is a story of a family where four brothers couldn't have kids naturally. The doctors found that one brother had a broken "engine" gene (DNAH17), while the other three had a broken "mold" gene (HIPK4) that made their sperm heads look like misshapen blobs. Thanks to this discovery, the brothers with the "mold" problem now know they can likely become fathers with the help of modern fertility treatments.

Technical Summary

1. Problem Statement

Male infertility affects a significant portion of couples, yet over 70% of cases lack a causal genetic diagnosis. While the genetic basis for sperm flagellar defects (specifically Multiple Morphological Abnormalities of the sperm Flagella, or MMAF) is increasingly understood, the genetic etiology of sperm head defects (teratozoospermia) remains poorly characterized.

• Specific Gap: Previous studies suggested a link between HIPK4 (Homeodomain-interacting protein kinase 4) and male infertility in mice, but human evidence was limited to variants of uncertain significance (VUS) or heterozygous states in azoospermic patients. The role of HIPK4 in human teratozoospermia and its inheritance pattern were unresolved.
• Study Context: The authors investigated a family with four infertile brothers exhibiting varying degrees of impaired spermatogenesis to disentangle the specific genetic causes.

2. Methodology

The study employed a multi-modal approach combining clinical phenotyping, genomic sequencing, and functional in vitro assays.

• Cohort: Four infertile brothers (M865, M1344, M1670, M1611), their parents, and one fertile brother (M1688) were recruited. Additionally, the MERGE cohort (>3,300 infertile men) was screened for replication.
• Genomic Analysis:
  • Exome Sequencing (WES): Performed on all five brothers and parents.
  • Variant Filtering: Focused on rare, high-impact variants (loss-of-function, splice, start-loss) in genes expressed in the testis.
  • Segregation Analysis: Determined inheritance patterns (homozygous vs. compound heterozygous) across the family.
• Phenotypic Characterization:
  • Semen Analysis: Standard WHO guidelines for count, motility, and morphology (strict criteria).
  • Immunofluorescence (IF): Staining for acetylated alpha-tubulin (flagella) and Hoechst (nuclei) to visualize structural defects.
  • Transmission Electron Microscopy (TEM): Used to assess axonemal ultrastructure (specifically outer dynein arms and microtubule doublets).
• Functional Validation (In Vitro):
  • Cell Culture: HEK293T cells were transfected with wild-type (WT) and mutant HIPK4 constructs.
  • Cloning: Generated constructs for WT and the patient-specific c.1A>G variant (start-loss).
  • Western Blot & Cycloheximide Chase: Assessed protein stability and translation initiation. The cycloheximide assay measured protein degradation rates over 24 hours.
  • Bioinformatics: Used AlphaFold for 3D structural modeling of the kinase domain and analyzed RNA-seq data (bulk and single-cell) to determine HIPK4 expression timing during spermatogenesis.
• Classification: Variants were classified according to ACMG/AMP guidelines, and gene-disease relationships were assessed using ClinGen criteria.

3. Key Contributions

• Discovery of a Second Genetic Cause: The study identified that within a single family, two distinct genetic mechanisms caused infertility: DNAH17 (a known MMAF gene) and HIPK4 (a novel candidate for sperm head defects).
• Resolution of Inheritance Pattern: The authors clarified that HIPK4-related infertility follows an autosomal recessive pattern, contradicting previous hypotheses that suggested heterozygous variants might be causative.
• Mechanistic Insight: Demonstrated that the HIPK4 start-loss variant leads to an N-terminally truncated protein lacking critical parts of the kinase domain, resulting in reduced stability and loss of function.
• Phenotypic Correlation: Linked HIPK4 deficiency specifically to 100% abnormal sperm head morphology (teratozoospermia) with variable acrosomal defects, distinct from the flagellar defects seen in DNAH17 cases.

4. Key Results

A. Genetic Findings

• Brother M865: Compound heterozygous for two loss-of-function (LoF) variants in DNAH17 (c.1076_1077dup and c.7752+2T>A). This caused Outer Dynein Arm (ODA) defects and MMAF.
• Brothers M1344, M1670, M1611: Homozygous for the HIPK4 c.1A>G variant. This variant disrupts the canonical start codon (p.Met1).
• Fertile Brother (M1688): Heterozygous for the HIPK4 variant, confirming autosomal recessive inheritance.
• Replication: No other individuals with biallelic HIPK4 variants were found in the MERGE cohort (n=3,339), highlighting the rarity of the condition.

B. Phenotypic Characterization

• DNAH17 Phenotype (M865): Severe asthenozoospermia (low motility) and MMAF (short, bent, coiled tails). TEM confirmed the absence of ODAs and axonemal disruption.
• HIPK4 Phenotype (M1344, M1670, M1611):
  • Morphology: 100% abnormal sperm head shapes (tapered, round, amorphous, acephalic "pinheads").
  • Motility/Count: Variable oligo- and asthenozoospermia, but distinct from the flagellar defects of M865.
  • Ultrastructure: TEM showed normal axonemal structure (unlike M865), confirming the defect is specific to head shaping.

C. Functional Validation

• Expression: HIPK4 is expressed specifically in late spermatogenesis (spermatids), consistent with its role in head shaping.
• Translation Shift: The c.1A>G mutation forces the ribosome to use an alternative downstream start codon (Met36), resulting in a protein truncated by 35 amino acids (p.(Met1_Glu35del)).
• Protein Stability: The truncated protein lacks part of the ATP-binding pocket within the kinase domain. Cycloheximide chase assays showed the mutant protein is significantly less stable than the wild-type (p=0.008).
• Structural Impact: The truncation removes residues essential for ATP binding, likely abolishing kinase activity.

D. Clinical Outcomes

• One brother with HIPK4 variants successfully fathered a child via Intracytoplasmic Sperm Injection (ICSI), proving that assisted reproduction is a viable treatment option despite the severe teratozoospermia.

5. Significance and Implications

• Diagnostic Utility: This study establishes HIPK4 as a candidate gene for male infertility characterized by sperm head defects. It highlights the necessity of exome sequencing in families to distinguish between different genetic causes that may present with similar clinical infertility.
• Genetic Heterogeneity: The case of the four brothers demonstrates that distinct genetic etiologies (DNAH17 vs. HIPK4) can co-occur in a single family, complicating genetic counseling if only one gene is tested.
• Therapeutic Guidance: The findings suggest that men with HIPK4-related infertility should be counseled for ICSI rather than standard IVF, given the severe head defects.
• Contraception Potential: Given that HIPK4 is testis-specific and its knockout causes infertility without other systemic effects in mice, it reinforces the potential of HIPK4 as a target for reversible male contraceptives.
• Current Limitations: The gene-disease relationship is currently classified as "moderate" evidence (pending independent replication of cases), as this is the first report of biallelic HIPK4 variants causing teratozoospermia in humans.

Conclusion: Koser et al. provide robust evidence that biallelic loss-of-function variants in HIPK4 cause autosomal recessive teratozoospermia in humans, characterized by 100% sperm head abnormalities and reduced protein stability. This expands the genetic landscape of male infertility and offers specific diagnostic and therapeutic pathways for affected families.