Clinical characteristics: X-linked hyper IgM syndrome (HIGM1), a disorder of abnormal T- and B-cell function, is characterized by low serum concentrations of IgG, IgA, and IgE with normal or elevated serum concentrations of IgM. Mitogen proliferation may be normal, but NK- and T-cell cytotoxicity can be impaired. Antigen-specific responses are usually decreased or absent. Total numbers of B cells are normal but there is a marked reduction of class-switched memory B cells. Defective oxidative burst of both neutrophils and macrophages has been reported. The range of clinical findings varies, even within the same family. More than 50% of males with HIGM1 develop symptoms by age one year, and more than 90% are symptomatic by age four years. HIGM1 usually presents in infancy with recurrent upper- and lower-respiratory tract bacterial infections, opportunistic infections including Pneumocystis jirovecii pneumonia, and recurrent or protracted diarrhea that can be infectious or noninfectious and is associated with failure to thrive. Neutropenia is common; thrombocytopenia and anemia are less commonly seen. Autoimmune and/or inflammatory disorders (such as sclerosing cholangitis) as well as increased risk for neoplasms have been reported as medical complications of this disorder. Significant neurologic complications, often the result of a CNS infection, are seen in 5%-15% of affected males. Liver disease, a serious complication of HIGM1 once observed in more than 80% of affected males by age 20 years, may be decreasing with adequate screening and treatment of Cryptosporidium infection.

Diagnosis/testing: The diagnosis of X-linked hyper IgM syndrome is established in a male proband with typical clinical and laboratory findings and a hemizygous pathogenic variant in CD40LG identified by molecular genetic testing.

Management: Treatment of manifestations: Hematopoietic stem cell transplantation (HSCT) (the only curative treatment currently available), ideally performed before age ten years, prior to evidence of organ damage; immunoglobulin replacement therapy (either intravenous or subcutaneous); appropriate antimicrobial therapy for acute infections; antimicrobial prophylaxis for opportunistic infection against Pneumocysitis jirovecii pneumonia; recombinant granulocyte colony-stimulating factor for chronic neutropenia; immunosuppressants for autoimmune disorders.

Agents/circumstances to avoid: Areas that place individual at risk of contracting Cryptosporidium including pools, lakes, ponds, or certain water sources; drinking unpurified or unfiltered water; live vaccines such as rotavirus, MMR, varicella, live attenuated polio, and BCG.

Surveillance: At least annually: CBC with differential to monitor for cytopenias, testing of IgG levels and lymphocyte subpopulations, pulmonary function tests after age seven years. Regular assessment of liver function, consider abdominal imaging; as well as polymerase chain reaction-based testing for the presence of enteric pathogens including Cryptosporidium. Monitor growth and general health with a low threshold for lymph node biopsy, given elevated oncologic risk.

Evaluation of relatives at risk: It is appropriate to clarify the genetic status of newborn at-risk relatives of an affected individual in order to identify as early as possible those who would benefit from early diagnosis and prompt initiation of treatment and prevention of infections.

Genetic counseling: By definition, X-linked hyper IgM syndrome (HIGM1) is inherited in an X-linked manner. Affected males transmit the pathogenic variant to all their daughters and none of their sons. Women with a CD40LG pathogenic variant have a 50% chance of transmitting the pathogenic variant in each pregnancy. Males who inherit the pathogenic variant will be affected. Female who inherit the pathogenic variant will typically be asymptomatic but may have a range of clinical manifestation depending on X-chromosome inactivation. Once the CD40LG pathogenic variant has been identified in an affected family member, heterozygote testing for at-risk female relatives, prenatal testing for a pregnancy at increased risk, and preimplantation genetic testing for HIGM1 are possible.