Blood XX, Skin XY, College

Masunaga and others, 2021, report a new patient whose body holds a mom-only cell line, made of cells carrying only maternal chromosomes.

Before you read the paper by Masunaga and others, make a guess. What percent of their new patient’s white blood cells came from the mom-only line? Write one sentence on why you think so.

Table 1 gives the mom-only share in two tissues for five of the six cases, and its last column is blank for your answers.

Genetic tests usually start with blood, and a blood test reads white blood cells. Table 1 gives a white blood cell share for every case except Case 5.

In Table 1, five cases have a mom-only share for two different tissues.

For Case 6, the authors propose that the egg kept its second polar body, which holds sister copies of the egg’s own chromosomes. In Figure 1 and the Results section, regions next to the centromeres carried one version from mom. Some regions farther out carried two different versions from mom.

The Case 6 patient has idiopathic hypersomnia, a sleep disorder. Exome sequencing found a rare variant in PER2, a circadian clock gene, in 55 of 66 sequencing reads from her and in 25 of 52 reads from her mother, who has no sleep symptoms.

Exome sequencing found the PER2 variant in 25 of 52 reads from the mother of the Case 6 patient.

The PER2 reads from the Case 6 patient come from a mix of mom-only cells and two-parent cells.

The Case 6 patient’s mother carries one copy of a recessive PER2 variant and has no sleep symptoms.

Masunaga and others note a bias in how doctors find people with a mom-only cell line. Genetic tests usually start with blood, and a mom-only line barely present in white blood cells would go unidentified.

Table 1 of Masunaga and others, 2021, lists all six known cases of a mom-only cell line.